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Characterization of geosynthetic reinforced airfield pavements at varying scales
By
TITLE PAGE
William Jeremy Robinson
Approved by:
Isaac L. Howard (Major Professor)
John K. Newman
John F. Rushing
Farshid Vahedifard (Committee Member/Graduate Coordinator)
Jason M. Keith (Dean, Bagley College of Engineering)
A Dissertation
Submitted to the Faculty of
Mississippi State University
in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
in Civil Engineering
in the Department of Civil and Environmental Engineering
Mississippi State, Mississippi
August 2020
Copyright by
COPYRIGHT PAGE
William Jeremy Robinson
2020
Name: William Jeremy Robinson
ABSTRACT
Date of Degree: August 7, 2020
Institution: Mississippi State University
Major Field: Civil Engineering
Major Professor: Isaac L. Howard
Title of Study: Characterization of geosynthetic reinforced airfield pavements at varying scales
Pages in Study 166
Candidate for Degree of Doctor of Philosophy
A large amount of research has been conducted to investigate the influence of
incorporating geosynthetics in highway pavements in laboratory-scale and full-scale
experiments, and performance improvement has been well documented. In most cases,
geosynthetics have been found to improve rutting resistance or reduce vertical pressure on the
subgrade. Airfield pavements are typically thicker than highway pavements and are subjected to
higher wheel loads and tire pressures. Thus, the benefit of geosynthetics within airfield
pavements may not be as pronounced as that observed in relatively thin highway pavements.
Prior to the writing of this dissertation, few documented studies focused on the performance of
geosynthetic inclusion in airfield pavements and existing Department of Defense (DOD)
guidance for geosynthetic inclusion had not been updated for several decades. The primary
objectives of this dissertation were to update the DOD geosynthetic design methodology, to
interpret results of laboratory-scale and full-scale experiments conducted specifically to evaluate
geosynthetic performance in airfield pavements, and to determine if a competitive market exists
for geosynthetic inclusion in airfield pavements.
The main body of this dissertation is a compilation of four complementary articles that
build upon the primary components of the main objectives. Chapter 1 and Chapter 2 present an
introduction and a literature review, respectively. Updates to the DOD design methodology are
presented in Chapter 3, results of laboratory-scale and full-scale evaluations are presented in
Chapter 4 and Chapter 5, respectively, and potential implications of geosynthetic inclusion in
airfield pavements are presented in Chapter 6. Chapter 7 presents overall conclusions and
recommendations.
Overall, it was found that, while some geosynthetics can be beneficial in airfield
pavements, more rutting than would typically be allowed on an operational airfield was required
to realize a meaningful performance benefit. In cases where geosynthetics were included in an
airfield pavement, it was found that an extension of service life rather than a reduction in
aggregate thickness was more optimal in assigning a geosynthetic value. Finally, the results of
this dissertation indicated that geosynthetic inclusion in airfield pavements did not yield the same
benefit level as that documented in the literature for highway pavements.
ii
DEDICATION
To my wife, Katie, and children, Nora, Rosie, and Gray, for their love and support.
iii
ACKNOWLEDGEMENTS
Many individuals deserve gratitude for the successful completion of this dissertation. I
would like to thank my major professor, Dr. Isaac L. Howard for his guidance and support. I
would also like to thank all the members of my committee, Dr. Farshid Vahedifard, Dr. John
Rushing, and Dr. Kent Newman.
I would like to thank Mr. Jeb Tingle, ERDC for providing guidance throughout the
process. In addition, I would like to thank Mr. Greg Norwood, Vulcan Materials (formerly of
ERDC), for providing portions of the data analyzed in this dissertation. I would like to thank
Benjamin Mahaffey, Burns and McDonnell (formerly of FAA) for providing technical oversight
of the work described in Chapter 4. I would like to thank Dr. Timothy Rushing, Chief of the
Airfields and Pavements Branch, ERDC for providing support throughout the process. I would
like to thank Lulu Edwards for providing assistance with geosynthetic inclusion in airfield
damage repair described in Chapter 6. Thanks are due to the team of technical staff at ERDC
Airfields and Pavements Branch that provided construction and data collection support.
I would like to thank Tensar International, the Federal Aviation Administration, and the
Air Force Civil Engineering Center for sponsoring the tests described and the resulting data
presented in Chapter 3.
I would like to thank the Federal Aviation Administration for sponsoring the tests
described and the resulting data presented in Chapter 4.
iv
Finally, I would like to thank the U.S. Air Force for sponsoring the tests described and
the resulting data presented in Chapter 5.
v
TABLE OF CONTENTS
DEDICATION ................................................................................................................................ ii
ACKNOWLEDGEMENTS ........................................................................................................... iii
LIST OF TABLES ....................................................................................................................... viii
LIST OF FIGURES .........................................................................................................................x
LIST OF SYMBOLS ..................................................................................................................... xi
CHAPTER
I. INTRODUCTION .............................................................................................................1
1.1 Introduction and Background ................................................................................1
1.1 Objectives and Scope ............................................................................................2
1.2 Organization of Study ............................................................................................4
II. LITERATURE REVIEW ..................................................................................................6
2.1 Overview of Literature Review .............................................................................6
2.2 Introduction ...........................................................................................................6
2.3 Laboratory-Scale Cyclic Plate Load Testing .........................................................8
2.3.1 Highway Loading ............................................................................................8
2.3.2 Airfield Loading ............................................................................................19
2.3.3 Observations from Laboratory Scale Plate Load Testing ..............................19
2.4 Full-Scale Load Testing ......................................................................................21
2.4.1 Highway Loading ..........................................................................................21
2.4.2 Aircraft Loading ............................................................................................32
2.4.3 Observations from Full-Scale Load Testing ..................................................34
2.5 Conclusions from Literature Review ...................................................................36
III. ASSESSMENT OF EQUIVALENT THICKNESS DESIGN PRINCIPLES FOR
GEOSYNTHETIC REINFORCED PAVEMENTS BY WAY OF ACCELERATED
TESTING ........................................................................................................................38
3.1 Introduction .........................................................................................................39
3.2 Objectives and Scope ..........................................................................................40
3.3 Pavement and Materials Properties for Lab and Full-Scale Testing ...................41
vi
3.4 Description of Previous Research Efforts ...........................................................44
3.5 Normalization of Data for Analysis ....................................................................47
3.5.1 Traffic Conversion .........................................................................................47
3.5.2 Asphalt Thickness Conversion ......................................................................48
3.5.3 Selection of Analysis Data ............................................................................49
3.6 Results .................................................................................................................53
3.7 Conclusions .........................................................................................................57
IV. CYCLIC PLATE TESTING OF GEOSYNTHETIC-REINFORCED AIRFIELD
PAVEMENTS .................................................................................................................59
4.1 Introduction .........................................................................................................60
4.2 Literature Review Pertinent to Cyclic Plate Load Testing ..................................61
4.2.1 Test Methods to Assess Geosynthetic Inclusion in Unbound Pavement
Layers ............................................................................................................61
4.2.2 Cyclic Plate Load Testing of Pavements Reinforced with Geosynthetics ....63
4.2.3 Geosynthetic-reinforced Airfields .................................................................67
4.3 Laboratory-scale Test Sections ............................................................................68
4.3.1 Material Properties ........................................................................................69
4.3.2 Instrumentation ..............................................................................................74
4.3.3 As-built Properties .........................................................................................75
4.4 Results .................................................................................................................78
4.4.2 Comparison of Unreinforced Sections ..........................................................79
4.4.3 Traffic Benefit Ratio ......................................................................................81
4.4.4 Interpretation of Permanent Surface Deformation Measurements ................82
4.4.4.1 Phase I Permanent Deformation ..............................................................82
4.4.4.2 Phase II Permanent Surface Deformation ...............................................84
4.4.5 Interpretation of EPC Measurements ............................................................84
4.4.5.1 Phase I EPC Response .............................................................................84
4.4.5.2 Phase II EPC Response ...........................................................................85
4.5 Discussion of Results ..........................................................................................86
4.5.1 Geosynthetic Performance in each Phase ......................................................86
4.5.2 Evaluation of Instrumentation Response and Placement Location ...............88
4.6 Conclusions .........................................................................................................91
V. ANALYSIS OF FULL-SCALE GEOSYNTHETIC REINFORCED AIRFIELD
PAVEMENT SUBJECTED TO ACCELERATED AIRCRAFT LOADING ................93
5.1 Introduction .........................................................................................................94
5.2 Objectives and Scope ..........................................................................................94
5.3 Literature Review Pertinent to Full-Scale Testing ..............................................95
5.4 Full-Scale Test Sections ......................................................................................99
5.4.2 Material Properties ......................................................................................101
5.4.3 As-built Properties .......................................................................................103
5.5 Results ...............................................................................................................105
5.5.1 Assessment of As Built Properties ..............................................................105
vii
5.5.2 Traffic Benefit Ratio ....................................................................................111
5.5.3 Interpretation of Average Rut Depth Measurements ...................................113
5.5.4 Interpretation of Subgrade Earth Pressure Cell (EPC) Measurements ........115
5.5.5 Falling Weight Deflectometer Measurements .............................................117
5.5.6 Statistical Analysis of Pavement Response Data ........................................120
5.5.6.1 Rutting ...................................................................................................121
5.5.6.2 Subgrade Pressure .................................................................................123
5.5.6.3 FWD Deflection Parameters ..................................................................127
5.6 Conclusions .......................................................................................................129
VI. IMPLICATIONS OF INCORPORATING GEOSYNTHETICS IN AIRFIELD
PAVEMENTS ...............................................................................................................131
6.1 Introduction .......................................................................................................132
6.2 Assessment of Existing Airfield Pavement Thickness ......................................133
6.3 DOD Pavement Design Methodology ...............................................................138
6.4 Cost/Value of Geosynthetics .............................................................................145
6.5 Other Uses of Geosynthetics in Airfield Pavements .........................................150
6.6 Conclusions .......................................................................................................152
VII. CONCLUSIONS AND RECOMMENDATIONS ........................................................154
7.1 Summary ............................................................................................................154
7.2 Conclusions .......................................................................................................154
7.3 Recommendations .............................................................................................156
REFERENCES ............................................................................................................................157
viii
LIST OF TABLES
Table 3.1 Asphalt mixture properties .........................................................................................49
Table 3.2 Cyclic plate-load testing .............................................................................................52
Table 3.3 Full-scale testing .........................................................................................................53
Table 4.1 Cyclic highway plate load tests and findings from literature .....................................64
Table 4.2 Laboratory material property test results ....................................................................72
Table 4.3 Geosynthetic properties as provided by manufacturer ...............................................72
Table 4.4 As-built properties (cyclic plate load tests) ................................................................77
Table 4.5 Cycles to failure and TBR ..........................................................................................78
Table 5.1 Full-scale highway load tests and findings from literature ........................................97
Table 5.2 Geosynthetic properties as provided by manufacturers ............................................102
Table 5.3 As-built properties (full-scale tests) .........................................................................104
Table 5.4 Measured and interpolated rutting data (mm) ..........................................................107
Table 5.5 Measured and interpolated pressure cell data (kPa) .................................................107
Table 5.6 Measured and interpolated BDI and BCI .................................................................108
Table 5.7 Measured and Interpolated MBDI and MBCI ..........................................................109
Table 5.8 Measured and interpolated AI4 and AAUP ..............................................................110
Table 5.9 Passes to failure and TBR .........................................................................................111
Table 5.10 Regression parameters from FWD data analysis ......................................................119
Table 5.11 Backcalculated layer modulus values .......................................................................120
Table 5.12 Paired t-test results for rutting ..................................................................................125
ix
Table 5.13 Paired t-test results for subgrade pressure ................................................................126
Table 5.14 Paired t-test results for BDI, BCI, MBDI, and MBCI ..............................................128
Table 5.15 Paired t-test results for AI4 and AAUP .....................................................................129
Table 6.1 Calculated Beta-values based on equivalent thickness methodology (DS2) ............142
Table 6.2 Geosynthetic value in terms of extended life ...........................................................148
Table 6.3 Geosynthetic value in terms of reduced aggregate thickness ...................................150
x
LIST OF FIGURES
Figure 3.1 Equivalent pavement thickness chart from ETL 1110-1-189 (USACE 2003) ...........40
Figure 3.2 Photographs of accelerated testing equipment ...........................................................42
Figure 3.3 Typical cross-section ..................................................................................................43
Figure 3.4 Effect of analysis variable on reinforced pavement regression ..................................51
Figure 3.5 Relationship between equivalent aggregate thickness and ESALs for 25.4
mm rutting ..................................................................................................................55
Figure 3.6 Relationship between unreinforced and reinforced aggregate thickness ...................57
Figure 4.1 Photographs of cyclic plate testing and asphalt paving ..............................................71
Figure 4.2 Photographs of geosynthetics .....................................................................................73
Figure 4.3 Schematic representation of the instrumentation of a typical test item ......................75
Figure 4.4 Instrumentation response ............................................................................................79
Figure 4.5 Surface deformation comparison for GEO1 and GEO3 .............................................88
Figure 4.6 Relationship between pressure and deformation ratios ..............................................91
Figure 5.1 Photographs of load test equipment .........................................................................100
Figure 5.2 Reinforced vs unreinforced rutting ...........................................................................115
Figure 5.3 Reinforced vs unreinforced subgrade pressure .........................................................117
Figure 6.1 Relative frequency of asphalt thickness ...................................................................136
Figure 6.2 Relative frequency of aggregate thickness ...............................................................137
Figure 6.3 Relative frequency of subgrade modulus values ......................................................137
Figure 6.4 Proposed geosynthetic modification to existing beta methodology .........................142
Figure 6.5 Data points comprising beta methodology ...............................................................143
xi
LIST OF SYMBOLS
AASHTO American Association of State Highway and Transportation Officials
AAUP Area under pavement profile
AC Asphalt concrete
AFCEC Air Force Civil Engineering Center
AI4 Fourth area index
AMD Average mean difference
ASTM ASTM International
BCI Base curvature index
BDI Base damage index
B/S Base/subgrade interface
B/2 Mid-depth of base
B/3 One-third from top of base
CBR California Bearing Ratio
CC Coefficient of curvature
C-130 Lockheed Martin C-130 Hercules aircraft
C-17 Boeing C-17 Globemaster aircraft
CH High-plasticity clay
CMD Cross-machine direction
CP Contact pressure
CU Coefficient of uniformity
dgs Depth of geosynthetic below surface
DL Dual layer
DOD Department of Defense
DOT Department of transportation
DS Data set
EPC Earth pressure cell
ERDC Engineer Research and Development Center
ESAL Equivalent single axle load
ESALF Equivalent single axle load factor
ETL Engineering Technical Letter
FAA Federal Aviation Administration
F-15 McDonnell Douglas F-15 Strike Eagle fighter jet
FHWA Federal Highway Administration
FWD Falling weight deflectometer
Gmm Theoretical maximum specific gravity
HMA Hot-mix asphalt
HVS Heavy vehicle simulator
xii
LEA Layered elastic analysis
LL Liquid limit
LVDT Linear variable displacement transducers
MBCI Modified base curvature index
MBDI Modified base damage index
MD Machine direction
MDD Maximum dry density
MDOT Mississippi Department of Transportation
ML-CL Silty clay
MVP Measured vertical pressure
NMAS Nominal maximum aggregate size
OD Oven dried
OMC Optimum moisture content
P200 Percent passing #200 sieve
Pb Asphalt binder content
PET Polyester
PG Performance grade
PGRAVEL Percent gravel
PL Plastic limit
PP Polypropylene
PSAND Percent sand
QC Quality control
RAP Recycled asphalt pavement
SaS Coarse sand
SDR Surface deformation reinforced
SDU Surface deformation unreinforced
S/S Subbase/subgrade interface
StS Stone screenings
tBase Base course thickness
TBR Traffic benefit ratio
tequiv Equivalent base course thickness
tHMA Asphalt thickness
TI Test item
TS Tensile strength
tsubbase Subbase thickness
USACE U.S. Army Corps of Engineers
USCS Unified Soil Classification System
VMA Voids in mineral aggregate
1
CHAPTER I
INTRODUCTION
1.1 Introduction and Background
In the early 1980s, geogrids were introduced in the United States for reinforcement
applications in railroad track ballast, unsurfaced (aggregate) pavements, surfaced flexible
(asphalt) pavements, and soil reinforcement. These products are incorporated in a pavement
system as a means of improving constructability over soft substrates, extending pavement service
life, or reducing overall pavement thickness.
The U.S. Army Engineer Research and Development Center (ERDC) has evaluated a
number of geosynthetic-reinforced pavements beginning in the early 1960s and continuing up to
today. One of the more comprehensive studies, conducted in the 1990s, developed a design
methodology to incorporate geosynthetics in base course for flexible pavements by using an
equivalent aggregate thickness concept, where the geosynthetic was assigned value in terms of
base course aggregate thickness. The study was limited to subgrade California Bearing Ratio
(CBR) values of 3 and 8 and was trafficked with relatively light airfield traffic loads.
Over the next 20 to 30 years, several laboratory-scale and full-scale geosynthetic
pavement investigations, using highway wheel loads and tire pressures, were performed at
ERDC to evaluate emerging geosynthetic products. These studies were generally stand-alone
investigations and the primary intent was to compare performance of newer or prototype
geosynthetic products to unreinforced pavement sections. No documented updates to the
2
original equivalent aggregate thickness concept were performed as new data become available,
leading the author to investigate the compilation of data from all geosynthetic pavement
investigations performed at ERDC to validate the decades old design procedure. Twenty-two
data points were added to the original equivalent aggregate thickness chart (which consisted of
eight data points). Simplifying assumptions were made, namely loading was converted to
equivalent single axle loads (ESALs) and hot mix asphalt (HMA) thicknesses were converted to
equivalent aggregate thickness.
A literature search revealed that a large amount of research has been conducted to
investigate the influence of incorporating geosynthetics in pavements (often thinner pavements),
particularly under highway loads and potential performance improvement has been well
documented. Numerous studies have documented laboratory-scale (cyclic plate load tests) and
full-scale (either accelerated testing facilities or in-service pavements) experiments on highway
pavements. Airfield pavements can be subjected to much higher gross wheel loads and tire
contact pressures and can be substantially thicker than highway pavements. Limited documented
studies can be found that investigate performance implications of including geosynthetics in
these thicker airfield pavements, therefore data are needed to quantify geosynthetic behavior in
airfield pavements. This dissertation contributes to the body of knowledge of thicker airfield
pavements.
1.1 Objectives and Scope
To address the lack of documented (and likely non-existent) geosynthetic performance in
airfield pavements, a laboratory-scale (hereinafter referred to as cyclic plate load test) and a full-
scale study were performed. Cyclic plate load test provide a relatively rapid construction and
testing timeframe when compared to full-scale testing and results can be typically obtained over
3
a duration of roughly six weeks. Full-scale testing provides a more realistic simulation of in-
service conditions; however, testing duration can extend for months or even years.
A series of eleven laboratory-scale representative airfield pavements incorporating
various geosynthetics were constructed and tested to gather data to advance the state-of-
knowledge of how geosynthetics affect flexible airfield pavement performance. The laboratory-
scale pavements consisted of a 3 CBR clay subgrade, 305 mm thick subbase, 178 mm thick base,
and 127 mm thick asphalt pavement surface. Geosynthetics were placed at two locations in the
pavement structure and simulated aircraft loading was applied until failure (defined by
permanent surface deformation).
Additionally, nine full-scale test items were constructed under shelter in ERDC’s Hangar
4 Pavement Test Facility and subjected to accelerated trafficking with a single C-17 aircraft
wheel using a Heavy Vehicle Simulator (HVS). The pavements consisted of an 8 CBR clay
subgrade, 360 mm thick base, and 100 mm thick asphalt pavement surface. Geosynthetics were
located at the base/subgrade interface and one item contained a geosynthetic at mid-depth of the
base course layer.
A generous amount of data were collected from these experiments and represent a
meaningful advancement in quantifying the behavior of geosynthetics in flexible airfield
pavements.
The research presented in this dissertation utilizes historical data collected at ERDC and
the results of laboratory-scale cyclic plate load test and full-scale pavement tests to advance the
state-of-knowledge for including geosynthetics in flexible pavements, with a unique focus on
airfield pavements. The primary objectives of this dissertation are to:
4
1. Validate and improve historical USACE equivalent thickness design criteria for
light duty aircraft and highway loading by expanding the limited original dataset.
2. Determine if meaningful performance improvement can be achieved with
geosynthetic inclusion in thick flexible airfield pavements by way of laboratory-
scale cyclic plate load tests and full-scale accelerated testing.
3. Determine if a sufficient number of geosynthetic products demonstrate an overall
performance improvement such that a competitive market exists for geosynthetic
inclusion in airfield pavements.
It is envisioned that the results of this study will be used to improve design guidance for
geosynthetic use in flexible airfield pavement systems. Specifically, the results will be used to
understand the potential performance implications in terms of base course thickness reduction
and/or extended service life.
1.2 Organization of Study
This dissertation is organized into seven chapters. The first and last chapters are an
introduction and conclusion, respectively. Chapter 2 presents a comprehensive review of
available literature related to full-scale and laboratory-scale evaluation of geosynthetic-
reinforced pavements. Chapter 3 presents the findings and relevant conclusions of an effort to
compile historical geosynthetic studies performed at ERDC. Chapter 4 presents construction and
performance data obtained from cyclic plate testing eleven laboratory-scale flexible airfield
pavements (both reinforced and unreinforced) in a box containment facility. Chapter 5 presents
the results and conclusions of trafficking nine full-scale flexible airfield pavements with a HVS.
Chapter 6 investigates modifications to the current Department of Defense (DOD) pavement
design methodology using data from Chapter 3, Chapter 4, and Chapter 5 and investigates the
cost/benefit of geosynthetic inclusion from the standpoint of extended service life and aggregate
5
thickness reduction. Chapter 7 summarizes conclusions and recommendations based on the data
collected for this dissertation.
At the time of the writing of this dissertation, the article presented in Chapter 3 has been
published as a peer reviewed journal article in the Transportation Research Record: Journal of
the Transportation Research Board, and Chapter 4 has been published as a peer reviewed journal
article in the Proceedings of the Institution of Civil Engineers-Ground Improvement. Chapter 5
has been accepted for publication as a peer reviewed journal article in the Journal of
Transportation Engineering Part B: Pavements, and Chapter 6 has been submitted to a peer-
reviewed journal for consideration. Some minor modifications have been performed to the
published and submitted documents to comply with the formatting requirements of this
dissertation; however, the technical content has not been altered. Permission (if needed) has
been obtained to reproduce the content in this document that is published in peer-reviewed
journals.
6
CHAPTER II
LITERATURE REVIEW
2.1 Overview of Literature Review
A literature review was performed to determine the extent to which accelerated pavement
testing at varying scales has been performed on representative highway and airfield pavements
incorporating geosynthetics. This literature review concentrated on geosynthetic placement in
aggregate layers, where performance improvement in terms of surface rutting and/or vertical
stress redistribution was the primary focus.
It is worthy to note that some literature (Austin and Gilchrist 1996; Abdesssemed, Kenai,
and Bali 2015; Buonsanti, Leonardi, and Scopelliti 2012; Von Quintas, Mallela, and Lytton
2009) documented geosynthetic inclusion in the upper portion of a flexible airfield pavement,
specifically, within asphalt layers, and reflective crack mitigation was the primary improvement
mechanism identified; however, this application is beyond the scope of this dissertation.
2.2 Introduction
A number of testing protocols are available to evaluate geosynthetic inclusion in
pavements, ranging in scale from bench-top testing to full-scale testing. Direct shear and shear
wave tests are examples of laboratory-scale experiments that can provide an indication of local
influence of geosynthetics on adjacent aggregate materials. Direct shear testing (Arulrajah,
Rahman, Priatheepan, Bo, and Imteaz 2014; Suddeepong, Sari, Horpibulsuk, Chinkulkijniwat,
and Arulrajah 2018) are examples of evaluations conducted to investigate the interface shear
7
strength of geosynthetic reinforced soils and aggregates. Some studies (Byun and Tutumluer
2017; Schuettpelz, Fratta, and Edil 2009) have used novel shear wave velocity measurement
techniques to investigate the localized zone of influence and stiffness enhancement properties of
geogrid inclusion in granular materials.
On the opposite end of the testing spectrum, field-testing is usually more expensive than
laboratory evaluations and presents logistical challenges; both have positive and negative
elements that should be considered. Full-scale field-testing is purely realistic but has replication
and variability challenges in many cases. Laboratory experiments of varying scales generally
permit more control on variables but with this control comes boundary condition, loading and
calibration challenges. Full-scale testing provides the opportunity to measure structural response
(e.g. stress, strain and deflection) via embedded instrumentation and environmental conditions
(e.g. temperature and moisture content) combined with monitoring of pavement distresses and
overall performance.
Cyclic plate load testing could be considered a balance between full-scale field testing
and smaller-scale laboratory experiments. Tested sections remain fairly large, instrumentation
can still be deployed, but their scale is still noticeably smaller than full-scale field experiments.
Additionally, direct correlation to full-scale test results can be problematic in some cases.
The literature review is organized into two sections: laboratory-scale cyclic plate load
testing and full-scale load testing. Each section is further divided into highway and airfield
loading conditions. Observations from each subsection are summarized and overall conclusions
from the literature review are presented at the end of the chapter.
8
2.3 Laboratory-Scale Cyclic Plate Load Testing
Laboratory-scale plate load tests are typically performed on smaller scale pavement
structures. Advantages of cyclic plate tests include reduced cost (when compared to full-scale
construction), increased construction and testing speed, and decreased variability. Inability to
simulate pavement response under a moving load and to emulate tire/pavement interactions
could be considered disadvantageous. However, as new geosynthetic products are developed,
cyclic plate load tests provide for a relatively rapid evaluation. Multiple cyclic plate load tests
have been performed to investigate performance benefits of incorporating geosynthetics in both
paved and unpaved applications. A summary of documented cyclic plate load tests are presented
in the following sections.
2.3.1 Highway Loading
Bauer and Abdelhalim (1987) evaluated an unsurfaced pavement with aggregate
thicknesses ranging from 75 to 300 mm using a 550 kPa contact pressure. It was found that the
number of load cycles to reach 28 mm of rutting was increased from 155,000 for the
unreinforced base to 233,000 for the geogrid-reinforced base. It was found that about 10,000
load cycles were required to mobilize full geogrid strength.
Haas, Walls, and Carroll (1988) performed a comprehensive research program with
asphalt thickness of 50, 75, and 100 mm, base thicknesses of 100, 150, 200, 254, and 300 mm,
and subgrade CBR values of <1, 1, 3, 5, and 8. It was found that geogrid reinforcement altered
stress distribution in flexible pavements, resulting in a reduced rate of permanent deformation.
In terms of geosynthetic placement location, it was suggested that the optimal location was
usually at the base/subgrade interface. For thicker bases, it was suggested that the optimal
placement location was near mid-height. Further, it was concluded that no benefits should be
9
expected when a single layer geogrid is placed near the top of a base layer under an asphalt
surface or at the midpoint or higher of a base layer over soft subgrades. It was recommended
that the geogrid be placed in a zone of elastic tensile strain ranging from 0.05 to 0.2 percent and
that maximum permanent strain in the grid over the design life should not exceed 1 to 2 percent.
Douglas and Valsangkar (1992) recommended the use of roadway stiffness rather than rut
depth to define failure in unpaved roads. Cyclic plate load tests were carried out using a pit run
gravel (to simulate a weak base) and compacted crushed rock (to simulate a strong base). A peat
material was used to simulate a very weak (CBR < 1) subgrade. Geotextile and geogrids were
evaluated, and a geotextile was placed at the subgrade/base interface and a geogrid was placed at
mid-depth of the 150 mm thick base course. A 4.5 kN load was applied to a 300 mm circular
plate at a frequency of 0.5 Hz. Stiffness was defined as applied pressure (kN/m2) divided by
displacement (m). The compacted crushed rock structure with geogrid at mid-depth was found
to have a stiffness 3.6 times that of a weak subgrade, and the pit run gravel was found to have a
stiffness 2.4 times that of a weak subgrade.
Kelly, Fairfield, and Sibbald (1995) investigated the difference in observed rut depths
when using anchored and unanchored geosynthetic installation techniques for unsurfaced
pavements. It was found that minimal improvement was observed in anchored installations
versus unanchored installations. Additionally, it was observed that a loss of interlock between
the geosynthetic and granular layer reduced the effect of lateral restraint under low
displacements, and increased geosynthetic tensile modulus reduced vertical displacement.
Douglas (1997) used a small-scale model to evaluate access roads used by forestry and
mining industries in Canada. The model tests were used to design unbound geosynthetic-built
roads using stiffness rather than rut depth as the failure criteria. It was argued that unsurfaced
10
roads, by nature, require periodic maintenance that would essentially eliminate surface
deformation. The preliminary model incorporated a dimensionless tension term to characterize a
geosynthetic.
Al-Qadi, Brandon, Valentine, Lacina, and Smith (1994) performed cyclic plate load
testing of a flexible pavement section constructed on a 4 CBR subgrade. The section consisted
of 70 mm of asphalt over 150 mm of base. Two sections were reinforced with geotextiles and
one was reinforced with a geogrid, with all placed at the base/subgrade interface. Little
improvement was observed from the geogrid; however, the geotextile improved performance up
to 35% and it was noted that separation provided by the geotextile appeared to be important to
improving structural capacity.
Laboratory model tests were performed by Das and Shin (1994) to investigate the
potential of using geogrids to improve bearing capacity in shallow foundations. Relatively small
box tests (915 mm long by 229 mm wide by 607 mm tall) were performed using a saturated
clayey soil reinforced with multiple layers of a biaxial geogrid. It was found that full depth
reinforcement may reduce permanent settlement under cyclic loading by 20-30% when
compared to a model test without reinforcement.
Cancelli, Montanelli, Rimoldi, and Zhao (1996) tested 75 mm thick asphalt and 300 mm
thick base over soils with CBR’s ranging from 1 to 18. Geogrids were placed at the
base/subgrade interface and by placing an additional layer at mid-height in the aggregate base.
Two layers of geogrid were found to provide a decrease in maximum settlement when compared
to one layer only. The percent reduction in rutting was found to increase as CBR decreased,
suggesting that potential improvement increases at lower CBR values. It was concluded that the
11
structural layer coefficient of the aggregate could be increased by 1.5 to 2 depending on the
subgrade CBR.
Montanelli, Zhao, and Rimoldi (1997) evaluated flexible pavement sections constructed
over subgrade soils with CBR’s ranging from 1 to 18. The results were used to modify the
AASHTO design method by increasing the base layer structural coefficient. It was found that
the base layer coefficient could be increased 1.5 to 2 times with geosynthetic inclusion.
Perkins (1999) found that significant improvement in surface rutting was observed in 75
mm thick asphalt pavement sections over various base thicknesses with inclusion of geosynthetic
reinforcement. It was found that a stiffer geogrid provided for better performance and that better
performance was observed when the geogrid was elevated in the base layer.
Leng and Gabr (2002) evaluated nine unsurfaced pavement sections consisting of base
course ranging from 150 mm to 274 mm thick constructed over a subgrade soil comprised of
85% sand and 15% kaolinite and CBR values ranging from 3 to 4. A 40 kN load was applied
through a 305 mm circular plate that resulted in an applied pressure of 550 kPa. Two biaxial
geogrids were evaluated, having the same aperture size of 25 mm by 33 mm, but different tensile
strength and modulus. Surface deformation and vertical stress were measured during load
application. It was found that geogrid reinforcement reduced surface deformation and improved
stress distribution to the subgrade. Further, it was found that, in terms of surface deformation,
the higher tensile strength and modulus geogrid performed better than the lower tensile strength
and modulus geogrid.
Tingle and Jersey (2005) performed laboratory-scale testing of geosynthetic reinforced
aggregate surfaced sections. Six sections were evaluated consisting of five sections with 356 mm
of crushed limestone and one section with 508 mm of crushed limestone. One of the five 356
12
mm aggregate sections was unreinforced and the 508 mm aggregate section was unreinforced.
Geosynthetic reinforcement consisted of a punched and drawn biaxial geogrid and a needle-
punched nonwoven polypropylene geotextile. A design subgrade CBR of 1 was targeted for all
sections. Cyclic plate load testing was accomplished through a 305 mm diameter steel plate with
a total load of 40 kN and a 551 kPa contact pressure. It was concluded that all reinforced
pavement sections showed an improvement in rutting performance when compared to the
unreinforced pavement sections as determined by Traffic Benefit Ratios (TBR) (ratio of load
cycles of a reinforced pavement structure to a defined failure state to load cycles of the same
unreinforced pavement structure at the same failure state) in excess of 1.0. It was hypothesized
that at very low subgrade strengths, the separation function provided by geotextiles was the
primary means of reinforcement and base reinforcement was a secondary function. Further, the
authors hypothesized that there may be a maximum geosynthetic depth of placement based on a
Boussinesq analysis indicating that the horizontal stress approaches zero at a depth of 406 mm
below the pavement surface.
Chen, Abu-Farsakh, and Tao (2009) investigated the effects of geogrid aperture shape on
pavement rutting. Two geogrids, one biaxial and one triaxial, where placed at the base/subgrade
interface. Five test sections were evaluated including: one unreinforced section on an 8 CBR
subgrade, two unreinforced sections on a 0.5 CBR subgrade, one biaxial geogrid reinforced
section on a 0.5 CBR subgrade, and one triaxial geogrid reinforced section on a 0.5 CBR
subgrade. The maximum applied load was 40 kN on a 305 mm diameter plate. It was observed
that subgrade strength significantly affected test section performance and that the section with an
8 CBR subgrade performed over 2,000 times better when compared to a 0.5 CBR section. It was
13
concluded that the triaxial geogrid performed similar to the biaxial geogrid with differences that
were insignificant.
Jersey and Tingle (2009) evaluated three different geogrids (two triaxial and biaxial)
placed over a 3 CBR subgrade. The pavement sections consisted of 152 mm crushed limestone
base over a 3 CBR CH subgrade that was surfaced with a 12.5 mm thick rubber mat. The 12.5
mm thick rubber mat was used to simulate a flexible pavement surface. Each item was subjected
to a series of load levels at the following magnitudes: 13 kN, 27 kN, 40 kN, 49 kN, 58 kN, and
67 kN for a duration of 5,000 cycles or until a permanent deformation of 12.5 mm. Relative
improvements were observed between the three different geogrids evaluated; however, an
unreinforced section was not tested. Failure at high loads was attributed to exceeding the bearing
capacity of the soft clay subgrade layer.
Dong, Han, and Bai (2010) evaluated the soil bearing capacity of geogrid-reinforced
bases subjected to static loading. Geogrids evaluated included three triangular shape aperture
products and two biaxial geogrid products. A poorly graded sand was used as the 200 mm thick
granular base, and the geogrids were placed at a depth of 50 mm or 100 mm below the surface.
Load was applied until total displacement exceeded 20 mm. It was found that the best
performance was observed when the geogrid was placed at a depth of 50 mm below the surface.
It was concluded that heavier and stiffer geogrids resulted in higher ultimate bearing capacity
and base stiffness.
Abu-Farsakh and Chen (2011) evaluated four different geogrids in flexible pavement
sections consisting of 51 mm thick asphalt pavement, 305 mm thick base course, and a 0.5 CBR
subgrade. Four sections contained a geogrid placed at the base/subgrade interface, two sections
contained a geogrid placed at the middle of the base layer, and one section contained a geogrid
14
placed at the upper one third of the base layer. A 40 kN load was applied to a 305 mm diameter
circular plate, yielding a contact pressure of 550 kPa. It was found that the inclusion of geogrid
significantly reduced rut depth, and TBR values of up to 15 were achieved at 19 mm of rutting.
Better performance was observed when a geogrid was placed in the upper one third of the base
layer than when a geogrid was placed in the middle of the base layer or at the base/subgrade
interface.
Gongora and Palmeira (2012) evaluated the use of geosynthetic-reinforced gravel and
recycled rubble fills in unpaved road applications over a 4 CBR subgrade. It was found that the
presence of reinforcement significantly increased the number of load repetitions, validating the
use of recycled rubble as a fill material in unpaved roads. It was suggested that performance was
a function of a combination of geosynthetic properties including aperture stability, modulus, and
tensile stiffness.
Qian, Han, Pokharel, and Parsons (2013) found that triangular aperture geogrids
improved performance of reinforced base courses with TBR values ranging from 1.0 to 13.0 at
permanent displacement from 25 to 75 mm, respectively. Twelve test sections were evaluated at
base thicknesses of 150, 230, and 300 mm constructed over a 2 CBR subgrade. Three geogrids
of varying stiffness were placed at the base/subgrade interface. Heavy-duty geogrid sections
were found to have higher TBR values than light-duty geogrid sections. It was found that
triangular aperture geogrids reduced maximum vertical stress on the subgrade. Further, it was
suggested that confinement provided by the geogrid aggregate interlock was the key mechanism
of improvement and that the tensioned membrane effect was recognized when permanent
deformation was larger than one-third the base thickness.
15
Tang, Stoffels, and Palomino (2013) used a medium-scale test apparatus to apply rolling-
wheel loads to a series of eight test sections. Three biaxial geogrids were evaluated in sections
consisting of 40 mm thick asphalt, 100 mm thick base, and subgrade CBR of 2.0 and 1.5.
Geogrids were placed at the base/subgrade interface, and resilient and permanent deformation
were monitored at the top of the subgrade by a linear variable displacement transducer (LVDT).
A 2.7 kN wheel load with a contact pressure of 690 kPa was applied to test tires with dimensions
of 30 cm diameter and 8 cm width. It was found that surface deflection of the asphalt layer in
the unreinforced section did not display a definitive trend when compared to the reinforced
sections, and it was suggested that the geogrid did not influence the unbound layers.
Additionally, direct measurement of the subgrade resilient deformation did not show significant
differences between the control and reinforced sections. However, it was observed that two of
the geogrids consistently reduced permanent deformation in the subgrade.
Sun, Han, Wayne, Parsons, and Kwon (2014) varied loading from 5 to 45 kN in a series
of cyclic plate load tests to investigate the performance of two triaxial geogrids in unsurfaced
pavement sections constructed over a 2 CBR subgrade. The base course was 230 mm thick, and
the geogrids were placed at the base/subgrade interface. Surface displacement, vertical pressure,
and horizontal pressure was monitored during load application. It was observed that the triaxial
geogrid reduced permanent deformation in the subgrade and base course, and that a heavier duty
geogrid was more beneficial.
Ghafoori and Sharbaf (2015) evaluated a triaxial geogrid placed in the middle of a 406
mm thick base layer with a 76 mm thick asphalt surface. The section was supported by a
relatively stiff 8 CBR subgrade. It was found that the geogrid improved performance in terms of
16
rutting, and that including the geogrid reduced the measured vertical stress in the middle of the
base by approximately 40%.
Sun, Han, Kwon, Parsons, and Wayne (2015) found that vertical stress at the base-
subgrade interface was reduced by geogrid inclusion. An increase in base course thickness was
found to decrease the vertical stress (as expected), but the reduction contributed by the presence
of the geogrid decreased, suggesting performance benefit decreases with increasing depth of
placement. Measured radial stresses away from the load plate decreased, indicating lateral
confinement provided by a geogrid changed the stress distribution.
Abu-Farsakh, Akond, and Chen (2016) investigated the effect of geogrid parameters
(tensile modulus, aperture shape, and geogrid location) on stress distribution in nine pavement
sections. Two sections were unreinforced, four contained a geogrid placed at the base/subgrade
interface, two contained a geogrid placed in the middle of the base layer, and one contained a
geogrid placed in the upper one third of the base layer. A 40 kN test load was applied to a 305
mm diameter circular plate. The design pavement section consisted of 51 mm thick asphalt, 305
mm thick base course, and a 0.5 CBR subgrade. It was found that the inclusion of geogrid base
reinforcement reduced stress concentration and improved vertical stress distributions on top of
the subgrade. It was observed that the pavement section with geogrid placed at the upper one-
third of the base layer provided the best stress improvement, and that as geogrid tensile modulus
increased, vertical stress at the top of subgrade decreased.
Abu-Farsakh, Hanandeh, Mohammed, and Chen (2016) subjected 76 mm thick asphalt
pavement, 254 and 457 mm thick base course, both over a 0.5 CBR subgrade, to a series of
increasing cyclic plate loads. Geosynthetics included a geogrid, a non-woven geotextile, and a
high-strength woven geotextile. Sections consisted of a non-woven geotextile at the
17
base/subgrade interface, a woven geotextile at the base/subgrade interface, a geogrid over a non-
woven geotextile at the base subgrade interface, and a geogrid at one third the base layer plus
geogrid over non-woven geotextile at the base subgrade interface. It was found that the single
layer geosynthetic at the base/subgrade interface resulted in TBR values up to 1.5. The best
rutting performance was observed in the section containing multiple geosynthetic layers.
Instrumentation response data suggested that the inclusion of geosynthetic reinforcement reduced
the stress concentration on top of the subgrade layer.
El-Maaty (2016) evaluated the effect of geosynthetic reinforcement on 10 cm, 15 cm and
25 cm thick base course material. Two geogrids and one woven geotextile were investigated at
various depths in the unsurfaced pavement section. Loading was applied at an initial static
pressure of 0.0875 N/mm2, held constant for approximately 20 minutes, and then increased to
0.35 N/mm2 at 0.0875 N/mm2 increments. Elastic and plastic deformation were measured using
a dial gauge on the center load plate. It was found that better performance was observed when
the geosynthetics were placed at the bottom of the section rather than the middle of the section.
For the 25 cm thick base, when the geogrid was elevated in the base course, it was found that
optimal placement location ranged from 40-60% of the base course thickness.
Sarici, Demir, Tutumluer, Demir, Gungor, Epsileli, Comez, and Ok (2016) concluded
permanent displacement of unpaved road sections over weak subgrade can be reduced by
geogrid inclusion. Six tests were performed on base thicknesses of 300, 400, and 450 mm.
Geogrid placement location was evaluated in the 450 mm thick base and included one-third
depth, two-third depth and at the base/subgrade interface. Geogrid placement at the upper one-
third of base thickness was recommended for best performance.
18
Ibrahim, El-Badawy, Ibrahim, Gabr, and Azam (2017) constructed five test sections
consisting of 50 mm thick asphalt, 150 mm thick base, and 300 mm thick clay subgrade. A
single uniaxial geogrid was evaluated and was placed at the base/subgrade interface, one-third of
the height of the base layer, mid-depth of the base layer, and at the base/asphalt interface. It was
found that geogrid reinforcement showed a reduction in measured tensile strain when compared
to a pavement without reinforcement. When a geogrid was placed at one-third to mid-height of
the base layer, reductions in tensile strain at the bottom of the asphalt and top of the subgrade
were observed. When a geogrid was placed at the base/subgrade interface, increased tensile
strain was observed at the bottom of the asphalt layer, while reduced strains were measured at
the bottom of the base layer. It was suggested that geogrid placement at the bottom of a base
layer may improve overall rutting performance but may result in reductions in cracking
resistance of the asphalt layer that could be attributed to overall stiffening of the granular base
layer.
Luo, Gu, Luo, Lytton, Hajj, Siddharthan, Elfass, Piratheepan, and Pournoman (2017)
performed cyclic plate load test on flexible pavements comprised of two base course thicknesses
(150 and 254 mm) and one asphalt thickness (150 mm). Both static and dynamic loading were
applied at load levels ranging from 27 to 72 kN. Geosynthetics (one biaxial geogrid and one
geotextile) were installed at the base/subgrade interface and at mid-depth of the base layer.
Stress distributions were measured above and below the geosynthetic. It was observed that lower
vertical stress under the center of applied load were measured in the sections that included
geosynthetics when compared to the unreinforced sections, and that the reductions were slightly
higher in sections with geogrid.
19
2.3.2 Airfield Loading
A literature search found that no documented laboratory scale plate load test have been
performed using representative airfield pavement sections or airfield loading conditions. None
of the 28 cyclic plate load references included a subbase aggregate course, and the maximum
base course thickness investigated for a HMA surfaced pavement was 457 mm (Abu-Farsakh,
Hanandeh, Mohammed, and Chen 2016).
Loading conditions (total load and contact pressure) were generally on the order of 40 kN
and 550 kPa, which were considerably lower than a range of present day military airfield loading
conditions. For example, a C-17 cargo aircraft may have loads of 200 kN and 965 kPa, and a F-
15 fighter jet may have loads of 155 kN and 2240 kPa. Only three of the 28 references (11%)
were found to investigate somewhat higher loading conditions (Sun, Han, Kwon, Parsons, and
Wayne 2015; Jersey and Tingle 2009; Abu-Farsakh, Hanadeh, Mohammed, and Chen 2016), and
it should be noted that Sun, Han, Kwon, Parson, and Wayne (2015) and Jersey and Tingle (2009)
evaluated unsurfaced pavement conditions.
The lack of representative airfield pavement structures and aircraft loading conditions
represent a meaningful gap in the body of knowledge. Chapter 4 of this dissertation endeavors to
address this gap by presenting the results of cyclic plate load tests conducted on representative
airfield pavements under much higher loading (128 kN) and contact pressure (1750 kPa)
conditions.
2.3.3 Observations from Laboratory Scale Plate Load Testing
A total of 28 cyclic plate load testing references were reviewed and the results of
laboratory scale plate load testing indicated that inclusion of geosynthetics generally improve
performance of highway pavements by increasing cycles to failure or reducing permanent
20
deformation. Reported TBR values at 25 mm surface deformation were found to range from 1.0
(Qian, Han, Pokharel, and Parsons 2013) to 20.3 (Gongora and Palmeira 2012). The average
reported TBR value at 25 mm surface deformation was found to be 7.0.
The overall average reported subgrade CBR was 4.8 and it was found that 19 of the 28
references (68%) evaluated geosynthetic test sections with subgrade CBR’s less than 8,
suggesting that geosynthetic improvement was more suited for softer subgrade soil applications.
Six of the 28 references (21%) had subgrade CBR greater than or equal to 8. Geosynthetic
improvement appeared to decrease with an increase in subgrade CBR, and it was reported
(Cancelli, Montanelli, Rimoldi, and Zhao 1996; Monanelli, Zhao, and Rimoldi 1997) that a
traffic improvement factor decreased from a value of 15 to 5 as subgrade CBR increased from 1
to 18.
Optimum geosynthetic placement location appears to be a parameter where conflicting
recommendations can be found. Some studies (Haas, Walls, and Carroll 1988; El-Maaty 2016;
Ibrahim, El-Badawy, Ibrahim, Gabr, and Azam 2107) suggested that optimal placement location
was at the base/subgrade interface, while others (Perkins 1999; Dong, Han, and Bai 2010; Sun,
Han, Kwon, Parsons, and Wayne 2015) suggested that to achieve the best results the
geosynthetic (particularly geogrids) should be elevated within the base layer. If a geosynthetic
was recommended to be elevated in the base layer, the upper one-third of the base thickness was
recommended by some (Abu-Farsakh and Chen 2011; Al-Qadi, Brandon, Valentine, Lacina, and
Smith 1994; Abu-Farsakh, Akond, and Chen 2016; Sarici, Demir, Tutumluer, Demir, Gungor,
Epsileli, Comez, and Ok 2016), and mid-height of the base thickness or multiple layers
(placement at mid-height of the base and the base/subgrade interface) were recommended by
21
others (Cancelli, Montanelli, Rimoldi, and Zhao 1996; Abu-Farsakh, Hanandeh, Mohammed,
and Chen 2016).
Eight references reported reduced vertical stress at the top of the subgrade attributed to
geosynthetic inclusion. Reductions ranged from 8% (Leng and Gabr 2002; Sun, Han, Kwon,
Parsons, and Wayne 2015) up to nearly 46% (Qian, Han, Pokharel, and Parsons 2013; Sun, Han,
Kwon, Parsons, and Wayne 2015). The average reported reduction in subgrade pressure was
found to be approximately 24%.
2.4 Full-Scale Load Testing
Full-scale load testing is advantageous for reasons including but not limited to the effect
of a rolling tire and the inherent wheel wander under traffic can be simulated. Further, typical
construction equipment (i.e. asphalt pavers, roller compactors, etc.) can be used to construct
representative pavement structures. Increased construction variability and generally slow traffic
speed (e.g. a heavy-vehicle simulator typically operates at speeds of approximately 7 kilometers
per hour) could be considered disadvantageous for some full-scale testing configurations.
2.4.1 Highway Loading
Chan, Barksdale, and Brown (1989) investigated the benefit of geosynthetic inclusion in
twelve different flexible pavement sections. Test sections consisted of asphalt thickness ranging
from 32 to 38 mm, base thickness ranging from 150 to 210 mm, and two base types (a
sand/gravel mixture and crushed limestone). Results suggested that geosynthetic placement
location depended on the quality and thickness of the base aggregate, and it was recommended
that the placement location be as high in the granular layer as practical. Further, it was found
22
that geogrids generally performed better than geotextiles, perhaps due to enhanced aggregate
interlock associated with a geogrid.
Collin, Kinney, and Fu (1996) evaluated the effect of a stiff biaxial geogrid in a series of
test sections with 50 mm thick HMA over a range of base thicknesses (150 to 460 mm). The
sections were constructed over a soft subgrade (1.9 CBR). It was found that for thicker
aggregate bases (e.g. 350 mm), the benefit of geogrid inclusion was diminished. TBR values
ranged from 2.1 to 10, and it was conservatively estimated that the geogrids tested could increase
pavement life 2 to 4 times.
Fannin and Sigurdsson (1996) described a series of field trials conducted to measure the
performance of geosynthetic stabilization in unsurfaced pavements. The sections were
constructed over an organic clayey silt and consisted of aggregate layer thicknesses ranging from
250 to 500 mm. One section was unreinforced, three contained geotextiles of varying tensile
strength and apparent opening size, and one contained a biaxial geogrid. A total of 500 passes
with a standard loaded truck (80 kN rear single axle load) were applied, and rut depths were
monitored (they did not include surface upheaval). It was found that the unreinforced section
displayed rapid rut development, regardless of base course thickness, but that thicker base
sections (>350 mm) had lower measured ruts at the same number of vehicle passes. It was
observed that all geosynthetic sections displayed improvement when compared to an equivalent
unreinforced section, and that the difference was greatest in the thinner base course layers. It
was suggested that the separation function provided by a geotextile was important in the thinner
base course layers and that increasing tensile strength improved traffic performance. A geogrid
was found to improve performance in the thicker base course layers, when compared to
23
geotextiles, and it was suggested that the reinforcement mechanism rather than separation
dominated this response.
Kinney, Abbot, and Schuler (1998) investigated the effect of varying base course
thickness, tire pressure, and geogrid properties on pavement rutting. Base course thickness
ranged from 150 to 530 mm, tire pressure ranged from 276 to 551 kPa, and the geogrids differed
in aperture size and tensile modulus. It was found that TBR decreased with increasing base
thickness, ranging from 10 at base thickness less than 254 mm to 1 at a base thickness of about
356 mm. Further, it was concluded that the effect of reinforcement with base thickness in excess
of 406 mm was minimal. It was found, in general, that lower tire pressure resulted in less
rutting. The authors acknowledged that rutting decreases attributed to reinforcement was a
function of base course thickness and geogrid properties; however, no definitive conclusions
regarding geogrid properties were drawn.
Appea and Al-Qadi (2000) used a falling weight deflectometer (FWD) to assess structural
deterioration of nine pavement sections on an instrumented roadway test section in Bedford
County, Virginia. Three test sections included a geogrid, three included a woven geotextile, and
three were not stabilized. Geosynthetics (a geotextile and a geogrid) were placed at the
base/subgrade interface. The ELMOD backcalculation program was used to determine a
subgrade layer modulus for the geotextile stabilized sections; HMA and base course layers were
fixed based on laboratory test results. The calculated subgrade modulus value was then used as
an input value for the unstabilized sections and the aggregate base layer modulus was calculated.
It was found that the unreinforced section had a structural capacity decrease of 33% over a 4 year
period when compared to a geotextile stabilized section, and the geogrid section had a structural
capacity decrease of 6% when compared to a geotextile stabilized section over the same time
24
period. Further, it was found that the lower 51 mm of the base layer had an increase in fines
(material passing a 75 um sieve) of 40% in the unreinforced section and 28% in the geogrid
stabilized section when compared to the geotextile stabilized section. It was concluded, through
FWD analysis and post-test excavation data, that the intrusion of fines weakened the base course
layer and that a geotextile provided protection against fine intrusion while a geogrid only
provided partial protection.
Edil, Benson, Bin-Shafique, Tanyu, Kim, and Senol (2002) investigated a variety of
stabilization techniques including alternative subbase materials, chemical stabilization, and
geosynthetic stabilization on a Wisconsin state highway. A total of twelve sections were
constructed: three sections were control sections, four test sections were constructed with
alternative subbase materials (foundry slag, foundry sand, bottom ash, and fly-ash treated
subbase), five test sections included geosynthetics (geocell, nonwoven geotextile, woven
geotextile, drainage geocomposite, and geogrid). Design thicknesses of the layers of interest
were determined such that the estimated structural number was equivalent to the control section.
All sections consisted of 125 mm of asphalt, 115 mm of crushed aggregate base, and 140 mm of
salvaged asphalt base. The geosynthetic test sections included a 300 mm thick rock subbase;
while the subbase thickness for alternative subbase materials ranged from 840 mm in the foundry
slag and foundry sand sections (as well as the control with a rock base) to 300 mm in the fly-ash
treated subbase. FWD measurements were made to compare stiffness of each test section and
visual inspections were performed. It was found that all sections provided adequate support
during construction and that stiffness was equal or better when compared to the control section,
with the exception of the foundry sand section, which contained an appreciable amount of
bentonite that was thought to be susceptible to freeze/thaw weakening. Further, it was found that
25
the fly ash stabilized and geosynthetic test sections provided equivalent support when compared
to the thicker alternative sections.
Perkins (2002) described four flexible pavement test sections constructed in the Frost
Effects Research Facility (FERF) at the U.S. Army Cold Regions Research and Engineering
Laboratory. The sections consisted of 75 mm thick asphalt, 300 mm thick base course, and an
AASHTO designation A-7-6 subgrade. Three sections included a geosynthetic placed at the
base/subgrade interface. A total load of 40 kN was applied to a dual wheel truck gear mounted
to a heavy vehicle simulator (HVS). In terms of rutting, the geosynthetic sections were found to
perform better than the unreinforced section. Instrumentation data suggested that the
geosynthetics reduced horizontal and vertical strain in the base and subgrade layers and
improved vertical stress distribution on the subgrade.
Al-Qadi and Appea (2003) further investigated performance of the nine pavement
sections described by Appea and Al-Qadi (2000). The sections consisted of an average 95 mm
thick asphalt surface over base thicknesses of 100, 150, and 200 mm. In-situ subgrade CBR
values ranged from 6 to 10. Field rutting measurements showed that the unreinforced control
sections had the highest amount of rutting, followed by the geogrid sections, and that the
geotextile sections had the least amount of rutting. It was suggested that the geotextile
performance improvement could be attributed to the reduction in intermixing of subgrade fines
and base course.
Holder and Andreae (2004) presented a case study of including a geogrid at mid-depth of
a 300 mm thick base course, 210 mm thick HMA surfaced Idaho city street. FWD
measurements were collected in an attempt to capture geogrid effects. It was found that the back
calculated base course modulus appeared to show a slight benefit from geogrid reinforcement.
26
Effective base layer structural coefficients were back calculated, and it was estimated that the
reinforced base layer had a structural layer coefficient of 0.18, compared to 0.14 normally used.
It was noted that no control section was constructed, and the authors acknowledged that a control
section is critical in evaluating the benefit of geogrid reinforcement.
Kim, Edil, Benson, and Tanyu (2005) evaluated the contribution of geosynthetics in the
construction of unsurfaced construction working platforms over soft subgrades. It was found
that reinforcing the working platforms with geosynthetics improved the aggregate layer
structural coefficient by 50 to 70 percent. Additionally, it was found that, for a typical pavement
structure, structural number increases ranging from 3 to 11 percent could be realized (i.e. modest
increases) and the largest increase was seen with a geogrid (when compared to woven and non-
woven geotextiles). In terms of accumulated deformation (Kim, Edil, Benson, and Tanyu 2006),
it was found that geosynthetics reduced the rate of deformation and that total deflections were
approximately one-half that observed in unreinforced sections.
Howard (2006) performed a full-scale field study of thirteen test sections constructed on
a low-volume frontage road in Arkansas. The test sections consisted of 50 mm thick asphalt, 150
and 254 mm thick crushed limestone base, and a highly plastic clay subgrade. Geosynthetics
evaluated included woven geotextile, nonwoven geotextile, and a biaxial geogrid that were
placed at the base/subgrade interface. Controlled traffic was applied via loaded dump trucks. It
was envisioned that data collection would occur during the wet season, which would highlight
the geosynthetic performance in a moisture-weakened subgrade condition, however the author
noted that unusually low amounts of rainfall occurred over the study period. Geosynthetic
contributions to the pavement structures could not be quantified, and it was concluded that the
lack of geosynthetic response could be attributed to the high strength of the subgrade under the
27
unseasonable dry weather conditions. This agreed with observations made by others that
geosynthetic benefit decreases with increasing subgrade strength.
Hufenus, Rueegger, Banjac, Mayor, Springman, and Bronnimann (2006) investigated
effects of including geosynthetics in relatively poor recycled rubble granular fills on compaction,
bearing capacity, and serviceability. Geosynthetics investigated included a geogrid, a non-
woven geotextile, and a woven slit tape geotextile that were selected to represent various raw
materials, types and manufacturing processes locally available. Geosynthetics were placed at the
subgrade/base interface. The base course was placed in three 200 mm compacted lifts, and test
traffic was applied to each lift prior to placement of subsequent lifts. It was suggested that the
thickness of the fill layer could be reduced approximately 30 percent by incorporating a
geosynthetic at the subgrade/base interface. It was found that stiffer geosynthetics increased the
bearing capacity and compactability of the fill layer.
A case study of performance of a geogrid reinforced Georgia state road was presented by
Aran (2006). Two HMA thicknesses and three base course thicknesses were evaluated over
subgrade CBR values ranging from approximately 8 to 10. It was found that thinner reinforced
sections performed comparable to the unreinforced sections, however the author concluded that
the pavement section subgrade was too strong (average subgrade CBR ranged from 7.7 to 10.4)
for the geogrid reinforcement to have a meaningful effect on performance.
Helstrom, Humphrey, and Labbe (2007) investigated the use of geosynthetics for
reinforcement and drainage in portions of a state route in Maine that had been noted to have local
bearing capacity failures and substantial pavement cracking. All test items consisted of a 150
mm thick asphalt surface and base course thickness of 300 or 600 mm. A geogrid was placed at
the base/subgrade interface and mid-depth of each aggregate thickness. The test sections were
28
fully instrumented and response measurements were collected during and after construction. It
was concluded that including geogrid in a 300 mm thick base increased the effective structural
capacity between 5 and 17% and that including geogrid in a 600 mm thick base had no
meaningful effect on effective structural number. It was found that measured response in the
geogrid (via strain gauges) increased over time in the 300 mm thick base while remaining
constant for the 600 mm thick base. It was suggested that traffic application is needed to fully
develop geogrid response as determined via strain gauge measurement. Additionally, it was
found that when the geogrid was placed at the base/subgrade interface, measured responses were
equal to or greater than responses measured when the geogrid was placed mid-depth.
Al-Qadi, Dessouky, Kwon, and Tutumluer (2008) constructed nine flexible pavement
sections consisting of two asphalt thicknesses (76 mm and 127 mm) and three base thicknesses
(203, 305, and 457 mm) over a 4 CBR subgrade. Geogrids were located at the base/subgrade
interface, one-third depth of the base, a double layer consisting of one geogrid at the
base/subgrade interface and one geogrid at one-third depth. The pavement sections were loaded
using an accelerated testing loading assembly (ATLAS) capable of applying a 44 kN load and
689 kPa tire pressure. Instrumentation response data indicated that geogrid reinforcement was
effective in reducing horizontal shear deformation of the aggregate layer, particularly in the
traffic direction. It was concluded that in thin base layers the optimum geogrid placement
location was at the base/subgrade interface. For thicker base layers, it was concluded that the
optimal placement location was at the upper third of the base layer, and that an additional
geogrid at the base/subgrade interface may be needed for stability. Further evaluation of these
experiments (Al-Qadi, Dessouky, Tutumluer, and Kwon 2011) noted that although a geogrid
29
tends to improve performance in low traffic volume pavement performance, it was not
recommended as a means to mitigate poor performance from an under-designed pavement.
Henry, Clapp, Davids, and Barna (2009) presented the results of trafficking eight test
sections consisting of 100 and 150 mm thick asphalt surfaces over 300 and 600 mm thick base
thickness. Design subgrade CBR was 4; however, construction testing with a FWD revealed that
the subgrade was substantially stiffer. Water was added to the subgrade via a piping system in
an attempt to achieve the 4 CBR subgrade strength and strengths were periodically monitored
prior to traffic initiation. Traffic was applied to each section using a dual truck gear with varying
wheel loads on a heavy vehicle simulator. One biaxial geogrid was evaluated in this study. It
was found that the grid-reinforced sections delayed development of surface rutting when
compared to the corresponding unreinforced control sections. Failure could not be achieved in
the 600 mm thick base sections; however, a benefit was observed for the test section with 100
mm of asphalt but not for the section with 150 mm asphalt.
Tingle and Jersey (2009) evaluated the performance of eight full-scale aggregate road
sections that included three different aggregate materials (crushed limestone, crushed chert
gravel, and rounded clay-gravel). Three of the eight 152 mm thick aggregate road sections were
unreinforced (control) sections for each aggregate type. Geosynthetics included in the study
consisted of a punched and drawn biaxial geogrid and a needle-punched nonwoven
polypropylene geotextile. All sections were constructed over a 4 CBR clay subgrade. The test
sections were trafficked with a dual-wheel tandem axle truck having a total gross weight of 194
kN and a tire contact pressure of 344 kPa. Results indicated that reinforced pavement sections
displayed improved rutting resistance when compared to unreinforced sections for all aggregate
types tested. The clay-gravel was found to be the best performer, followed by the crushed
30
limestone, and finally the crushed chert gravel. The authors attributed the clay gravel sections
improved performance to natural cementation stemming from drying of the clay gravel base,
noting that moisture susceptibility was not a test variable. It was found that the geogrid-
reinforced crushed limestone section outperformed the geotextile-reinforced crushed limestone
section, whereas the geotextile- and geogrid-reinforced crushed limestone section performed the
best overall.
Al-Qadi, Dessouky, Kwon, and Tutumluer (2012) further investigated optimal placement
location based on data obtained from Al-Qadi, Dessouky, Kwon, and Tutumluer (2008).
Instrumentation data suggested that geogrid placement at one third base layer thickness was
equivalent to geogrid placement at both one third and bottom of the base layer. Further, for
pavements with 300 mm base thickness or less, increasing HMA thickness was found to be more
effective than including a geogrid.
Norwood and Tingle (2014a) constructed two test items with structural cross-sections of
100 mm HMA over 200 mm crushed limestone base and 76 mm HMA over 150 mm crushed
limestone base reinforced with a triaxial aperture geogrid. The design subgrade CBR was 6 for
both sections. Trafficking of the section was accomplished using a HVS outfitted with a dual-
tandem truck gear. The unreinforced section was subjected to a total of 811,200 equivalent
single axle loads (ESALs); however, a malfunction in the HVS environmental chamber caused
the pavement temperature of the reinforced section to rise well beyond the specified range. It is
noted that the rate of surface rutting dramatically increased at the time of the environmental
control malfunction. It was stated that the reinforced section performed equally as well as the
unreinforced section until the significant temperature increase.
31
Performance of geosynthetic reinforced pavement sections surfaced with a double
bituminous surface treatment were evaluated by Norwood and Tingle (2014b). The test section
contained one item with a 150 mm thick crushed limestone base reinforced with a triaxial
aperture geogrid and one item with a 200 mm thick crushed limestone unreinforced base both
placed over a 6 CBR subgrade. The 25.4 mm thick, double bituminous surface treatment was
considered typical for low-volume road placement. Loading was accomplished by a single-axle
load cart outfitted with a single-axle dual wheel truck gear loaded to a nominal load of 44.5 kN
and 827.4 kPa tire pressure. It was concluded that both items performed equally well through the
application of 60,400 ESALs despite the unreinforced pavement section having an additional 50
mm of base. FWD tests showed that the stiffness of the unreinforced item decreased more
rapidly when compared to the stiffness of the reinforced item.
Saghebfar, Hossain, and Lacina (2016) investigated the performance of six test sections
consisting of HMA thicknesses of 102, 127, and 152 mm, base thicknesses of 203, 229, 254, and
305 mm over a 5 CBR subgrade. Three types of woven geotextiles were placed at the
base/subgrade interface. Traffic testing was accomplished using a dual tire gear loaded to 80 kN
at 620 kPa tire pressure on an accelerated pavement testing (APT) machine. Rutting results
indicated that reinforced base sections outperformed control sections with similar cross sections.
Instrumentation response data indicated that the reinforced base layer reduced vertical pressure at
the top of the subgrade, and FWD data suggested that the reinforced base layer was significantly
stronger than the unreinforced control section after loading.
Chen, Hanandeh, Abu-Farsakh, and Mohammad (2017) investigated six full-scale
sections that included a geogrid, a non-woven geotextile, and a high-strength woven geotextile.
Base thickness were 254 mm and 457 mm, which were overlain with a 76 mm thick asphalt
32
layer. Subgrade resilient modulus values were estimated from dynamic cone penetrometer
(DCP) tests and ranged from 17.6 to 20.3 MPa. Traffic was applied by an accelerated loading
facility (ALF) and consisted of an initial load of 43.4 kN that was incrementally increased to
63.8 kN. Tire pressure was maintained at 724 kPa. A variety of instrumentation was installed
including earth pressure cells, linear variable displacement transducers, potentiometer, and time-
domain reflectometers. It was found that the geosynthetics were successful in reducing the
surface rutting of the test sections. Further, it was observed that as the load intensity was
increased, the reduction in maximum vertical pressure on top of the subgrade became more
pronounced. It was concluded that placing geosynthetics at the base/subgrade interface
improved performance, but an additional layer of geogrid in the upper one-third of the base layer
amplified performance improvement.
Robinson, Tingle, Norwood, Wayne, and Kwon (2018) evaluated full-scale sections
consisting of 75 mm thick HMA, 150 mm thick limestone base, and a 6 CBR clay subgrade. The
sections contained two triangular aperture geogrids that were trafficked with a dual-tandem gear
configuration on a HVS. It was concluded that the multi-axial geogrids improved rutting
resistance when compared to an unstabilized test section and that the inclusion of a geogrid
provided a benefit at least equivalent to the benefit provided by 25 mm of additional HMA and
50 mm of base course. Effective base structural layer coefficients were estimated to increase
from a baseline of 0.14 to 0.29 with geogrid inclusion.
2.4.2 Aircraft Loading
Decades ago, Haliburton, Lawmaster, and King (1980) performed a literature review and
laboratory study to investigate the use of geotextiles in flexible airfield pavements. It was
33
concluded that geotextiles had potential to improve airfield pavement performance, but that
additional research was required to fully understand performance improvement.
Webster (1993) evaluated the inclusion of geosynthetics for light-duty airfield
pavements. The intent of the study was to evaluate flexible pavements for light aircraft. The
study consisted of twelve individual test items at subgrade CBR values of 3 and 8. The
pavement structure consisted of 152, 254, 305, 356, and 457 mm thick crushed limestone base
layers. All sections were surfaced with a 50 mm thick HMA layer. The geosynthetics were
punched and drawn, as well as woven, biaxial geogrids located at the base/subgrade interface in
all sections with one product located in the middle of a 356 mm thick base section. Control
(unreinforced) items were constructed at base thicknesses of 152 mm and 254 mm over an 8
CBR CH subgrade and 305 mm, 356 mm, and 457 mm base thicknesses over a 3 CBR CH
subgrade. Each test item was trafficked with a single-wheel C-130 load cart at a total load of
133.5 kN and a tire contact pressure of 468 kPa.
The study concluded that the inclusion of geogrid reinforcement in base courses for flexible
pavements could provide structural improvement. It was noted that geogrid performance was a
function of depth of placement, and it was recommended that the minimum placement depth
should be 152 mm (as measured from the pavement surface). Importantly, a thickness reduction
chart was developed presenting a relationship between unreinforced total pavement thickness
and equivalent reinforced total pavement thickness. This relationship is presented in US Army
Corps of Engineers ETL-1110-1-189 (USACE 2003) as the means of determining pavement
thickness for reinforced flexible pavement sections surfaced with asphalt. It is noted that the
procedure is valid for subgrade CBR values equal to or less than 8. The ETL recommends that
for subgrade strengths greater than a CBR of 8, a full-scale test section be constructed to
34
determine the effect of geogrid on base course reduction as the benefit of geogrid inclusion was
expected to diminish with increasing subgrade strength.
Cancelli, Recalcai, and Shin (2000) and Shin, Oh, and Kyu-Jin (1999) document analysis
and field performance of geogrid reinforcement of base course of a runway at Inchon
international airport. The airport was developed on marine sediment and settlement of the airfield
pavement system was a concern, particularly around rigid drainage structures. Finite-element
analysis indicated that inclusion of a geogrid would reduce potential settlement under traffic. The
follow-on case study demonstrated that the geogrid was effective in reducing differential
displacements around the runway’s drainage structures.
2.4.3 Observations from Full-Scale Load Testing
A total of 24 references were reviewed; 17 references focused on rutting behavior of
geosynthetic reinforced pavements; and 12 of 17 (70%) found that geosynthetics improved
rutting performance. Three of seventeen (18%) found that geosynthetics provided similar rutting
performance to sections without geosynthetics, and the lack of improvement was attributed to a
strong subgrade (Howard 2006; Aran 2006). Full-scale test results generally suggested that the
observed benefits under highway loading tend to decrease with an increase in base course
thickness (Chan, Barksdale, and Brown 1989; Collin, Kinney, and Fu 1996; Fannin and
Sigurdsson 1996; Kinney, Abbot, and Schuler 1998) or with an increase in asphalt thickness
(Henry, Clapp, Davids, and Barna 2009; Al-Qadi, Dessouky, Kwon, and Tutumluer 2012). This
suggests that as a flexible pavement becomes stiffer, the ability of a geosynthetic to engage either
through lateral restraint of base course aggregate and/or tensioned membrane effect becomes less
pronounced.
35
Reductions in vertical pressure at the top of the subgrade were observed to range from
approximately 12.5% (Chen, Hanandeh, Abu-Farsakh, and Mohammad 2017) up to
approximately 25% (Saghebfar, Hossain, and Lacina 2016; Chen, Hanandeh, Abu-Farsakh, and
Mohammad 2017). Al-Qadi, Dessouky, Kwon, and Tutumluer (2008) monitored changes in
vertical subgrade pressure from initial values to the end of traffic application and found that
changes in vertical subgrade pressure were higher in an unreinforced section than changes
observed in reinforced sections.
Evaluation of stiffness was conducted in six references and it was generally observed that
improvement in stiffness was realized with geosynthetic inclusion. Recommended increases in
AASHTO base course structural coefficient ranged from 0.18 (Holder and Andreae 2004) up to
0.29 (Robinson, Tingle, Norwood, Wayne, and Kwon 2018). Some studies (Edil, Benson, Bin-
Shafique, Tanyu, Kim, and Senol 2002; Saghebfar, Hossain, and Lacina 2016) found that initial
stiffness in reinforced sections was as good as or better than companion unreinforced sections,
and others found that stiffness under traffic in reinforced sections decreased at a slower rate than
unreinforced sections (Appea and Al-Qadi 2000; Norwood and Tingle 2016b).
Limited evaluations of geosynthetic inclusion in airfield pavements have been performed.
Webster (1993) represents the most comprehensive evaluation found to date, although the
loading conditions applied are much lower than would be expected on modern day military and
international airfields. Current DOD geosynthetic design criteria makes use of an equivalent
thickness chart where the geosynthetic is assigned an equivalent aggregate thickness. A review
of historical development of the equivalent thickness chart revealed that limited data (8 data
points) where used to develop the relationship. Multiple evaluations have been conducted since
this chart was developed and revalidation of this relationship is needed. Chapter 3 of this
36
dissertation verifies the equivalent thickness chart by expanding the limited original dataset to
include nine studies and 27 data points from historical geosynthetic evaluations conducted at
ERDC.
The lack of controlled full-scale evaluations for airfield pavements subjected to airfield
loading conditions is a gap in current geosynthetic pavement design. Performance data and
instrumentation response data of full-scale airfield pavements are needed to extend the body of
knowledge and determine if similar observations to highway conditions (e.g. improved rutting
performance and reduction in observed stresses) can be realized. Chapter 5 of this dissertation
addresses this gap by presenting the results of a full-scale airfield pavement evaluation subjected
to airfield loading conditions.
2.5 Conclusions from Literature Review
A review of cyclic-plate load and full-scale testing (45 total references) indicated that
including geosynthetics in flexible pavement base course subjected to highway loads can provide
a performance benefit, and it was found that 40 of the 45 references (89%) showed some level of
surface rutting improvement. Experiments that did not show a meaningful performance
improvement or a reduced performance improvement were attributed to 1) strong subgrade soils,
2) thicker base course layers or, 3) thicker asphalt layers. This is an important observation
because airfield pavements are often much thicker than highway pavements, therefore it may be
hypothesized that geosynthetic performance improvement may be less than that observed in
highway pavements.
Improvements in vertical stress at the top of the subgrade attributed to geosynthetic
inclusion were observed to range from approximately 8% to nearly 46%. Current Department of
Defense (DOD) flexible pavement design methodology (as described by Gonzalez 2015) is based
37
on providing sufficient pavement structure to limit vertical stress on the subgrade (as a function
of subgrade strength). If similar vertical stress reductions could be realized in thicker airfield
pavements, then modifications to current DOD design procedures could be made to adjust design
procedures to account for geosynthetic inclusion.
It was observed that the optimal geosynthetic placement location in an aggregate base
course was not well defined. For thinner aggregate bases, general recommendations suggested
that optimal placement location was at the base/subgrade interface. As base course thickness
increased, it was generally recommended that the geosynthetic be elevated in the base course
layer. However, recommendations range anywhere from one-third depth, mid-depth, or even as
high as practical in the base layer. If a typical stress distribution in a pavement is considered,
then this could suggest that placement location is a function of stress (potentially vertical and/or
horizontal stress) and that some minimum stress level at the geosynthetic location is required to
full engage the geosynthetic and realize a meaningful benefit.
Current DOD geosynthetic design methodology has not been updated in decades and,
while providing a relatively simple means to account for geosynthetic inclusion in light aircraft
and highway pavements, requires validation. Further, a literature review revealed that limited to
no documented evaluation of geosynthetic inclusion in thick airfield pavements subjected to
modern-day airfield loading conditions have been performed, representing a meaningful gap in
the body of knowledge.
38
CHAPTER III
ASSESSMENT OF EQUIVALENT THICKNESS DESIGN PRINCIPLES FOR
GEOSYNTHETIC REINFORCED PAVEMENTS BY WAY OF ACCELERATED TESTING
This chapter has been previously published as a journal article in Volume 2672 Issue 40
of the Transportation Research Record: Journal of the Transportation Research Board (TRB).
The original paper may be accessed at http://dx.doi.org/10.1177/ 0361198118781682. In
accordance with Sage Publishing Reuse Guidelines, the paper (Robinson, Tingle, Norwood and
Howard 2018) has been reformatted and reproduced herein with minor modifications, i.e.,
portions of the literature review in the paper have been moved to Chapter 2, to suit the objectives
of this dissertation.
The Engineer Research and Development Center (ERDC) of the U.S. Army Corps of
Engineers has performed multiple laboratory and full-scale evaluations of geosynthetic
reinforced pavements. One result from early geosynthetic reinforced pavement evaluations was a
pavement design methodology implemented in ETL 1110-1-189: Use of Geogrids in Pavement
Construction (USACE 2003). Since that time, the evaluations have been primarily focused on
comparing performance between varying types of geosynthetic products. While the studies have
independently compared the discrete performance of single geosynthetic reinforced sections to
unreinforced sections, a comprehensive analysis of available data has not been performed to
validate or refine the implemented design methodology. The objective of this effort was to
assemble available data from laboratory and full-scale testing conducted at ERDC for the
39
primary purpose of assessing the flexible pavement design methodology presented in ETL 1110-
1-189. Simplifying assumptions were made to allow comparison of varying loading and
pavement structure conditions. This assessment found that the combined dataset supports the
original design curve produced with the equivalent thickness methodology described in ETL
1110-1-189. The updated dataset would reduce the equivalent reinforced thickness by
approximately 25.4 mm at unreinforced thicknesses less than 356 mm, providing a slightly more
conservative result. The adjusted data converged with the original equivalency chart at an
unreinforced thickness of approximately 406 mm.
3.1 Introduction
The Engineer Research and Development Center (ERDC) of the U.S. Army Corps of
Engineers (USACE) has performed multiple laboratory and full-scale evaluations of geosynthetic
reinforced pavements. One result from early geosynthetic reinforced pavement evaluations was a
pavement design methodology based on two subgrade CBR values. This method is implemented
in ETL 1110-1-189 Use of Geogrids in Pavement Construction (USACE 2003) in the form of an
equivalent pavement thickness chart (Figure 3.1) that is considered valid for subgrade CBR
values ranging from 0.5 to 8.0. Simply, an unreinforced flexible pavement consisting of an
asphalt (AC) and base layer (denoted on the y-axis in Figure 3.1) is designed for the given
subgrade conditions. A reinforced pavement thickness is determined by entering the chart with
the unreinforced pavement thickness, intersecting the equivalency curve, and drawing a vertical
line to the equivalent reinforced pavement thickness (denoted on the x-axis in Figure 3.1). The
reinforced aggregate thickness is determined by subtracting the unreinforced pavement thickness
and equivalent reinforced pavement thickness, while maintaining the same asphalt layer
thickness, thereby reducing the aggregate layer thickness only. After development of the
40
equivalent pavement thickness chart, evaluations have been primarily focused on comparing
performance between varying types and manufacturer’s geosynthetics. While the studies have
independently compared the performance of discrete geosynthetic reinforced sections to
unreinforced sections, a comprehensive analysis of available data has not been performed for one
purpose, in particular to evaluate the current state of practice within USACE that was developed
with considerably less data than is currently available.
Figure 3.1 Equivalent pavement thickness chart from ETL 1110-1-189 (USACE 2003)
3.2 Objectives and Scope
The objectives of this effort were to assemble available data from laboratory and full-
scale testing conducted at ERDC, document the various material parameters that have been
investigated, and draw appropriate conclusions from the combined dataset. The data utilized
herein spans a time frame beginning in the early 1990s when USACE began investigating
inclusion of geosynthetics into pavement structures and incorporates data collected as recently as
41
2016. The additional data collected since implementation of ETL 1110-1-189 (USACE 2003) is
useful to assess the current USACE design methodology or determine if adjustments to the
design methodology are required. A total of nine different studies were assessed consisting of a
total of thirty-one different test sections.
3.3 Pavement and Materials Properties for Lab and Full-Scale Testing
Inclusion of geosynthetics in pavement structures by ERDC has been investigated from
the early 1960s to as recently as 2016. However, this summary only includes ERDC
geosynthetic experiments conducted since the 1990s. As new geosynthetic products became
available, laboratory scale evaluations used a 1.8-m by 1.8-m box with a cyclic plate load setup
as shown in Figure 3.2a and Figure 3.2b. Full-scale test sections where trafficked using a single-
wheel load cart as shown in Figure 3.2c and Figure 3.2d and/or a Heavy Vehicle Simulator
(HVS) as shown in Figure 3.2e and Figure 3.2f.
In summarizing the materials used in past experiments, it is difficult to provide
comprehensive descriptions of each material used. However, this section provides a general
summary of material properties while detailed descriptions can be found in references (Webster
1993, Tingle and Jersey 2005, Tingle and Jersey 2009, Jersey and Tingle 2009, Norwood and
Tingle 2014a, Norwood and Tingle 2014b, Robinson, Tingle, and Norwood 2017). A typical
cross-section showing the various layers and construction quality control (QC) parameters is
presented in Figure 3.3.
42
Figure 3.2 Photographs of accelerated testing equipment
(a) Cyclic plate load laboratory equipment
(b) Close-up of cyclic plate test
(c) Single-wheel load cart
(d) Inside single-wheel load cart
(e) Heavy-vehicle simulator
(f) Dual-tandem truck gear on HVS
43
Figure 3.3 Typical cross-section
For the experiments included in this chapter, the design subgrade consisted of a locally
available high-plasticity clay having liquid limits (LL) ranging from 65 to 90, plastic limits (PL)
ranging from 22 to 29, and plasticity indices (PI) ranging from 38 to 62 (ASTM 2017b).
According to the ASTM D2487 Unified Soil Classification System (USCS) (ASTM 2017a), the
soil was classified as a high-plasticity clay (CH) and an A-7-6 according to the American
Association of State and Highway Transportation Officials (AASHTO) M 145 classification
system (AASHTO 2012). The percent fines (P200) ranged from 95 percent to 98 percent. The
review of modified proctor data for the CH subgrade soil showed that the maximum dry density
ranged from 1474 to 1666 kg/m3 at optimum moisture contents ranging from 19 percent to 23
percent (ASTM 2012). Design California Bearing Ratios (CBR) of the CH subgrade for the
different experiments ranged from 1 to 8% as determined by ASTM D4429 (ASTM 2009).
44
A subbase material was used in only one study (Norwood 2016, personal
communication), and consisted of a locally available granular material comprised of
approximately 98 percent sand. The subbase material was classified as a poorly-graded sand
(SP) according to D2487 and an A-1-b according to M 145. Additionally, the material selected
was found to meet the subbase requirements of the Federal Aviation Administration (FAA) P-
154 specification.
Multiple base course materials have been investigated consisting of crushed limestone,
crushed chert gravel, and rounded clay gravel. Modified proctor maximum dry densities for the
base materials ranged from 2387 kg/m3 for the crushed limestone base to 2195 kg/m3 for the
rounded clay gravel base. Optimum moisture contents ranged from 3.8 percent to 5.9 percent.
Pavement surfacing has consisted of no surfacing (aggregate-surfaced roadways), rubber
mat surfacing (simulated flexible pavement surfaces), double-bituminous surface treatments
(DBST), and hot-mix asphalt surfacing. Hot-mix asphalt surfacing thickness ranged from 50 to
127 mm and were typical of local 9.5 mm nominal maximum aggregate size (NMAS) mixtures
used by the Mississippi Department of Transportation (MDOT). Voids in mineral aggregate
(VMA) ranged from 13 to 15 percent and asphalt binder content (Pb) ranged from 5.2 to 5.7
percent. Recycled asphalt pavement (RAP) content was up to 20 percent by weight of the
aggregate blend for cases where data was available.
3.4 Description of Previous Research Efforts
A review of internal ERDC documentation found that a number of previous research
efforts had been well documented. Those published efforts have been summarized in Chapter 2
of this dissertation and are listed below:
45
• Webster 1993: Geogrid reinforced base courses for flexible pavements for light
aircraft: Test section construction, behavior under traffic, laboratory tests, and
design criteria
• Tingle and Jersey 2005: Cyclic plate load testing of geosynthetic-reinforced
unbound aggregate roads
• Tingle and Jersey 2009: Full-scale evaluation of geosynthetic-reinforced
aggregate roads
• Jersey and Tingle 2009: Cyclic plate testing of geogrid-reinforced highway
pavement
• Norwood and Tingle 2014a: Performance of geogrid-stabilized flexible pavements
• Norwood and Tingle 2014b: Performance of geogrid-stabilized gravel flexible
base with bituminous surface treatment
• Robinson, Tingle, Norwood, Wayne, and Kwon 2018: Performance of multi-axial
geogrid stabilised flexible pavements
Further internal review found that two studies had been conducted but little to no post-
test analysis had been performed. Limited data from those two studies were used in the analysis
effort discussed in this chapter and are summarized below.
Note: Discovery of the previously unanalyzed and unpublished study described below led
the author to perform a full analysis and documentation of the work, which comprises Chapter 5
of this dissertation.
Norwood (2017, personal communication) constructed nine full-scale test items with
structural cross-sections of 101 mm HMA over 356 mm crushed limestone base on an 8 CBR
subgrade to evaluate the inclusion of geosynthetics in relatively thick airfield pavement sections.
Biaxial and multiaxial geogrids were installed at the base/subgrade interface, with the exception
of an unreinforced item and one item where the biaxial geogrid was installed mid-depth in the
base layer. Trafficking of the test section was accomplished using a Heavy Vehicle Simulator
46
(HVS) outfitted with a single C-17 wheel. The nominal wheel load was 200 kN at a tire pressure
of 978 kPa.
Test results were mixed with the unreinforced control section performing both better and
worse than the reinforced sections. An approximate 30% improvement in rutting resistance was
observed in the best performing reinforced section when compared to the control. Conversely,
the worst performing reinforced item experienced 20% more rutting when compared to the
unreinforced control item. It was suggested that the overall stiffness of the sections may have
contributed to the minimal improvement observed by geogrid inclusion. It was noted that all
geogrids may not provide the same reinforcement benefit.
Note: The data described below were used to supplement additional research conducted
by the author and fully documented in Chapter 4 of this dissertation.
Norwood (2016, personal communication) performed cyclic plate load tests to investigate
the effect of geosynthetic reinforcement airfield pavements. The laboratory-scale sections
consisted of 305 mm subbase, 178 mm base, and 127 mm HMA surfacing. A subgrade CBR of
3% was targeted for all sections. Four test items were evaluated; three items reinforced with
geogrid at the base/subbase interface and an unreinforced control item. Plate loading consisted
of a total load of 128 kN and a contact pressure of 1750 kPa.
It was found that rutting resistance improved with geogrid inclusion when compared to
the unreinforced control item evidenced by TBR values ranging from 20 to 30. Post-test forensic
investigation found that subgrade rutting was observed in the unreinforced control item, whereas
no measurable rutting was observed in any of the geogrid reinforced items.
47
3.5 Normalization of Data for Analysis
The studies described in the previous section were performed to evaluate, at the time,
emerging geosynthetic products or to address specific sponsor questions. Therefore, each study
test plan was designed as a stand-alone project; at the time, consideration was not given to future
compilation of individual datasets. Loading conditions and structural thicknesses were selected
for each specific study, and as such, significantly differing loading conditions and structural
thicknesses were observed.
In order to compile previous studies into an overall data set, varying loading conditions
and varying structural thicknesses were normalized to common parameters. Specifically, tire
and/or plate loading configurations were converted to Equivalent Single Axle Loads (ESALs)
and hot mix asphalt thicknesses were converted to equivalent base course thicknesses (tequiv).
Note the term tequiv is considered the same as the pavement thickness term shown in Figure 3.1
due to maintaining the same asphalt thickness in the ETL 1110-1-189 procedure (USACE 2003).
One can observe a similar approach in the results of the AASHO Road Test and subsequently the
1993 AASHTO Guide for Design of Pavement Structures (AASHTO 1993) , which has been
used extensively by state DOTs. The authors acknowledge that this approach is a substantial
simplification based on the load-dependency nature of the paving materials utilized. The
procedure used for each conversion is described below.
3.5.1 Traffic Conversion
Various loading configurations and conditions were applied in the studies compiled for
this effort. A majority of the test items were trafficked or loaded with loads and tire pressures
generally in the range of that anticipated in a highway condition. It was determined that in order
to provide a reasonable comparison of the various studies the applied loading would be
48
converted to equivalent single axle loads (ESALs). Previous studies requiring traffic conversion
were Webster (133.5 kN at 468 kPa (1993) and Norwood (200 kN at 978 kPa) (2017, personal
communication). The Asphalt Institute’s equivalent axle load factors (Huang 2004) were
plotted and a best-fit polynomial trend line was fitted through the data. The trend line equation
was then used to extrapolate the aircraft loading to an equivalent single axle load factor
(ESALF). The ESALF calculated for Webster’s loading condition was 114 and for Norwood’s
loading condition was 634. The author acknowledges that the EALF’s obtained are well beyond
published values and represent a very large and approximate extrapolation beyond the AASHO
Road Test.
3.5.2 Asphalt Thickness Conversion
Asphalt thicknesses were converted to equivalent base thicknesses using the AASHTO
structural coefficients of 0.44 for asphalt and 0.14 for crushed stone base, which are typically
specified by MDOT for the local materials used in each study. Each asphalt thickness was
multiplied by 0.44 and then divided by 0.14 to obtain an equivalent base thickness. The
equivalent thickness were then added to the underlying base thickness to calculate an overall
equivalent aggregate thickness (tequiv). It is noted that variations in the properties of asphalt
placed may have occurred when the MDOT transitioned from the Marshall Mix design method
to the Superpave mix design method (approximately 1998): however for simplicity a standard
HMA structural coefficient was used. Use of a universal asphalt coefficient of 0.44 is
approximate, and it should be noted that modern asphalt pavements have values used above 0.44
in some cases (e.g. 0.50 to 0.54).
49
Limited asphalt mixture properties were available, as the asphalt surfacing was not
typically considered a test variable for each individual study. Key properties documented from
each study are presented in Table 3.1.
Table 3.1 Asphalt mixture properties
Reference Design
Method
NMS
(mm) RAP (%) Pb (%)
VMA
(%) P200 (%)
Webster 1993 Marshall 9.5 NR NR NR NR
Norwood 2017, personal
communication Superpave 9.5 NR NR 15.8 NR
Norwood and Tingle
2014b Superpave 9.5 15 5.7 15.4 3.8
Robinson, Tingle, and
Norwood 2017 Superpave 9.5 20 5.7 15.1 5.1
Norwood 2016, personal
communication Superpave 12.5 NR 5.2 13.0 5.0
NMS = nominal maximum aggregate size; RAP = recycled asphalt pavement; Pb = percent
binder by mass of the mixture; VMA = voids in mineral aggregate; P200 = percent passing #200
sieve size; NR = not reported
3.5.3 Selection of Analysis Data
The relationship between equivalent aggregate thickness (tequiv) and loading (ESALS) for
all reinforced test items is presented in Figure 3.4a. Variations in base course type, geosynthetic
placement location, and geosynthetic type were investigated by removing each group from the
overall dataset and observing changes in the regression.
It was found that a majority of the pavement structures consisted of crushed limestone
base course material. Pavement structures that did not include crushed limestone were excluded
from the analysis (four test items) due to significantly different documented strength
characteristics. Figure 3.4b presents the relationship for reinforced sections containing crushed
limestone base course material. An upward shift was observed in the regression (intercept of
+0.8 to +43.4) indicating that base course type could influence performance.
50
Al-Qadi, Dessouky, Kwon, and Tutumluer (2012) and Abu-Farsakh and Chen (2011)
concluded that pavement performance was improved when the geosynthetics were installed in
the upper one-third of thick base layers. Haas, Walls, and Carroll (1988) suggested that the
optimum placement location was the mid-point of the base layer. It was found that for a majority
of the test items, the geosynthetic was placed at the subgrade/base interface. Two items were
found to have a geosynthetic installed mid-depth in the base course and four items had a
geosynthetic installed at the base/subbase interface and were excluded from the analysis as
shown in Figure 3.4c. Of the two items where a geosynthetic was installed mid-depth in the base
course, one item (constructed on a 3 CBR subgrade and tequiv. of 508 mm showed improved
performance when compared to the unreinforced item. The second item (constructed on an 8
CBR subgrade and tequiv. of 686 mm showed poor performance when compared to the
unreinforced item. It was found that including only items with a geosynthetic installed at the
subgrade/base interface did not significantly change the observed trend.
It has been suggested that the reinforcement mechanisms of geotextile and geogrids are
different, and that different performance improvement could be observed (Barksdale, Brown, and
Chan 1989, Al-Qadi, Brandon, Valentine, Lacina, and Smith 1994). Test items (seven total)
containing a geotextile or geogrid/geotextile combination were excluded due to potentially
differing performance enhancement characteristics (reinforcement vs. separation). Figure 3.4d
presents the relationship for reinforced items containing only geogrid, and a downward shift was
observed in the regression (intercept of +0.8 to -59.5) indicating that geosynthetic type could
influence performance.
51
Figure 3.4 Effect of analysis variable on reinforced pavement regression
(a) All reinforced test items
(b) All limestone base test items
(c) All items reinforce at base/subgrade
(d) All geogrid test items
52
Data selected for analysis is shown in Table 3.2 for laboratory-scale test sections and
Table 3.3 for full-scale test sections. It is noted that the loading condition in Norwood (2016,
personal communication) (four items) was not considered in this analysis due to contact pressure
of 1750 kPa being well outside that anticipated in a highway scenario. A total of 31 different test
items were included in the analysis (4 from plate-load testing and 27 from full-scale testing).
Table 3.2 Cyclic plate-load testing
Reference HMA
(mm)
Base
(mm)
Subbase
(mm)
Subgrade
CBR Grid Type
dgs
(mm)
tequiv.
(mm)
ESALS at 25.4
mm Rutting
Jersey and
Tingle
2009
0 150 0 3 Triaxial 150 150 22,737
0 150 0 3 Biaxial 150 150 10,229
0 150 0 3 Triaxial 150 150 72,197
Tingle and
Jersey 2005 0 350 0 1 Biaxial 350 350 1,212
HMA = hot mix asphalt; CBR = California bearing ratio; dgs = depth of geosynthetic below
surface; ESAL = equivalent single axle load extrapolated as described earlier
53
Table 3.3 Full-scale testing
Reference HMA
(mm)
Base
(mm)
Subbase
(mm)
Subgrade
CBR Grid Type
dgs
(mm)
tequiv.
(mm)
ESALS at
25.4 mm
Rutting
Robinson,
Tingle, and
Norwood 2017
75 150 0 6 Triaxial 1 225 375 811,2001
75 150 0 6 Triaxial 2 225 375 811,2001
Norwood and
Tingle 2014b
25 200 0 6 None -- 275 60,4001
25 150 0 6 Triaxial 175 225 60,4001
Norwood and
Tingle 2014a 100 200 0 6 None -- 525 1,996,800
Jersey 2009,
personal
communication
50 200 0 3 None -- 350 13,000
75 200 0 3 None -- 425 27,870
50 200 0 3 Triaxial 250 350 100,000
Norwood
2017, personal
communication
100 350 0 8 None -- 675 44,976,650
100 350 0 8 Biaxial 450 675 63,737,000
100 350 0 8 Triaxial 450 675 37,335,860
100 350 0 8 Biaxial 450 675 58,126,869
100 350 0 8 Biaxial 450 675 43,774,572
100 350 0 8 Biaxial 450 675 113,451,860
100 350 0 8 Biaxial 450 675 29,445,219
100 350 0 8 Biaxial 450 675 114,726,600
Tingle and
Jersey 2009
0 150 0 4 None -- 150 23
0 150 0 4 Geogrid 150 150 2,205
Webster 1993
50 250 0 8 None -- 400 1,702,650
50 250 0 8 Biaxial 300 400 11,351,000
50 150 0 8 Biaxial 200 300 1,702,650
50 150 0 8 None -- 300 76,052
50 450 0 3 None -- 600 128,380
50 450 0 3 Biaxial 500 600 162,546
50 300 0 3 Biaxial 350 450 32,010
50 350 0 3 Biaxial 400 500 56,755
50 350 0 3 Biaxial 400 500 32,350
HMA = hot mix asphalt; CBR = California bearing ratio; dgs = depth of geosynthetic below
surface; ESAL = equivalent single axle load extrapolated as described earlier; 1Test terminated
prior to 25.4 mm rutting
3.6 Results
The relationship between equivalent aggregate thickness (tequiv) and loading (ESALS)
was plotted for both reinforced and unreinforced test items using the approach described in
54
Webster (1993) and is shown in Figure 3.5. It is noted that the aggregate equivalency chart
derived by Webster (1993) and implemented in ETL 1110-1-189 (USACE 2003) was derived
from a total of eight data points (two points per line) and as such overall variability could not be
determined. When the overall dataset is combined, it is observed that the fitted equations have
R2 values of approximately 0.64 and 0.59 and standard error (S) of 91.4 and 124.5 for
unreinforced and reinforced items, respectively.
Data were reviewed to determine if an increase in subgrade CBR provided for an increase
in rutting performance within the range of CBR values investigated (1 to 8). It would be
expected that items with higher subgrade CBR would fall above the regression line and lower
subgrade CBR would fall below the regression line. It was found that no clear trend existed
between subgrade CBR and the derived regression line, with CBR values falling both above and
below each regression line.
55
Figure 3.5 Relationship between equivalent aggregate thickness and ESALs for 25.4 mm
rutting
Using the derived trends for unreinforced and reinforced test items, equivalent aggregate
thickness values were calculated at levels ranging from 100 to 2,000,000 ESAL in approximately
logarithmic intervals. The calculated unreinforced thickness were then plotted on the y-axis and
the calculated reinforced thickness were plotted on the x-axis as shown in Figure 3.6. The
combined data show relatively good agreement with the original equivalency chart. It is noted
that the updated dataset reduces the equivalent thickness by approximately 25.4 mm at
unreinforced thickness less than 356 mm and converges with the original equivalency chart at an
unreinforced thickness of approximately 406 mm. Additionally, it was found that as the geogrid
depth of placement increases, the performance benefit provided by the geogrid decreases.
Review of the relationship in Figure 3.6 indicates that after a depth of approximately 560 mm
56
from the surface little to no performance improvement is realized. A 406-mm depth was
suggested by Tingle and Jersey (2005) and Kinney, Abbott, and Shuler (1998). Tingle and
Jersey (2005) hypothesized that the performance benefit decrease could be attributed to reduced
horizontal stress and/or deflection at increasing depth resulting in failure to mobilize tensile
behavior in the geosynthetics. Kinney, Abbott, and Shuler (1998) found that the traffic benefit
ratio (ratio of cycles for a reinforced section to an identical unreinforced section) decreased from
a value in excess of 10 for base thicknesses of 254 mm or less to a value of 1 at base thicknesses
of about 356 mm. Al-Qadi, Dessouky, Kwon, and Tutumluer (2012) found that for aggregate
layer thicknesses ranging from 203 mm to 457 mm geogrid was effective in reducing horizontal
shear deformation. Further (Al-Qadi, Dessouky, Kwon, and Tutumluer 2012) concluded that for
thicker base layers, optimal geogrid placement location was at the upper third of the layer,
suggesting that performance benefit decreases with placement depth.
57
Figure 3.6 Relationship between unreinforced and reinforced aggregate thickness
3.7 Conclusions
Multiple test sections have been constructed and trafficked at ERDC over the course of
approximately 25 years to assess geosynthetic inclusion in pavement structures. The pavement
test sections were constructed in a uniform manner with minor variability utilizing similar
materials and construction techniques. The uniformity of construction and materials allow for
meaningful comparisons between test sections to be made. Varying geogrid products in varying
pavement structures have been evaluated using a range of loading conditions. A majority of the
evaluations performed at ERDC have consisted of a pavement structure comprised of HMA
surface, crushed limestone aggregate base, and a high-plasticity clay subgrade. Combination of
the test data yielded the following:
58
1. Review of studies indicates that in terms of rutting performance, geogrid
stabilized sections performed equal to or better than companion unstabilized
sections.
2. Improvement appears to diminish with increasing depth of geogrid placement and
approaches no distinguishable improvement around approximately 560 mm.
3. The combined data supports the equivalent thickness chart recommended by
Webster (1993) and implemented in ETL 1110-1-189 (USACE 2003). The
adjusted data reduces the equivalent reinforced thickness by approximately 25.4
mm at unreinforced thickness less than 356 mm, providing a slightly more
conservative result. The adjusted data converged with the original equivalency
chart at an unreinforced thickness of approximately 406 mm.
59
CHAPTER IV
CYCLIC PLATE TESTING OF GEOSYNTHETIC-REINFORCED AIRFIELD PAVEMENTS
This chapter has been previously published as a journal article in the Proceedings of the
Institution of Civil Engineers (ICE)-Ground Improvement. The original paper may be accessed
at http://dx.doi.org/10.1680/jgrim.18.00106. In accordance with ICE Publishing Guidelines, the
paper (Robinson, Mahaffay, Howard and Norwood 2019) has been reformatted and reproduced
herein with minor modifications to suit the objectives of this dissertation.
Numerous cyclic plate load tests have been performed over the past several years to
investigate potential performance benefits of including geosynthetics in paved and unpaved
applications. A majority of these studies have focused on relatively thin pavement structures
subjected to highway loads. Airfield pavements can be substantially thicker and include multiple
aggregate layers, and data are needed to quantify geosynthetic contributions under high contact
pressures. Eleven representative airfield pavement structures (four unreinforced sections, four
containing geosynthetics at the subbase/subgrade interface, and three containing geosynthetics at
the base/subbase interface) were constructed in a laboratory containment facility and subjected to
cyclic loading under 1750 kPa simulated aircraft contact pressure. Permanent surface
deformation and vertical pressure response data were collected to determine relative
improvement when compared to an unreinforced pavement structure and to evaluate the
influence of geosynthetics. Some geosynthetics increased cycles to failure, and it was found that
some level of permanent deformation (e.g. 25 mm) may be required to engage reinforcing
60
benefits when placement occurs at the subgrade/subbase interface. Changing subbase material
from a California Bearing Ratio of 15-18 to 55 resulted in 1.5 orders of magnitude more cycles
to 25 mm of permanent deformation than any of the geosynthetic reinforced sections evaluated.
4.1 Introduction
Deteriorating infrastructure and shrinking fiscal budgets lead transportation agencies to
seek innovation in pavement design and construction practices. For instance, the Federal
Aviation Administration (FAA) in their 10-year research and development plan, are proposing to
investigate methodologies that extend the anticipated design life of airfield pavements from 20 to
40 years. One possible solution could be the use of geosynthetics, provided they can increase
service life or reduce up-front material and/or construction costs.
Numerous studies have been undertaken to determine potential performance
improvement gained from incorporating geosynthetics in flexible pavements. A majority of these
studies have focused on relatively thin pavement structures subjected to highway loads. Airfield
pavements can be substantially thicker and include multiple aggregate layers when compared
with highway pavements. Data are needed to quantify geosynthetic behavior within airfield
pavement structures to determine if they can be one viable approach to, for example, assist the
FAA increase airfield pavement design lives to 40 years.
The literature presented in the next section suggests that while multiple cyclic plate load
tests have been conducted on geosynthetic inclusion in relatively thin paved and unsurfaced
highway pavements, little work has been documented evaluating thicker airfield pavements,
including subbase layers and high contact pressures (CPs). Further, few full-scale research
studies for airfield pavements have been conducted, with Webster (1993) representing one of the,
if not the, most comprehensive studies, albeit at relatively lower load and tire pressures than
61
would be expected on an international or military airfield. This paper presents results from a
multi-phase series of cyclic plate load tests conducted at the Engineer Research and
Development Center (ERDC) of the U.S. Army Corps of Engineers (USACE) to investigate
behavioral characteristics of relatively thick flexible pavements incorporating geosynthetics
under high CP loading. Testing aimed to assess whether geosynthetics could be effective in
thicker pavements, and whether characteristics of different geogrids could be detected under
these conditions.
4.2 Literature Review Pertinent to Cyclic Plate Load Testing
The literature review presented herein summarizes findings relevant to cyclic plate load
testing only in support of the observations made in this chapter. A more detailed description of
cyclic plate load testing literature can be found in Chapter 2.
This paper made use of cyclic plate load testing, but it is worth noting that several other
testing protocols can, and have, been used to evaluate geosynthetics in flexible pavements. A
sampling of such methods is provided in the following sub-section, and thereafter another sub-
section provides specific content for cyclic plate load testing, which is the testing approach of
interest for this paper, for a variety of pavement configurations. The final subsection provides
data of direct relevance to geosynthetic-reinforced airfields.
4.2.1 Test Methods to Assess Geosynthetic Inclusion in Unbound Pavement Layers
Full-scale evaluations of geosynthetic-reinforced pavements are a testing option that has
been employed by multiple US state departments of transportation (DOTs), and by several other
research groups (e.g. Brandon, Al-Qadi, Lacina, and Bhutta 1996; Collin, Kinney, and Fu 1996;
Hayden, Humphrey, Christopher, Henry, and Fetten 1999; Perkins and Lapeyre 1996; Warren
62
and Howard 2007). Field-testing is usually more expensive than laboratory evaluations; both
have positive and negative attributes. Full-scale field-testing is purely realistic but has replication
and variability challenges in the majority of cases, whereas laboratory experiments of varying
scales have progressively more control on variables but with this control comes boundary
condition, loading and calibration challenges. Full-scale testing provides the opportunity for
adding instrumentation to measure structural response (e.g. stress, strain and deflection) and
environmental conditions (e.g. temperature and moisture content) alongside monitoring of
pavement distresses and overall performance.
At the smaller end of laboratory-scale experiments, procedures such as direct shear and
shear wave tests provide a means to measure local influence of geosynthetics on adjacent
aggregate materials. Direct shear testing (Arulrajah, Rahman, Priatheepan, Bo, and Imteaz 2014;
Suddeepong, Sari, Horpibulsuk, Chinkulkijniwat, and Arulrajah 2018) has been conducted to
investigate the interface shear strength of geosynthetic-reinforced soils and aggregates. Shear
wave velocity measurements have been used to quantify the zone of influence and stiffness
enhancement properties of geogrid inclusion in granular materials (Byun and Tutumluer 2017;
Schuettpelz, Fratta, and Edil 2009).
Cyclic plate load testing could be considered to be a balance between full-scale field
testing and smaller-scale laboratory experiments described in the previous two paragraphs.
Tested sections remain fairly large, instrumentation can still be used, but their scale is still
noticeably smaller than field experiments. Overall, the authors elected to take a balanced
approach and employ cyclic load testing herein, and the following section reviews literature from
cyclic load experiments.
63
4.2.2 Cyclic Plate Load Testing of Pavements Reinforced with Geosynthetics
Cyclic plate load tests have advantages over full-scale testing, namely reduced cost,
increased construction and testing speed and decreased variability. Inability to simulate
pavement response under a moving load and to emulate tire/pavement interactions are
disadvantages. Multiple cyclic plate load tests have been performed to investigate performance
benefits of incorporating geosynthetics in paved and unpaved applications.
Table 4.1 presents a summary of cyclic plate load tests performed to measure
geosynthetic performance for highways. Asphalt thicknesses ranged from 0 cm (unsurfaced) up
to 100 mm. Base thickness for studies incorporating an asphalt layer ranged from 100 to 450
mm, and unsurfaced base thickness was up to 760 mm. None of the studies included a subbase
layer. CP (e.g. tire pressure) was generally in the range expected for highways. The highest CP
was observed to be 1100 kPa.
64
Table 4.1 Cyclic highway plate load tests and findings from literature
Reference Asphalta
(mm)
Basea
(mm)
Subgrade
CBR
Plate
Diameter
(mm)
Total
Load
(kN)
Pressure
(kPa) Key Findings
Abu-Farsakh and
Chen 2011
50 310 0.5 305 40 550 TBR values up to 15.3 at 19.1 mm of rutting. Better performance
observed when the grid was placed in the upper one-third of the
base layer vs. placement at the middle of base layer or top of
subgrade.
Abu-Farsakh,
Hanandeh,
Mohammed, and
Chen 2016
76 250
&
450
0.5 305 40-80 550-
1100
Single layer geosynthetic placed at the base-subgrade interface
resulted in TBR values up to 1.52. Best performance was observed
in a section with double reinforcement layers.
Al-Qadi,
Brandon,
Valentine,
Lacina, and
Smith 1994
70 150 4 300 40 550 Geotextile placed at the base-subgrade interface improved
performance up to 35% over control section after load seating. It
was noted that separation via geotextile appeared to be important to
improving structural capacity.
Bauer and
Abdelhalim
1987
0 80-
300
NR 305 40 550 Load cycles to reach 28 mm were 155,000 for unreinforced base vs.
233,000 for reinforced base. It was found that about 10,000 load
cycles were required to develop full grid strength.
Cancelli,
Montanelli,
Rimoldi, and
Zhao 1996
75 300 1, 3, 8,
18
300 40 570 Two layers of geogrid provided a decrease in maximum settlement
when compared to one layer only. The percent reduction in rutting
increased as CBR decreased.
Douglas 1997 0 250-
760
NR Beam 0.12 -
0.16
NR Presented a model to design unbound geosynthetic-built roads using
stiffness rather than rut depth based on results of repeated load tests.
Incorporated a dimensionless tension term to characterize
geosynthetic.
Douglas and
Valsangkar 1992
0 150 <1 300 4.5 64 Recommended use of roadway stiffness rather than permanent rut
depth to define failure in unpaved roads. A compacted crushed rock
structure with geogrid at mid-depth had a stiffness 3.6 times that of
the weak subgrade. aReported thicknesses are design thicknesses; b13 mm rubber mat used to simulate asphalt; NR = not reported; TBR defined as ratio of the number of load
cycle of a reinforced pavement structure to reach a defined failure state to the number of load cycles of an identical reinforced pavement structure
65
Table 4.1 (continued) Cyclic highway plate load tests and findings from literature
Reference Asphalta
(mm)
Basea
(mm)
Subgrade
CBR
Plate
Diameter
(mm)
Total
Load
(kN)
Pressure
(kPa) Key Findings
Ghafoori and
Sharbaf 2015
76 410 8 305 40 550 Geogrid placed in middle of 410 mm thick base layer. Improved
performance in terms of rutting. Geogrid inclusion reduced vertical
stress in the middle of base by approximately 40%.
Gongora and
Palmeira, 2012
0 230 4.2 200 18 560 Use of recycled rubble was validated with geosynthetic
reinforcement. Performance was a function of a combination of
factors, including aperture stability modulus and tensile stiffness.
Haas, Walls,
and Carroll
1988
50-100 100-
300
<1, 1,
3.5,8
305 40 550 Optimum grid location considered to be at the base-subgrade
interface for thin bases. For thicker bases, it was suggested that the
optimal location is in the middle portion.
Jersey and
Tingle, 2009
13b 150 3 305 13-67 180-900 Relative improvements were observed in three different geogrids;
however, an unreinforced section was not tested. Failure at high
loads were based on exceeding the bearing capacity of the soft clay
subgrade layer.
Kelly, Fairfield,
and Sibbald
1995
0 200 NR 200 15 306 Loss of interlock between the geosynthetic and aggregate layer
reduced the lateral restraint effect. Increased geosynthetic tensile
modulus reduced vertical displacement.
Montanelli,
Zhao, and
Rimoldi 1997
75 300 1, 3, 8,
18
300 40 570 Modified the 1981 AASHTO pavement design method by adjusting
base layer coefficient to account for increased performance. Found
that aggregate structural layer coefficient ratio ranged from 2 to 1.5.
Perkins, 1999 75 200-
380
1.5 & 15 305 40 550 Significant improvement in surface rutting observed with inclusion
of geosynthetic reinforcement. Stiffer geogrid provided for better
performance. Significantly better performance observed when
geogrid was elevated in the base.
Qian, Han,
Pokharel, and
Parsons 2013
0 150-
300
2 300 40 550 TBR values range from 1.0 to 13.0 at permanent displacement from
25 to 75 mm. Tensioned membrane effect was recognized when
permanent deformation was larger than one-third the base thickness.
aReported thicknesses are design thicknesses; b13 mm rubber mat used to simulate asphalt; NR = not reported; TBR defined as ratio
of the number of load cycle of a reinforced pavement structure to reach a defined failure state to the number of load cycles of an
identical reinforced pavement structure
66
Table 4.1 (continued) Cyclic highway plate load tests and findings from literature
Reference Asphalta
(mm)
Basea
(mm)
Subgrade
CBR
Plate
Diameter
(mm)
Total
Load
(kN)
Pressure
(kPa) Key Findings
Sarici, Demir,
Tutumluer, Demir,
Gungor, Epsileli,
Comez, and Ok
2016
0 300-
450
4 300 40 550 Geogrid placement at the upper one-third of base thickness
recommended for best performance.
Sun, Han, Kwon,
Parsons, and
Wayne 2015
0 150-
300
2 300 5-50 70-700 Vertical stress at the base-subgrade interface reduced by the
inclusion of geogrid. Radial stresses away from the load plate
decreased, indicating lateral confinement of geogrid changed the
stress distribution.
Tingle and Jersey
2005
0 360
&
510
1 305 40 550 TBR values range from 1.3 to 36.5 at deformation from 12 to 50
mm. Suggested a maximum placement depth of 400 mm below the
surface.
aReported thicknesses are design thicknesses; b13 mm rubber mat used to simulate asphalt; NR = not reported; TBR defined as ratio
of the number of load cycle of a reinforced pavement structure to reach a defined failure state to the number of load cycles of an
identical reinforced pavement structure
67
4.2.3 Geosynthetic-reinforced Airfields
Limited airfield work was found in the literature. Some literature (Abdesssemed, Kenai,
and Bali 2015; Buonsanti, Leonardi, and Scopelliti 2012; Von Quintas, Mallela, and Lytton
2009) documented geosynthetic inclusion in the upper portion of a flexible airfield pavement,
specifically, within asphalt layers. Reflective crack mitigation was the primary improvement
mechanism identified. A complete review of geosynthetic placement within asphalt is outside the
scope of this paper; rather the focus is directed at geosynthetic placement in aggregate layers.
Decades ago, Haliburton, Lawmaster, and King (1980) investigated the use of geotextiles
in flexible airfield pavements. It was concluded that geotextiles had potential to improve airfield
pavement performance, but that additional research was required to fully understand
performance improvement.
Webster (1993) constructed and trafficked 16 flexible pavement sections, 11 of which
included geosynthetic products. While this work was used to develop current USACE design
criteria, it is noted that traffic simulated relatively light aircraft (133.5 kN single wheel load at
486 kPa tire pressure).
Tirado, Carrasco, Nazarian, Norwood, and Tingle (2014) used a three-dimensional finite-
element model to estimate geogrid-reinforced flexible pavement performance improvement
under C-17 and F-15 aircraft loading conditions. It was found that geogrid reinforcement was
more beneficial under F-15 loading, mainly attributed to differences in loading and gear
configurations. It is noted that the model results were not calibrated with field test results,
therefore comparisons are relative.
Cancelli, Recalcai, and Shin (2000) and Shin, Oh, and Kyu-Jin (1999) document analysis
and field performance of geogrid reinforcement of base course of a runway at Inchon
68
international airport. The airport was developed on marine sediment and settlement of the airfield
pavement system was a concern, particularly around rigid drainage structures. Finite-element
analysis indicated that inclusion of a geogrid would reduce potential settlement under traffic. The
follow-on case study demonstrated that the geogrid was effective in reducing differential
displacements around the runway’s drainage structures.
4.3 Laboratory-scale Test Sections
Eleven test items, TI1−TI11, were constructed in a 1.8 m cube steel containment box.
TI1−TI7 comprised phase I where geosynthetics were placed at the subbase−subgrade interface.
Phase II included TI8−TI11 where geosynthetics were placed at the base−subbase interface. All
items were loaded using a 222 kN capacity hydraulic actuator controlled with a material testing
system control unit (Figures 4.1a and 4.1b). A 128 kN applied load was transmitted to the
pavement by a 305 mm diameter plate with a 6 mm rubber pad (Figure 4.1c) yielding a 1750 kPa
simulated aircraft contact pressure. Contact pressure was selected to simulate aircraft such as a
Boeing 787 or Airbus 350, although it is noted that the aircraft pressure would be applied on a
tire contact area over twice the size of the laboratory plate. Further, the aircraft noted have dual
tandem gear configurations that would likely introduce stress interactions not simulated by single
plate loading. Loading was applied sinusoidally and each pulse had a total duration of 1.2 s.
Load was applied for a 0.3 s duration followed by a 0.9 s rest period (Figure 4.1d). During the
rest period, a 0.4 kN surcharge was maintained to ensure the plate remained in contact with the
pavement surface while allowing elastic rebound of the pavement. Previous research using the
same hydraulic equipment (Tingle and Jersey 2005) found that the equipment was not capable of
sustaining applied loads under a more rapid load rate. The load rate was reduced to ensure that
the hydraulic equipment did not have difficulty in sustaining the design load.
69
Due to dimensional constraints and inherent variability in placing and compacting HMA
inside a containment facility, a 3.6 m wide by 30.5 m long HMA section was constructed over a
substrate of 38 mm thick plywood as shown in Figures 4.1e–4.1g. After the HMA had
sufficiently cooled, 172 cm by 172 cm slabs that were roughly 127 mm thick were saw cut and
transported (Figure 4.1h) to a non-climate-controlled covered storage facility where ageing
occurred for approximately one month prior to testing. Placement and compaction outside the
facility allowed for normal paving and compaction procedures to be utilized. It is noted that by
placing the HMA layer outside the box facility, the underlying soils are not subjected to
construction stresses during HMA compaction and that the interface bonding condition likely
differs from what would be expected in full-scale construction.
4.3.1 Material Properties
Laboratory tests were performed to determine gradation, plasticity and moisture density
relationship of component materials (Table 4.2). Subbase material for TI-1 was stone screenings
(StS), and subbase for TI-2 through TI-11 was a locally available coarse sand (SaS). Both
materials met the requirements for FAA-P154 subbase.
The 127 mm thick asphalt layer for each test item was constructed using a locally
available 12.5 mm nominal maximum aggregate size (NMAS) HMA mixture placed in two lifts.
The mixture was a gravel/limestone aggregate blend with an unmodified PG 67-22 binder, 11%
reclaimed asphalt pavement (RAP), and 5.4% total asphalt content. Design gyrations were 85,
representing a high traffic volume mixture for the area.
Geosynthetics evaluated in this study (Table 4.3 and Figure 4.2) consisted of a punched
and drawn biaxial geogrid (GEO1), a geogrid composed of interlocking polypropylene yarns
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coated with polymer (GEO2 and GEO5), a multiaxial geogrid consisting of a series of concentric
triangles (GEO3), and a woven polypropylene filament geotextile (GEO4).
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Figure 4.1 Photographs of cyclic plate testing and asphalt paving
(a) Test Box (Front Removed)
(b) Cyclic Plate Test Set-up
(c) Close-up of Cyclic Plate Test (After Test)
(d) Typical Load Pulses of 128 kN
(e) Plywood Substrate
(f) Loaded Asphalt Truck Backing On Plywood
(g) Asphalt Paver on Plywood
(h) Saw-Cut Slabs Transported to Test Box
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Table 4.2 Laboratory material property test results
Material LL PL PG
(%)
PS
(%)
P200
(%) Cc Cu AASHTO USCS
MD
(kg/m3)
OMC
(%) Item
Subgrade 84 29 0 1.5 98.5 -- -- A-7-6 CH 1640 22.8 All
StS
Subbase -- -- 6 94 0 1.2 7.9 A-1-b SW 2192 8.1 TI-1
SaS
Subbase -- -- 1 99 0 0.7 6 A-3 SW 1786 1.9
TI2-
TI11
LL = liquid limit; PL = plastic limit; PG = percent gravel; PS = percent sand; P200 = percent passing 200 sieve; Cc =
coefficient of curvature; Cu = coefficient of uniformity; USCS = Unified Soil Classification System determined by
ASTM D2487; MDD = maximum dry density determined by ASTM D1557; OMC = optimum moisture content
determined by ASTM D1557
Table 4.3 Geosynthetic properties as provided by manufacturer
Property Method GEO1 GEO2 GEO3 GEO4 GEO5
Geosynthetic type -- Biaxial Biaxial Triaxial Woven Biaxial
Aperture Size MD
(mm) Measured 25 25 -- -- 15
Aperture Size CMD
(mm) Measured 33 25 -- -- 15
TS @ 2% Strain MD
(kN/m)
ASTM D6637/ASTM
D4595 6.1 15.1 -- 7.9 8.1
TS @ 2% Strain CMD
(kN/m)
ASTM D6637/ASTM
D4595 9.1 15.1 -- 31.5 13.1
TS @ 5% Strain MD
(kN/m)
ASTM D6637/ASTM
D4595 11.81 32.1 -- 22.8 20.1
TS @ 5% Strain CMD
(kN/m)
ASTM D6637/ASTM
D4595 19.61 32.1 -- 71.8 27.1
Ultimate TS MD
(kN/m) ASTM D6637 19.2 40 -- -- 27
Ultimate TS CMD
(kN/m) ASTM D6637 28.8 40 -- -- 35
Rib Pitch-Longitudinal
(mm) Measured -- -- 40 -- --
Rib Pitch-Transverse
(mm) Measured -- -- 40 -- --
Radial stiffness kN/m
@ 0.5% Strain ASTM D6637 -- -- 225 -- --
MD = machine direction; CMD = cross-machine direction; TS = tensile strength
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Figure 4.2 Photographs of geosynthetics
(a) GEO1
(b) GEO2
(c) GEO3
(d) GEO4
(e) GEO5
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4.3.2 Instrumentation
Sensors were placed in the subgrade, subbase and at the pavement surface to quantify the
response of each test item during loading (Figure 4.3). Two 100 mm diameter earth pressure
cells (EPCs) capable of measuring earth pressures up to 400 kPa were placed in the subgrade
directly under the center of the loading plate; the upper EPC was placed approximately 25 mm
below the subbase−subgrade interface and the lower EPC was placed approximately 50 mm
above the bottom of the test section. One 100 mm diameter EPC capable of measuring earth
pressure up to 1013 kPa was placed approximately 25 mm below the subbase surface. Six linear
variable displacement transducers (LVDTs) were placed at the surface of the asphalt layer to
monitor deformation/upheaval outside the load plate, and one LVDT was placed on the load
plate to monitor surface deformation directly at the loading site. Permanent surface deformation
data referred to in this paper represent measurements obtained from the plate LVDT.
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Figure 4.3 Schematic representation of the instrumentation of a typical test item
4.3.3 As-built Properties
Quality control tests were performed during construction of each material lift to ensure
target values were achieved and to monitor material consistency. Dry density and moisture
content (two tests per 150 mm thick lift) were measured using a nuclear device in accordance
with ASTM D6938 (ASTM 2017d) to verify the uniformity of each material lift. In-place
California bearing ratio (CBR) tests (three tests per 150 mm thick lift) were performed in general
accordance with ASTM D4429-09a (ASTM 2009) on each compacted lift to ensure target values
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were achieved. Dynamic cone penetrometer tests were conducted after base course placement to
approximate CBR values or the SaS subbase in TI-2 through TI-11. Asphalt cores densities were
determined in accordance with AASHTO T166 (AASHTO 2016) and reported as a percentage of
theoretical maximum specific gravity (% Gmm). As-built properties are provided in Table 4.4.
Table 4.4 As-built properties (cyclic plate load tests)
Item TI-1 TI-2 T1-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9 TI-10 TI-11
Geosynthetic None None None GEO1 GEO3 GEO2 GEO4 None GEO1 GEO3 GEO5
Phase I I I I I I I II II II II
CH Subgrade Properties
% of D1557 MD 82.4 79.9 76.8 80.4 78 79.4 79.6 81.2 83.8 82.7 84.2
Dry Density (kg/m3) 1351 1310 1259 1318 1279 1302 1305 1331 1374 1357 1381
Nuclear Moisture (%) 34.4 36 38.1 36.2 39.6 36.1 36.8 32.7 32.6 33.4 32.5
OD Moisture (%) 36.8 38.2 40.7 38.6 38 39.2 38 35.4 36.9 38.3 36.8
In-Place CBR (%) 3 2.9 1.8 2.9 3 2.9 2.8 3.6 3.2 3.4 3.1
Thickness (mm) 708 706 711 711 708 713 713 706 721 734 711
Subbase Properties
Subbase Type StS SaS SaS SaS SaS SaS SaS SaS SaS SaS SaS
% of D1557 MD 77
96.2 95.1 98.9 95 99.3 97 97 98.8 98.1 97.3 98.4
Dry Density (kg/m3) 2108 1699 1766 1697 1774 1731 1733 1764 1752 1738 1757
Nuclear Moisture (%) 6.6 4.1 4 3.8 2.8 2.4 3.3 4.9 3.2 4.5 3.6
OD Moisture (%) 3.1 4.3 5 4.1 2.3 3.5 3.2 5.8 5.9 5.8 4.2
In-Place CBR (%) 55 18 18 16 18 17 17 15 18 15 15
Thickness (mm) 302 307 304 304 304 302 307 310 312 315 315
Crushed Limestone Base Properties
% of D1557 MD 93.6 91.4 92.5 92 92.4 92.2 92.9 91.5 93 94 94
Dry Density (kg/m3) 2200 2149 2173 2162 2172 2167 2184 2151 2185 2209 2209
Nuclear Moisture (%) 2.8 2.8 2 4.4 2.6 2.2 2 5.3 2.1 1.7 1.7
OD Moisture (%) 1.9 2.5 1.3 2.1 2 0.9 1.3 5.1 5.1 4.8 4.8
In-Place CBR (%) 100+ 100+ 100+ 100+ 100+ 100+ 100+ 88 100+ 100+ 100+
Thickness (mm) 182 172 172 175 175 177 175 180 170 152 170
Hot-Mix Asphalt Properties
% of Gmm 94.5 94.5 94.5 94.5 94.5 94.5 94.5 91.2 91.2 91.2 91.2
Thickness (mm) 119 134 124 132 124 132 124 122 130 127 130
MD = maximum dry density; OD = oven dried; CBR = California Bearing Ratio; Gmm = theoretical maximum specific gravity
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4.4 Results
Table 4.5 and Figures 4.4a–4.4d provide test results that are interpreted later in this paper.
Table 4.5 arranges results by test item (TI), geosynthetic type (none is reported for unreinforced
control sections), and Table 4.2 subbase type (StS or SaS). These interpretations are divided into
several categories, due largely to the nature of data collection. There are distinct differences
between phases I and II that can be observed in the as-built properties of Table 4.4.
Table 4.5 Cycles to failure and TBR
Item
Cycles at 25
mm Permanent
Deformation
Cycles at 50
mm Permanent
Deformation
TBR at 25 mm
Permanent
Deformation
TBR at 50 mm
Permanent
Deformation
Phase I – Geosynthetics at Subbase/Subgrade Interface
TI-1 (None-StS) 150,720 Not Achieved -- --
TI-2 (None-SaS) 3,700 16,990 1 1
TI-3 (None-SaS) 3,000 14,000 --- ---
TI-4 (GEO1-SaS) 9,720 44,960 2.6 2.6
TI-5 (GEO3-SaS) 1,460 9,890 0.4 0.6
TI-6 (GEO2-SaS) 4,100 17,600 1.1 1
TI-7 (GEO4-SaS) 2,575 20,830 0.7 1.2
Phase II – Geosynthetics at Base/Subbase Interface
TI-8 (None-SaS) 304 1,280 1 1
TI-9 (GEO1-SaS) 6,325 25,960 20.8 20.3
TI-10 (GEO3-SaS) 1,000 4,300 3.3 3.4
TI-11 (GEO5-SaS) 9,025 34,900 29.7 27.3
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Figure 4.4 Instrumentation response
(a) Permanent deformation with load cycles
(b) Subbase pressure response with load cycles
(c) Top of the subgrade pressure response with load cycles
(d) Bottom of the subgrade pressure response with load cycles
4.4.2 Comparison of Unreinforced Sections
TI-1, TI-2, TI-3 and TI-8 were unreinforced, and all had comparable subgrade, subbase
and base layer thicknesses with the maximum difference in any layer being 10 mm. Asphalt
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thickness in TI-1 was 15 mm less than TI-2, with TI-3 and TI-8 falling between these values. In
place subbase CBR values were much higher for TI-1 (55, built with StS), against the other three
sections where CBR values were similar (16−18, built with SaS). Subgrade CBR values ranged
modestly (2−4), while base CBR values were 100+ for the TI-1 to TI-3, but only 88 for TI-8. TI-
8 also had lower density asphalt (91.2% of Gmm) than TI-1 to TI-3 (94.5% of Gmm).
TI-1 experienced at least 40 times the cycles to 25 mm deformation as the other three
controls. The variability in as-built properties described in the previous paragraph and shown in
Table 4.4 do not account for such a drastic difference, and as such the data suggest the improved
subbase CBR was the primary contributing factor. No geosynthetic in either test phase provided
improved rutting resistance within 1.5 orders of magnitude of the improved subbase layer.
TI-2 and TI-8 were intended to be replicates of each other with a subgrade CBR of 3,
while TI-3 targeted a subgrade CBR of 2 instead of 3 but was otherwise intended to replicate TI-
2 and TI-8. TI-2 and TI-3 have comparable cycles at 25 mm deformation that are logical relative
to as-built properties. TI-2 has a 10 mm thicker asphalt layer, a CBR of 2.9 against 1.8, and it
withstood 700 additional cycles. TI-8 behaved an order of magnitude worse than TI-2; TI-2 had a
12 mm thinner asphalt layer, 3.2% less %Gmm (a major difference), 8 mm more base compacted
to a lower density leading to a 12% (or more) CBR reduction and a subgrade CBR increase of
roughly 1%.
Laboratory rutting tests were conducted using an asphalt pavement analyzer to determine
the expected performance impact that may be attributed to asphalt density. Six specimens were
prepared at target air void contents of 5.5% (to approximate density of TI-2) and 8.5% (to
approximate density of TI-8), and tests were conducted at a 64°C chamber temperature, 445 N
load and 689 kPa hose pressure. Both air void levels were subjected to 8000 test cycles and it
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was found that specimens prepared at 5.5% air voids (TI-2) had an average rut depth of 2.6 mm,
while specimens prepared at 8.5% air voids (TI-8) had an average rut depth of 6.1 mm. The
relative magnitude of the laboratory rut test data (over two times the rutting in TI-8 compared
with TI-2), while not accounting completely for the differences observed in the two items,
provides an indication of the influence of asphalt density when comparing test phases.
Comparing TI-8 to all test items (neglecting geosynthetics), its subgrade was probably
the best of all items, but by a modest margin, its base layer had the lowest CBR of any test item,
and it had the thinnest and lowest density asphalt layer (TI-1 thickness excluded). As such, any
relative comparison to TI-8 should be understood in the context that TI-8 is observed to be the
worst of the test items in terms of its intended as-built properties, so direct comparisons of
geosynthetic improvement if viewed solely against TI-8 are likely inflated to some extent.
4.4.3 Traffic Benefit Ratio
Traffic benefit ratio (TBR), which is defined by AASHTO R 50-09 as the ratio of the
number of load cycles of a reinforced pavement structure to reach a defined failure state to the
number of load cycles of an identical unreinforced pavement structure, were calculated at 25 and
50 mm of permanent surface deformation based on the respective unreinforced item in each
phase (Table 4.5). TBR values in Table 4.5 should be viewed in the context of the as-built
properties (TI-8 in particular) presented in the previous section.
Phase I TBR values are much lower than phase II, but with TI-8’s as-built properties,
TBR values are not a source that the authors relied on heavily herein. Relying on literature,
placing geosynthetics deeper in relatively thick sections has the tendency to reduce potential
benefits under highway loading conditions. A 406 mm depth of placement below the top of the
pavement was suggested by Tingle and Jersey (2005) and Kinney, Abbot, and Schuler (1998).
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Kinney, Abbot, and Schuler (1998) found that the TBR decreased from a value in excess of 10
for base thicknesses of 254 mm or less to a value of 1 at base thicknesses of about 356 mm. Al-
Qadi, Dessouky, Kwon, and Tutumluer (2012) found that for aggregate layer thicknesses ranging
from 203 to 457 mm geogrid was effective in reducing horizontal shear deformation. Further Al-
Qadi, Dessouky, Kwon, and Tutumluer (2012) concluded that for thicker base layers, optimal
geogrid placement location was at the upper third of the layer, suggesting that performance
benefit decreases with placement depth. Robinson, Tingle, Norwood, and Howard (2018)
assembled data from test sections constructed with varying geogrid products in varying
pavement structures over a range of loading conditions. It was concluded that performance
improvement appeared to diminish with increasing depth of geogrid placement and approached
no distinguishable improvement at a depth of approximately 560 mm.
4.4.4 Interpretation of Permanent Surface Deformation Measurements
Interpretation of permanent surface deformation is divided by test phase to better account
for as-built properties. In phase I, TBR values are a resource since TI-2 is a reasonable
benchmark relative to the reinforced sections. In phase II, TBR values are not relied on for
assessments in this section.
4.4.4.1 Phase I Permanent Deformation
For the phase I study, in terms of cycles to failure at the 50 mm failure criteria, TI-4
(GEO1) was found to be the best performer withstanding 44,960 cycles (TBR of 2.6), followed
by TI-7 (GEO4). It is noted that TI-7 only took 2,575 cycles to reach 25 mm of permanent
deformation, suggesting that some level of deformation is required to mobilize a performance
benefit (TI-7 had a TBR below 1 at 25 mm of deformation, but a TBR above 1 at 50 mm of
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deformation). These two sections had essentially the same as-built properties except TI-7 had 8
mm (6%) less asphalt thickness, which would be expected to modestly affect this section’s
ability to withstand load cycles. TI-6 was found to perform slightly better than the control,
having a TBR of 1.1 and 1.0 at 25 and 50 mm deformation, respectively. TI-5 was found to have
a TBR of 0.6 when compared with the control. Review of construction data indicate that TI-5
had lower subgrade density and an asphalt layer on the thinner end of the range relative to other
phase I items. The combination of these two material properties could explain some of the
reduced performance in TI-5 when compared with the unreinforced section.
Permanent deformation data collected directly at the load plate are shown in Figure 4.4a.
In the items containing coarse sand subbase (SaS), TI-4 (GEO1) was found to be the best
performer at all load cycles, while TI-5 (GEO3) was found to be the worst performer. TI-6 was
found to have comparable performance to the unreinforced item at all load levels. TI-7 was
found to underperform the unreinforced item up to approximately 10,000 cycles (38 mm of
permanent deformation), after which some performance improvement was observed, suggesting
that some level of permanent deformation is required to engage the geotextile reinforcing benefit.
For airfields, one of the observations from phase I is the need for a noticeable amount of
permanent deformation (e.g. 25 mm) to occur for some geosynthetic products to engage in a
more productive manner. TBR values, overall, trended higher at higher deformation levels. If a
large and costly airfield is designed and constructed that requires damage to mobilize the
geosynthetics, this may not be as desirable as moving the geosynthetics closer to the pavement
surface. Recall that all of the phase I testing occurred with geosynthetics at the
subbase−subgrade interface. A second overall observation is that geosynthetic properties were
noticeable even when placed deep in the pavement structure.
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4.4.4.2 Phase II Permanent Surface Deformation
TI-11 (GEO5) was found to be the best performer in the phase II study, followed by TI-9
(GEO1). TI-10 (GEO3) underperformed relative to GEO1 by a factor of 6, and relative to GEO5
by a factor of 8−9. A review of construction data indicates that TI-10 was constructed with
approximately 25 mm (14%) less base course than other test items and it could be assumed that
performance would be higher had the item had the full 178 mm base course thickness. An
interesting observation in phase II is that GEO1 and GEO3 behaved similarly to each other
relative to phase I, suggesting that the inherent geosynthetic properties are manifesting
themselves at different depths. Another observation from phase II is that GEO3 and GEO5 when
placed at the base−subbase interface were able to mitigate some of the performance deficiencies
from less than desired base course and asphalt compaction.
4.4.5 Interpretation of EPC Measurements
4.4.5.1 Phase I EPC Response
Measured EPC maximum pressure at the top of the subbase is shown in Figure 4.4b.
Subbase pressures were found to slightly increase and then decrease after initial loading, likely
attributed to aggregate shakedown (Werkmeister, Dawson, and Wellner 2001). After a duration
of constant or decreased pressure with loading, it was found that pressure increased. Maximum
pressure inflection points were observed at around 25 mm of permanent deformation. The
general shape of subbase pressure curves was found to be consistent for all items. It is noted that
for TI-1 subbase pressures were found to be relatively consistent for the duration of loading, after
initial aggregate shakedown.
The measured maximum pressure at the top of the subgrade is shown in Figure 4.4c and
was found to be relatively constant up to 1000 cycles for all items. Thereafter, the pressure was
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found to increase, which could be attributed to an increase in subgrade rutting. The general shape
of the subgrade maximum pressure curves was found to be generally consistent for all items. The
pressure response for TI-1 was found to be relatively consistent throughout the test duration.
Pressure response data at the bottom of the subgrade are presented in Figure 4.4d. Review of the
response data indicate that two distinct behavior shapes were identified. It was observed that TI-
4 and TI-7 displayed similar response behavior over loading duration, and TI-4 had lower
measured pressures than TI-7, which was consistent with permanent deformation observations.
Additionally, pressures for these two items were found to display a relatively constant increase
with an increase in surface deformation.
TI-2, TI-3, TI-5 and TI-6 displayed similar behaviors that were found to be different from
the two best performers. It was observed that a considerable increase in the bottom of the
subgrade pressure occurred early in loading (around 20−60 cycles), which was found to be at
approximately 4.75 mm of permanent surface deformation. Pressure was observed to remain
constant or slightly increase after this point.
4.4.5.2 Phase II EPC Response
Subbase pressures were found to be relatively consistent during loading. TI-9
experienced a gradual pressure increase throughout loading with a more drastic increase
approaching 50 mm of permanent surface deformation. TI-10 had higher subbase pressures than
the other items, which could be a function of reduced base thickness. For the test items having
approximately equal base thicknesses, it was observed that subbase pressures were lower in
items containing geosynthetics, suggesting that the geosynthetics influenced pressure
distribution.
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Pressure response data at the top of the subgrade indicated that the geosynthetic-
reinforced items had higher measured pressures than the unreinforced item with the exception of
TI-11 (which was the best performer). The general shape of the top of the subgrade response
curves were found to be consistent for all reinforced items, marked by an initial pressure
increase, followed by a duration of constant or decreased pressure, and ending with a pressure
increase near the failure point.
Pressure at the bottom of the subgrade in TI-8 was found to increase rapidly during
loading and progress in a linear nature until test termination. TI-9 and TI-10 displayed similar
response with a constant pressure up to approximately 100 cycles, followed by a general increase
with increased loading. TI-11 displayed a different behavior from the other reinforced items,
showing constant pressure up to approximately 20 load cycles, followed by a rapid increase
approaching pressure values observed in TI-8, and then a more consistent pressure response over
load duration.
4.5 Discussion of Results
4.5.1 Geosynthetic Performance in each Phase
GEO1 and GEO3 were evaluated in both phases; therefore, direct comparison of
permanent deformation performance can provide meaningful insight to placement location.
Permanent deformation data were plotted as shown in Figure 4.5, and phase I deformation values
were plotted on the x-axis while phase II deformation values were plotted on the y-axis. Phases I
and II unreinforced items (TI-2 and TI-8) were included to observe how differences in as-built
properties influenced permanent deformation performance. Recall TI-8 had a lower base CBR,
thinner asphalt and lower asphalt density than TI-2, which was found to lead to substantial
permanent deformation increases that began almost immediately. When GEO1 is considered,
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equivalent performance was observed up to approximately 18 mm of permanent deformation
although TI-9 had lower asphalt density than TI-4. Similar observations can be made for GEO3,
noting that TI-10 had lower asphalt density and thinner base than TI-5. Thereafter, it can be
observed that deformation trended upwards for both geosynthetics, suggesting that placing the
geosynthetic closer to the surface may be more beneficial.
GEO1 was observed to be a slightly better performer than GEO3, particularly at higher
levels of permanent deformation. The data suggest that some level of permanent deformation
may be required to engage the reinforcing benefit of geosynthetics when placed at the
subgrade−subbase interface of relatively thick airfield pavements, which might not be ideal for
some aircraft. This could be especially noteworthy relative to the desire of the FAA potentially to
extend airfield pavement design life from 20 to 40 years, in that the level of deformation required
to realize a benefit (18 mm) may exceed that which would be allowed on an operational airfield.
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Figure 4.5 Surface deformation comparison for GEO1 and GEO3
4.5.2 Evaluation of Instrumentation Response and Placement Location
An attempt was made to determine if pavement response (i.e. measured vertical pressure
(MVP)) could provide insight to the effect of placement location using data from the two studies,
particularly since performance in the unreinforced items was different. It can be observed from
the literature that optimum depth of placement recommendations vary (recommendations range
from the bottom of the base up to one-third base thickness), therefore relating depth of placement
to pavement response in terms of applied load could be a useful design parameter. MVP is a
function of applied load, depth and individual layer material properties. In order to mitigate the
influence of these variables, the ratio of CP (1750 kPa) to MVP in each respective test item at a
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number of load cycle levels was determined (pressure ratio). To quantify behavior at each
measured pressure ratio, the ratio of surface deformation for a reinforced item to surface
deformation for an equivalent (or as close to equivalent as possible – note TI-8 discussion
earlier) unreinforced item for each study was determined (deformation ratio). As surface
deformation ratio increases towards 1, the deformation of a reinforced section approaches
deformation observed in a companion unreinforced section; hence, lower deformation ratios
indicate more desirable behavior.
Relationships between deformation ratio and pressure ratio at each EPC location were
investigated. It was found that no identifiable trend between pressure and deformation ratios was
observed for the top of the subbase and the bottom of the subgrade pressure cell data. The top of
the subbase pressure ratio data were in the range of values of approximately 4−8 and
deformation ratio data ranged from approximately 0.2 to 1.5. The bottom of the subgrade
pressure ratio data ranged from approximately 20 to 80 and a few values were found to extend to
800. While the lack of trend in the bottom of the subgrade data was expected, due to the EPC
being located approximately 127 cm from the applied load, the lack of trend in subbase pressure
data was not expected. Improvement was observed in items where the geosynthetic was located
at the base−subbase interface. A slight trend was observed in the top of the subgrade pressure
data; in particular, items with a geosynthetic placed at the base−subbase interface displayed
improvement when compared with those with a geosynthetic placed at the subbase−subgrade
interface.
The pressure ratio data at the location of the geosynthetic were plotted and are shown in
Figure 4.6. It is observed that as pressure ratio increases, deformation ratio increases, which
supports limiting geosynthetic placement depth in relatively thick pavement structures. A
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deformation ratio of 0.75 or 25% improvement was arbitrarily selected as an upper threshold for
discussion purposes herein, as the concept presented in Figure 4.6 is relatively new and needs
additional investigation before firm statements are warranted. The data suggest that geosynthetic
placement at a pavement depth where the pressure ratio ranges from approximately 4 to 8
provide the best level of behavior improvement. It is noted that GEO1 data showed some level of
improvement at high pressure ratios (≈25−30), which could be a function of geosynthetic
physical properties. Additional data are needed to determine the effect of geosynthetic inclusion
at pressure ratios greater than 8 (i.e. at some depth within a base or subbase layer). Further, the
data presented are based on one subgrade CBR value, a single layer of geosynthetic, and some
test sections that were not compacted as much as would be ideal, thus data from additional
subgrade CBR values and multi-layer geosynthetic systems are needed to further characterize the
relationship.
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Figure 4.6 Relationship between pressure and deformation ratios
4.6 Conclusions
Cyclic plate load tests were performed to assess the behavior of relatively thick
unstabilized and geosynthetic stabilized flexible pavement sections under aircraft tire CPs. It can
be concluded that the inclusion of some geosynthetics in airfield pavements, tested in medium-
scale cyclic plate loading, display a deformation resistance benefit evidenced by increased cycles
to failure and TBR greater than one when compared with an unreinforced control pavement.
Overall, there was modest evidence to suggest that placing geosynthetics closer to the surface
than the subgrade−subbase interface may be desirable for thick airfield pavements. Changing
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subbase material type and consequently CBR from 15–18 to 55 resulted in performance
improvement well beyond that observed from geosynthetic inclusion. No geosynthetic-reinforced
section evaluated improved rutting resistance within 1.5 orders of magnitude of the CBR 55
subbase.
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CHAPTER V
ANALYSIS OF FULL-SCALE GEOSYNTHETIC REINFORCED AIRFIELD PAVEMENT
SUBJECTED TO ACCELERATED AIRCRAFT LOADING
This chapter has been accepted for publication in the ASCE Journal of Transportation
Engineering, Part B: Pavements. The original paper may be accessed at
http://dx.doi.org/10.1061/JPEODX.0000212. In accordance with ASCE Publishing Guidelines,
the paper (Robinson, Howard, Tingle, and Norwood 2020) has been reformatted and reproduced
herein with minor modifications to suit the objectives of this dissertation.
A full-scale airfield pavement section was constructed and trafficked by the U.S. Army
Engineer Research and Development Center (ERDC) to evaluate the performance of geogrid
reinforced flexible airfield pavement structures subjected to simulated aircraft traffic. The test
section included nine different test items (one unreinforced test item and eight comparable
reinforced test items) containing seven different commercial geosynthetic products. The primary
objective of this experiment was to determine if the collective behavior of a variety of
geosynthetic products demonstrated an overall performance improvement in thicker airfield
pavements carrying heavy aircraft loads. For the Department of Defense (DOD) to make optimal
use of geosynthetics, then the ideal would be for there to be multiple products in a competitive
market that improve overall airfield performance. The rutting behavior, pressure response, and
falling weight deflectometer response of each pavement section were monitored at selected
traffic intervals, and the data were analyzed to determine the potential benefit of using
94
geosynthetics to reinforce flexible airfield pavements. The results indicated that the inclusion of
less than half of the commercial geosynthetics improved rutting performance suggesting the
overall marketplace of geosynthetics should not be expected to, by default, improve thicker
airfield rutting performance.
5.1 Introduction
Geosynthetics have been successfully incorporated into a number of applications over the
years; e.g. railroad track ballast, unsurfaced and surfaced highway pavements, and soil
reinforcement for walls or other earthen structures. A significant amount of research has been
conducted to investigate the influence of incorporating geosynthetics in pavements; however, the
majority of full-scale research conducted to date has focused on geosynthetic inclusion in
roadway pavement systems with a narrow focus on airfield pavements. There is a concern that
the benefit of geosynthetics within thick airfield pavements may not be as pronounced as that
observed in relatively thin highway pavements. Full-scale data are needed to extend the body of
knowledge regarding geosynthetic inclusion in relatively thick airfield pavements subjected to
higher wheel loads and tire pressures than would be anticipated in highway pavements.
5.2 Objectives and Scope
This research was conducted to evaluate the performance implications of incorporating
geosynthetic reinforced base course into flexible airfield pavements. Nine full-scale flexible
airfield pavement sections (one unreinforced section and eight geogrid-reinforced sections) were
constructed and trafficked at the U.S. Army Engineer Research and Development Center
(ERDC) with a Heavy Vehicle Simulator (HVS) using a C-17 aircraft wheel. Rutting
measurements, subgrade pressures, and falling weight deflectometer (FWD) measurements were
95
analyzed to determine the potential benefit of using geosynthetics to reinforce flexible airfield
pavements. This assessment of geogrid reinforcement potential was performed mostly from a
global or marketplace perspective; i.e. does the collective behavior of several geogrid products
demonstrate an overall improvement to key response properties for thicker airfield pavements
carrying heavy aircraft loads? For the Department of Defense (DoD) to make optimal use of
geogrids, the ideal would be for there to be multiple products existing in a competitive market
that if used on several projects would improve overall airfield performance. A secondary effort
during analysis was to compare the different geogrid products to each other and attempt to
associate their properties to pavement response.
5.3 Literature Review Pertinent to Full-Scale Testing
The literature review presented herein summarizes findings relevant to full-scale testing
only in support of the observations made in this chapter. A more detailed description of full-
scale testing literature can be found in Chapter 2.
Literature review revealed a substantial amount of full-scale testing to evaluate
performance of geosynthetics, mainly in highway pavements. Table 5.1 presents a summary of
full-scale tests performed to measure geosynthetic performance for highways. Contact pressure
(e.g. tire pressure) ranged from 276 to 830 kPa. Asphalt thickness ranged from 0 to 150 mm and
base course thickness ranged from 75 to 600 mm. Subgrade strength in terms of California
Bearing Ratio (CBR) values ranged from very soft (approximately 1) to relatively stiff
(approximately 12).
An overall observation from literature is that geosynthetics can improve rutting
performance and that improvement appears to diminish with increasing base course thickness
and increasing subgrade stiffness. Geosynthetic placement location recommendations
96
(particularly in thick base courses) are mixed. Some research (Al-Qadi, Dessouky, Kwon, and
Tutumluer 2008, Chen, Hanandeh, Abu-Farsakh, and Mohammad 2017) recommended
placement at the base/subgrade interface, while others (Al-Qadi, Dessouky, Kwon, and
Tutumluer 2012, Chan, Barksdale, and Brown 1989) recommended placement at the upper one-
third of the base thickness. Dual-layer placement was also suggested, depending on base layer
thickness.
In terms of airfield pavements, Webster (1993) trafficked 16 pavement sections (11
included geosynthetics), using a 133.5 kN single wheel load at a 486 kPa tire pressure to
simulate relatively light aircraft. A range of base thicknesses (150 to 460 mm) were surfaced
with a 50 mm thick asphalt pavement. These light-duty airfield pavement structures and light
aircraft loading conditions generally fall within those evaluated for highway applications (Table
5.1).
Recently, Robinson, Mahaffay, Howard, and Norwood (2019) employed laboratory scale
cyclic plate load tests to evaluate geosynthetic inclusion in representative airfield pavement
structures whose loading conditions were more severe than those in Webster (1993). Eleven
airfield pavement structures were subjected to cyclic loading under simulated 1750 kPa aircraft
contact pressures. The pavement structures were composed of 127 mm thick asphalt, 178 mm
thick base, 305 mm thick P-154 subbase, and a 3 CBR subgrade. Some (not all) geosynthetics
reduced surface deformation.
97
Table 5.1 Full-scale highway load tests and findings from literature
Reference Asphalta
(mm)
aBase
(mm)
Subgrade
CBR Load (kN)
Pressure
(kPa)
Geosynthetic
Location Key Findings
Al-Qadi and Appea
2003 95
100-
200 6-10 ** ** B/S
Results indicated that geosynthetic stabilized pavements with a 100 mm base
course could have almost double the life of an unreinforced pavement. A
geotextile section had less rutting than a geogrid section.
Al-Qadi, Dessouky,
Kwon, and Tutumluer
2008
76 &
127
200-
460 4 44 689
B/S, B/3, DL
(B/S & B/3)
Optimal placement location for thin aggregate layer is at base/subgrade interface.
In thicker base layers, optimal location is at the upper third of the layer, and
another at the base/subgrade interface may be needed.
Al-Qadi, Dessouky,
Kwon, and Tutumluer
2012
76 &
127
200-
460 4 26- 44 550-789
B/S, B/3, DL
(B/S & B/3)
Instrumentation data suggested geogrid placement at 1/3 base layer was
equivalent to geogrid placement at both 1/3 and bottom of 457 mm base layer.
Increasing HMA thickness is more effective than geogrid in thin base.
Appea and Al-Qadi
2000 95
100-
200 6-10 ** ** B/S
FWD data showed a 33% reduction in base course modulus in a non-stabilized
road section when compared to a geosynthetic stabilized section, resulting from
fines migration of subgrade material into the base layer.
Aran 2006 110, 160 150-
280 7.7-10 ** ** B/S, B/2
Thinner reinforced sections performed comparable to the control sections.
Pavement sections and subgrades were too strong for geogrid reinforcement of the
aggregate bases to have a significant effect on performance.
Chan, Barksdale, and
Brown 1989 32-38
150-
210 2.6 6.6-9 460-500 B/S,B/2
Effective placement location appeared to depend on both quality and thickness of
granular material. Recommended placing the geosynthetic as high up in the
granular layer as practical.
Chen, Hanandeh, Abu-
Farsakh, and
Mohammad 2017
76 250,
460 ≈1 43-64 724 B/S, B/3
Reduction of vertical stress from geosynthetic on top of subgrade are more
distinguishable at higher loads. Geosynthetics placed at the base/subgrade
interface improve performance.
Collin, Kinney, and Fu
1996 50
150-
460 1.9 20 550
B/S, DL (B/S
& B/2)
Thick aggregate base showed a diminished reinforcement benefit. TBR values
ranged from 2.1 to 10 and it was conservatively estimated that the geogrids tested
would increase pavement life by 2 to 4 times.
Fannin and Sigurdsson
1996 0
250-
500 NR 80 620 B/S
Reinforced sections exhibited improved performance that was greatest on thinner
base course sections and diminished with increasing base course thickness.
Henry, Clapp, Davids,
and Barna 2009
100
150
& 300
600
& 4 48.9- 93.4 689 B/S
For 600 mm base thickness, benefit was observed in the 100 mm asphalt section
but not in the 150 mm asphalt section. In the 300 mm base sections, somewhat
less base compression was observed.
Hufenus, Rueegger,
Banjac, Mayor,
Springman, and
Bronnimann 2006
0 600 ≈3-12 280 850 B/S
Significant improvement in bearing capacity of a geosynthetic reinforced layer
was found to be true for thin layers over weak subgrades. Influence on stiffer
subgrade was marginal. Thickness reduction of approx. 30% expected.
Kim, Edil, Benson,
and Tanyu 2005 125 250 NR NR NR S/S
Quantified improvement of geogrid and geotextiles in gravel working platforms.
Results indicated that geogrid and geotextiles increased the gravel layer structural
coefficient 50% to 70%.
aDesign thickness; NR = Not reported; TBR = traffic benefit ratio; B/S = base/subgrade interface; B/3 = 1/3 from top of base; B/2 = mid-depth of
base; DL = dual layer; S/S = subbase/subgrade interface
98
Table 5.1 (continued) Full-scale highway load tests and findings from literature
Reference Asphalta
(mm)
Basea
(mm)
Subgrade
CBR Load (kN)
Pressure
(kPa)
Geosynthetic
Location Key Findings
Kinney, Abbott, and
Schuler 1998 60
150-
530 1 18.2 & 91
276 &
551
B/S
TBR decreases with increasing base layer thickness (10 at depths of less than
254 mm to 1 at a depth of about 356 mm). Concluded that the effect of
reinforcement with base thicknesses greater than 406 mm is minimal.
Norwood and Tingle
2014b 75 & 100
150 &
200 6 89 830 B/S
Comparable performance observed in thinner geogrid section when compared
to unreinforced section. Measured deflections were higher in geogrid section
but did not seem to influence subgrade shearing.
Norwood and Tingle
2014a 20
150 &
200 6 45 830 B/S
Geogrid stabilized item had less stiffness degradation than the unstabilized
pavement. Initial stiffness was not a good indicator of rutting performance.
Robinson, Tingle,
Norwood, Wayne,
and Kwon 2018
75 150 6 89 830 B/S
Effective base structural coefficient increased from 0.14 to 0.29 with multi-
axial geogrid. Inclusion of geogrid equivalent to 25 mm of HMA and 50 mm
of crushed limestone base course.
Safhebgar, Hossain,
and Lacina 2016 100-150
200-
300 5 80 620 B/S
Reinforced base sections with similar cross-sections outperformed the control
in terms of rutting. The reinforced base layer reduced pressure on the
subgrade.
Tingle and Jersey
2009 0 150 4 176 354 B/S
Geosynthetics improved performance in both high-quality and marginal base.
Stiffness became a better indicator of performance after densification and
mobilizing the reinforcement.
Howard and Warren
2009 51
152 &
254 ≈1-2 89 620 B/S
Developed a finite element model of thin flexible pavements to stress/strain
response in thin flexible pavements incorporating geosynthetics.
aDesign thickness; NR = Not reported; TBR = traffic benefit ratio; B/S = base/subgrade interface; B/3 = 1/3 from top of base; B/2 =
mid-depth of base; DL = dual layer; S/S = subbase/subgrade interface
99
5.4 Full-Scale Test Sections
Nine test sections, TI-1 to TI-9, each 3.6 m wide by 15.2 m long, were constructed under
an open-ended but covered hangar to limit sunlight and inclement weather exposure. TI-1 was
an unreinforced section. TI-2 through TI-6 and TI-8 through TI-9 incorporated geosynthetics at
the base/subgrade interface. TI-7 incorporated a geosynthetic at mid-depth within the base layer.
Construction initiated with the excavation of an 11 m wide by 58 m long by 1.1 m deep
trench. In-situ soils at the bottom of the excavation were classified as ML-CL, with an in-place
CBR ranging from 7 to 10 at in-situ moisture content of roughly 17%. Prior to subgrade soil
placement, the excavation was lined with a plastic membrane to minimize moisture variation
during testing. Subgrade soil was a high-plasticity clay (CH) pulverized and processed outside
the test section before being placed in approximately 220 mm thick loose lifts and compacted
with a pneumatic tire and smooth drum roller to target an in-place CBR of 8. Crushed limestone
was used to construct the 360 mm thick base course layer (placed in two lifts), and a 9.5 mm
nominal maximum aggregate size (NMAS) dense-graded asphalt mixture was used to construct
the 100 mm thick surface layer that was placed in two lifts. Geogrids were not anchored and
were placed on top of a fully compacted lift of either subgrade or crushed limestone that was
relatively smooth, and thereafter the next layer of materials was placed and compacted.
Accelerated trafficking was completed via a HVS outfitted with a single C-17 aircraft
wheel (Figure 5.1) loaded to a gross load of approximately 200 kN and a tire pressure of 979
kPa. An actual C-17 aircraft consists of a dual wheel nose gear and two sets of body gears with
six wheels per side. Gonzalez (2015) observed performance differences when accelerated traffic
was applied with a HVS outfitted with a single C-17 aircraft wheel and a dual wheel C-17
aircraft gear. It was observed that passes to failure for a dual wheel C-17 gear was
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approximately three times that of a single C-17 wheel, and that performance improvement could
be attributed to self-healing of the asphalt surface from a kneading action provided by the dual
tires. Traffic was applied bi-directionally in an approximate normally distributed loading pattern
with a total wander width of 1.2 m until 25 mm of surface rutting occurred. Bi-directional traffic
was implemented to simulate an airfield operation scenario (where takeoffs/landings/taxing
could occur from both directions) and to increase accelerated traffic efficiency. Temperature
inside the HVS was maintained at 25ºC ± 3ºC during trafficking.
Figure 5.1 Photographs of load test equipment
(a) Heavy vehicle simulator
(b) C-17 wheel configuration
Earth pressure cells were placed approximately 50 mm below the top of the subgrade of
each test section to collect vertical pressure data at selected traffic intervals. Rutting was
measured from the top of the pavement surface using a 3 m straightedge and included any
upheaval along the wheel path’s edge. Upheaval was defined as upward displacement of asphalt
due to shear flow, therefore rutting measurements were considered the difference in peak
101
elevation (outside the wheel path) to minimum elevation (inside the wheel path) at the time of
measurement.
5.4.2 Material Properties
Geosynthetics evaluated in this study (Table 5.2) were selected based upon a review of
available base reinforcement products encompassing a variety of manufactured types, aperture
sizes, and strengths. It is noted that some mechanical properties were not publicly available, thus
they were omitted from Table 5.2. GEO1 and GEO3 were also evaluated in Robinson,
Mahaffay, Howard, and Norwood (2019), and therefore a similar numbering convention was
used to allow for comparisons between the two efforts. GEO6 through GEO10 were not
previously evaluated in airfield pavements by the authors.
Asphalt layers were produced with a 9.5-mm NMAS mixture with an unmodified PG 67-
22 binder (per Mississippi Department of Transportation specifications) at 5.3% total asphalt
content. The mixture consisted of 25% gravel, 60% limestone, and 15% sand that was
compacted to 75 design gyrations.
The base material had a maximum aggregate size of 37.5 mm, a D85 (85% passing) of
20.3 mm, a D50 (50% passing) of 8.4 mm, and 7% passing a 0.075 mm sieve (P200). ASTM
D2487 (ASTM 2017a) and AASHTO M145 (AASHTO 2012) classified this material as GP-GM
and A-1-a, respectively, and the overall gradation curve had 32% sand sized particles, a
coefficient of curvature (Cc) of 3.1, and a coefficient of uniformity (Cu) of 50.8. Maximum dry
density (MD) and optimum moisture content (OMC) were 2420 kg/m3 and 3.8% when measured
via ASTM D1557 (ASTM 2012).
102
Table 5.2 Geosynthetic properties as provided by manufacturers
Property GEO1 GEO3 GEO6 GEO7 GEO8 GEO9 GEO10
Geosynthetic type Biaxial Triaxial Biaxial Biaxial Biaxial Biaxial Biaxial
Manufacturing Process Extruded Extruded Woven Extruded Laser
Welded Extruded Knitted
Material Type PP PP PET PP PP PP PP
Aperture Size MD (mm) 25 -- 25 30 44 33 36
Aperture Size CMD (mm) 33 -- 25 30 43 46 36
TS @ 2% Strain MD (kN/m) 6 -- 7.3 -- -- -- --
TS @ 2% Strain CMD
(kN/m) 9 -- 10.9 -- -- -- --
TS @ 5% Strain MD (kN/m) 11.8 -- 13.4 -- 20 -- 24
TS @ 5% Strain CMD
(kN/m) 19.6 -- 19.7 -- 28 -- 24
Ultimate TS MD (kN/m) 19.2 -- 29.2 -- 30 -- 30
Ultimate TS CMD (kN/m) 28.8 -- 58.4 -- 32 -- 30
Rib Pitch-Longitudinal (mm) -- 40 -- -- -- -- --
Rib Pitch-Transverse (mm) -- 40 -- -- -- -- --
Radial stiffness kN/m @
0.5% Strain -- 225 -- -- -- -- --
MD = machine direction; CMD = cross-machine direction; TS = tensile strength; PET = polyester; PP =
polypropylene
The design subgrade had a liquid limit (LL) of 79, a plastic limit (PL) of 23, and 99%
P200. ASTM D2487 (ASTM 2017a) and AASHTO M145 (AASHTO 2012) classified this
material as CH and A-7-6, respectively. Maximum density and OMC were 1660 kg/m3 and 19%
when measured via D1557 (ASTM 2012).
The relationship between aggregate and geosynthetic aperture size has been found to
affect the ability of aggregate to interlock with a geosynthetic (Brown, Kwon, and Thom 2007;
Indraratna, Hussaini, and Vinod 2011; Tutumluer, Huang, and Bian 2010). It has been
recommended (FHWA 2008) that the minimum aperture size be at least equal to the particle size
103
represented by D50 and that the maximum aperture size should be less than two times D85. Thus,
the minimum recommended aperture size for this base course aggregate would correspond to 8.4
mm and the maximum aperture size would correspond to 40.6 mm. A review of the Table 5.2
geosynthetic properties indicates that all met the recommended criteria, with the exception of
GEO8 and GEO9 that exceeded the recommended maximum aperture size.
5.4.3 As-built Properties
Quality control tests were performed during construction and produced the values shown
in Table 5.3. Dry density and moisture content were measured using a nuclear device in
accordance with ASTM D6938 (ASTM 2017d), and in-place CBR tests were performed in
general accordance with ASTM D4429 (ASTM 2009). Asphalt cores were obtained from the as-
built sections and core densities were determined in accordance with AASHTO T166 (AASHTO
2016).
104
Table 5.3 As-built properties (full-scale tests)
Item TI-1 TI-2 T1-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9
Geosynthetic None GEO6 GEO3 GEO1 GEO7 GEO8 GEO1-MID GEO9 GEO10
CH Subgrade Properties
% of D1557 MD 89.0 88.5 88.3 90.3 89.6 93.4 89.3 88.4 91.1
Dry Density (kg/m3) 1477 1469 1466 1499 1488 1551 1482 1467 1512
Nuclear Moisture (%) 29.9 27.0 28.6 28.0 29.6 25.0 29.3 29.1 27.9
OD Moisture (%) 28.9 29.2 29.1 32.7 27.8 29.0 28.2 28.1 27.4
In-Place CBR (%) 7.6 7.1 7.2 7.9 8.2 7.4 7.5 7.6 7.4
Crushed Limestone Base Properties
% of D1557 MD 92.6 91.3 91.7 93.7 91.4 90.5 90.0 90.7 93.1
Dry Density (kg/m3) 2241 2210 2220 2268 2211 2191 2179 2195 2252
Nuclear Moisture (%) 2.2 2.1 2.2 2.2 2.4 2.4 2.1 2.2 2.4
OD Moisture (%) 1.9 1.9 2.0 2.0 2.3 2.2 2.5 2.3 2.2
In-Place CBR (%) 100+ 100+ 100+ 100+ 100+ 100+ 100+ 100+ 100+
Thickness (mm) 363 363 371 363 348 368 351 363 371
Hot-Mix Asphalt Properties
% of Gmm 94.2 94.1 93.3 94.6 94.9 94.3 94.7 94.5 93.6
Thickness (mm) 107 104 99 107 104 104 104 102 104
MD = maximum dry density; OD = oven dried; CBR = California Bearing Ratio; Gmm = theoretical maximum specific gravity
105
5.5 Results
5.5.1 Assessment of As Built Properties
Tables 5.4 through 5.8 provide all data assessed qualitatively and statistically throughout
the remainder of this manuscript. It should be noted that differing (but relatively close) pass
interval measurement locations were used for TI-1 when compared to the remaining eight test
items. For example, data were collected at pass 1,520 for TI-1, while data were collected at pass
1,640 for all other test items. In order to populate a complete paired dataset (based on pass
level), response data at the unmeasured pass level were linearly interpolated based on known
data points at adjacent pass levels. Interpolated data values are underlined in Tables 5.4 through
5.8.
Performance results, based on aircraft wheel passes (Table 5.9), should be interpreted in
terms of as-built properties presented in Table 5.3, particularly when comparisons are made to
TI-1 (Control). Overall, average subgrade CBR for the test series was 7.5 and values ranged
from 8.2 to 7.1. Subgrade densities (based on maximum dry density) ranged from 93.4 to 88.3%,
and the overall average subgrade density was 89.8%. Average base course density was 91.7%,
with observed values ranging from 93.7 to 90.0%, and base thicknesses ranged from 371 to 348
mm, with an average thickness of 362 mm. Asphalt thicknesses ranged from 107 to 99 mm, and
the average asphalt thickness was 104 mm. Asphalt densities were found to be consistent and
ranged from 94.9 to 93.3% of theoretical maximum density.
TI-1 had the thickest asphalt layer (107 mm), base thickness near the middle of observed
values, and subgrade CBR values that were comparable to most reinforced items. Base density
was near the higher end of the observed range and subgrade density was near the middle of the
observed range. These observations suggest that the strength of the unreinforced section should
106
have been equivalent (or at best slightly stronger based on asphalt thickness) than the reinforced
sections. Therefore, it could be assumed that performance improvement observed when
compared to TI-1 could be attributed to geosynthetic inclusion.
TI-3 and TI-7 sustained less passes than TI-1. TI-3 had a lower subgrade CBR, lower
base density, but a thicker base than TI-1. TI-7, which included GEO1 at mid-depth of the base,
had comparable subgrade properties to TI-1. Base density, base thickness, and asphalt thickness
were lower than the unreinforced section.
TI-5 had the highest subgrade CBR in the test series, a base density lower than TI-1 (but
comparable to most other sections), and a base thickness lower than all other test sections. In
terms of passes to failure, TI-5 generally displayed equivalent performance to TI-1.
Comparison of TI-4 and TI-7 (both contain GEO1) show that TI-4 had a slightly higher
subgrade density (1%) and CBR (5%), almost 4% higher base density and approximately 10 mm
thicker base (3%). TI-4 asphalt thickness was 3 mm (3%) higher than TI-7.
107
Table 5.4 Measured and interpolated rutting data (mm)
Pass Level
Item 1 330 1520 1640 5927 6540 7898 15770 16380 23642 26220 31514 33800 44000 60000 100066 140000 146000 180000
TI-1 0 2.6 4.2 4.4 8.8 9.5 11.1 14.3 14.4 16.1 17.2 19.6 17.8 20.8 24.3 30.5 31.8 32.0 33.3
TI-2 0 4.5 4.7 4.8 7.7 8.2 9.2 15.4 15.9 19.1 20.2 18.7 17.5 19.0 22.9 25.4 32.3 33.3 32.4
TI-3 0 0.5 3.5 3.9 7.0 7.5 8.3 13.0 13.4 16.2 17.2 18.7 20.0 21.9 26.8 33.1 39.6 40.6 ND
TI-4 0 1.8 4.5 4.8 7.7 8.2 8.7 11.8 12.0 14.5 15.4 15.7 15.9 17.6 21.8 25.4 28.2 28.6 ND
TI-5 0 1.8 4.3 4.5 9.1 9.8 10.3 13.6 13.8 16.2 17.0 18.7 20.2 21.5 24.7 32.2 35.0 35.4 ND
TI-6 0 0.0 3.1 3.4 6.3 6.7 7.0 9.2 9.4 10.3 10.7 10.9 11.1 12.0 14.2 18.8 21.0 21.3 25.8
TI-7 0 1.9 4.0 4.2 11.0 11.9 12.5 15.7 15.9 18.9 19.6 21.8 23.7 25.3 29.4 31.9 34.8 35.3 37.3
TI-8 0 2.7 4.8 5.0 8.0 8.4 9.2 13.8 14.2 18.4 20.0 21.9 23.6 25.1 29.0 36.5 40.2 40.8 44.9
TI-9 0 1.6 3.9 4.1 6.1 6.4 7.2 12.1 12.5 13.6 14.1 15.9 17.5 17.9 18.8 20.4 23.6 24.0 25.6
ND = no data available; Underlined values were interpolated from adjacent data
Table 5.5 Measured and interpolated pressure cell data (kPa)
Pass Level
Item 1 330 1520 1640 5927 6540 7898 15770 16380 23642 26220 31514 33800 44000 60000 100066 140000 146000 180000
TI-1 183 226 242 243 260 260 261 266 266 273 275 280 281 285 288 293 288 288 284
TI-2 210 232 267 270 294 297 302 326 327 316 312 318 323 316 300 306 313 314 306
TI-3 237 244 285 289 262 258 264 295 297 302 303 294 290 288 290 305 302 302 333
TI-4 174 203 229 231 270 276 278 287 288 283 282 280 279 281 286 293 301 302 305
TI-5 144 166 187 189 209 212 219 257 260 282 283 284 285 288 293 321 316 315 320
TI-6 153 175 189 190 221 225 225 225 225 252 261 265 268 271 279 295 283 281 294
TI-7 194 243 259 261 299 301 305 307 307 313 315 320 325 323 318 333 323 322 313
TI-8 203 239 263 265 279 281 283 298 299 300 300 300 301 302 303 302 299 298 304
TI-9 232 257 267 268 275 276 277 282 282 284 285 286 288 299 327 408 ND ND ND
ND = no data available; Underlined values were interpolated from adjacent data
108
Table 5.6 Measured and interpolated BDI and BCI
Pass Level
Item 1 330 1520 1640 5927 6540 7898 15770 16380 23642 26220 31514 33800 44000 60000 100066 140000 146000
Base Damage Index (BDI) (D2-D3, µm) (LOI: Base)
TI-1 164.8 258.2 268.5 269.0 287.0 280.3 265.4 299.7 299.1 291.8 289.3 284.0 282.2 274.4 262.2 251.4 266.0 268.1
TI-2 141.9 233.2 279.3 283.9 282.3 282.1 281.6 278.6 278.4 310.4 321.8 310.7 301.2 302.4 305.3 340.2 311.9 307.7
TI-3 167.0 241.4 255.8 257.2 278.9 282.0 285.1 302.8 304.2 310.8 313.1 309.4 306.1 304.6 300.7 326.2 377.4 385.1
TI-4 152.8 253.7 264.5 265.5 275.7 277.2 281.1 303.5 305.2 301.7 300.4 296.9 293.9 312.4 358.7 303.4 359.6 368.0
TI-5 174.6 242.7 268.4 271.0 294.8 298.2 297.7 294.6 294.3 309.0 314.2 303.6 294.5 298.3 307.9 296.7 333.8 339.3
TI-6 176.1 231.3 247.7 249.4 246.3 245.8 248.1 261.3 262.4 262.4 262.4 263.0 263.5 282.5 330.4 333.7 357.2 360.8
TI-7 181.5 267.7 283.1 284.6 318.9 323.7 325.0 332.5 333.1 340.0 341.6 347.5 351.8 348.6 340.5 309.9 330.6 334.1
TI-8 ND ND ND 276.3 368.5 381.7 379.8 369.0 368.1 358.1 354.6 343.6 334.2 330.9 322.8 334.3 333.8 333.7
TI-9 158.6 229.6 268.2 272.1 275.7 276.2 277.4 284.1 284.7 272.9 268.7 298.7 324.5 318.6 303.9 303.3 326.9 330.5
Base Curvature Index (BCI, µm) (D3-D4) (LOI:Subgrade)
TI-1 79.9 125.2 130.0 130.4 142.4 141.1 138.1 148.4 148.5 149.7 150.2 151.1 150.2 146.3 140.0 135.7 140.9 141.7
TI-2 80.8 124.0 143.9 145.9 144.2 143.9 143.4 140.1 139.9 157.0 163.0 156.5 150.9 150.0 147.9 167.9 148.9 146.1
TI-3 84.2 116.2 120.6 121.1 134.9 136.9 138.2 145.6 146.2 151.5 153.4 149.3 145.7 146.3 147.7 155.7 165.9 167.4
TI-4 79.7 122.8 133.5 134.5 141.6 142.6 144.4 154.8 155.6 158.1 159.0 154.4 150.4 160.1 184.3 154.2 178.7 182.3
TI-5 95.1 117.3 114.8 114.6 145.1 149.5 149.0 146.6 146.4 154.6 157.5 152.9 148.9 151.5 158.0 148.3 168.3 171.3
TI-6 92.4 125.2 133.4 134.3 126.8 125.7 127.3 136.7 137.4 137.6 137.7 137.6 137.6 147.4 172.1 169.3 177.9 179.2
TI-7 99.4 144.5 158.5 159.9 181.8 184.9 185.7 190.4 190.7 195.0 196.0 199.6 202.3 202.3 202.5 172.4 186.6 189.0
TI-8 ND ND ND 148.0 185.9 191.3 191.0 189.1 188.9 187.2 186.5 180.4 175.2 174.1 171.4 170.8 172.5 172.8
TI-9 88.3 119.1 138.5 140.5 140.6 140.6 141.5 146.8 147.2 142.9 141.3 153.0 163.1 163.3 163.8 161.2 169.4 170.7
ND = no data available; LOI = layer of interest; Underlined values were interpolated from adjacent data
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Table 5.7 Measured and Interpolated MBDI and MBCI
Pass Level
Item 1 330 1520 1640 5927 6540 7898 15770 16380 23642 26220 31514 33800 44000 60000 100066 140000 146000
Modified Base Damage Index (MBDI, µm) (D2-D4) (LOI: Base)
TI-1 244.9 383.4 398.5 399.3 429.5 421.4 403.5 448.1 447.6 441.6 439.4 435.1 432.4 420.7 402.3 387.1 406.9 409.8
TI-2 234.7 357.1 423.2 429.8 426.5 426.0 424.9 418.7 418.3 467.4 484.9 467.2 452.1 452.4 453.2 508.1 460.9 453.8
TI-3 251.2 357.5 376.4 378.3 413.8 418.9 423.3 448.4 450.4 462.3 466.5 458.6 451.9 450.9 448.3 481.9 543.3 552.5
TI-4 232.4 376.5 397.9 400.1 417.3 419.8 425.4 458.2 460.8 459.8 459.5 451.3 444.3 472.5 543.1 457.6 538.2 550.3
TI-5 269.6 360.0 383.2 385.6 439.9 447.7 446.7 441.1 440.7 463.6 471.7 456.5 443.5 449.8 465.8 444.9 502.1 510.6
TI-6 268.5 356.5 381.1 383.6 373.1 371.6 375.5 398.0 399.8 400.0 400.1 400.6 401.0 429.9 502.4 502.9 535.1 539.9
TI-7 280.9 412.2 441.5 444.5 500.7 508.7 510.8 522.9 523.8 535.0 537.7 547.1 554.1 550.9 543.0 482.3 517.1 523.1
TI-8 ND ND ND 424.3 554.4 573.0 570.8 558.1 557.1 545.3 541.1 524.0 509.3 505.0 494.2 505.1 506.3 506.5
TI-9 247.0 348.7 406.8 412.6 416.3 416.8 418.9 430.9 431.8 415.7 410.0 451.8 487.6 481.9 467.7 464.4 496.4 501.2
Modified Base Curvature Index (MBCI, µm) (D4-D6) (LOI:Subgrade)
TI-1 61.4 74.2 77.1 77.3 86.1 86.2 86.3 90.1 90.5 94.3 95.6 98.4 97.9 95.8 92.5 95.3 96.1 96.2
TI-2 66.5 88.3 95.2 95.9 95.2 95.2 94.9 93.7 93.6 99.1 101.0 98.2 95.8 95.2 93.8 104.4 91.3 89.3
TI-3 67.4 77.7 79.8 80.0 91.4 93.0 93.8 98.5 98.8 103.5 105.2 102.2 99.7 100.2 101.6 109.7 109.8 109.8
TI-4 65.5 82.3 86.3 86.7 90.4 90.9 91.9 97.4 97.9 93.5 91.9 94.5 96.8 100.1 108.4 96.3 108.1 109.9
TI-5 62.3 78.3 84.2 84.8 94.4 95.7 95.9 96.6 96.6 99.8 101.0 98.9 97.1 98.5 102.0 97.6 107.9 109.4
TI-6 71.5 89.5 96.3 97.0 90.7 89.8 91.0 98.4 99.0 99.1 99.1 98.5 98.0 103.8 118.3 114.8 114.4 114.3
TI-7 78.2 100.3 111.4 112.5 135.9 139.2 140.0 144.7 145.1 149.4 150.4 154.0 156.7 159.5 166.4 135.3 143.2 144.5
TI-8 ND ND ND 108.7 122.5 124.5 125.9 134.2 134.9 142.6 145.3 130.5 117.8 119.0 121.8 127.2 118.6 117.3
TI-9 73.2 86.2 98.1 99.3 99.2 99.2 99.7 102.3 102.5 102.9 103.0 107.4 111.1 112.1 114.5 112.2 114.5 114.8
ND = no data available; LOI = layer of interest; Underlined values were interpolated from adjacent data
110
Table 5.8 Measured and interpolated AI4 and AAUP
Pass Level
Item 1 330 1520 1640 5927 6540 7898 15770 16380 23642 26220 31514 33800 44000 60000 100066 140000 146000
Fourth Area Index (AI4) (D4+D5/2*D1) (LOI:Subgrade)
TI-1 0.2430 0.1710 0.1810 0.1808 0.1740 0.1780 0.1870 0.1750 0.1747 0.1705 0.1690 0.1660 0.1686 0.1800 0.1980 0.2060 0.1980 0.1968
TI-2 0.2690 0.2310 0.2119 0.2100 0.2085 0.2083 0.2079 0.2052 0.2050 0.2013 0.2000 0.2011 0.2020 0.1992 0.1920 0.1850 0.1893 0.1900
TI-3 0.2440 0.2170 0.2106 0.2100 0.2126 0.2130 0.2116 0.2036 0.2030 0.2060 0.2070 0.2054 0.2040 0.2046 0.2060 0.2030 0.1917 0.1900
TI-4 0.2390 0.1950 0.1995 0.2000 0.2000 0.2000 0.1985 0.1897 0.1890 0.1875 0.1870 0.1908 0.1940 0.1912 0.1840 0.1930 0.1713 0.1680
TI-5 0.2250 0.2020 0.2029 0.2030 0.1934 0.1920 0.1935 0.2023 0.2030 0.2030 0.2030 0.2003 0.1980 0.1971 0.1950 0.1920 0.1807 0.1790
TI-6 0.2510 0.2460 0.2405 0.2400 0.2251 0.2230 0.2249 0.2361 0.2370 0.2311 0.2290 0.2295 0.2300 0.2260 0.2160 0.2090 0.1933 0.1910
TI-7 0.2250 0.2060 0.2042 0.2040 0.2075 0.2080 0.2078 0.2064 0.2063 0.2051 0.2048 0.2038 0.2030 0.2081 0.2210 0.1980 0.1985 0.1985
TI-8 ND ND ND 0.2230 0.1906 0.1860 0.1872 0.1944 0.1950 0.2016 0.2040 0.2051 0.2060 0.2074 0.2110 0.2030 0.1943 0.1930
TI-9 0.2650 0.2330 0.2203 0.2190 0.2146 0.2140 0.2137 0.2121 0.2120 0.2201 0.2230 0.2079 0.1950 0.2016 0.2180 0.2130 0.1913 0.1880
Area Under Pavement Profile (AAUP, µm) (5*D1-2*D2-2*D3-D4/2) (LOI: Entire pavement)
TI-1 798.2 1301.4 1156.7 1156.5 1149.3 1148.3 1146.0 1316.3 1315.8 1309.7 1307.5 1303.0 1288.8 1225.2 1125.6 1094.5 1158.6 1168.2
TI-2 690.5 996.6 1179.8 1198.2 1203.9 1204.7 1206.6 1217.0 1217.9 1329.2 1368.7 1318.1 1296.2 1311.6 1335.8 1483.2 1273.7 1242.2
TI-3 855.6 1088.2 1152.5 1159.0 1247.1 1259.7 1277.8 1382.4 1390.5 1397.2 1399.6 1375.0 1364.3 1364.5 1364.9 1509.1 1619.5 1636.1
TI-4 755.0 1145.6 1124.1 1121.9 1157.0 1162.0 1184.9 1317.5 1327.8 1315.7 1311.5 1270.5 1252.8 1338.7 1473.4 1299.9 1553.1 1591.2
TI-5 853.9 1115.0 1171.1 1176.8 1303.9 1322.1 1309.0 1233.3 1227.4 1246.0 1252.6 1277.7 1288.5 1310.3 1344.5 1313.0 1485.2 1511.1
TI-6 816.3 965.9 1034.8 1041.8 1094.8 1102.4 1100.5 1089.4 1088.5 1114.1 1123.2 1117.4 1115.0 1228.2 1405.8 1466.3 1582.1 1599.5
TI-7 835.1 1091.2 1188.8 1198.7 1263.1 1272.3 1285.8 1364.4 1370.5 1442.9 1468.6 1521.4 1544.2 1465.6 1342.2 1319.9 1379.5 1388.4
TI-8 ND ND ND 1169.5 1570.7 1628.0 1621.6 1584.4 1581.5 1547.2 1535.0 1436.3 1393.7 1402.5 1416.3 1455.0 1491.4 1496.9
TI-9 762.9 1012.4 1183.5 1200.8 1209.5 1210.8 1218.7 1264.9 1268.5 1189.5 1161.4 1367.0 1455.8 1380.8 1263.1 1299.6 1533.2 1568.3
ND = no data available; LOI = layer of interest; Underlined values were interpolated from adjacent data
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Table 5.9 Passes to failure and TBR
Item
Passes at
6.3 mm
Rutting
Passes at
12.5 mm
Rutting
Passes at
25 mm
Rutting
TBR at
6.3 mm
Rutting
TBR at
12.5 mm
Rutting
TBR at
25 mm
Rutting
TI-1 (None) 3,530 10,247 70,318 -- -- --
TI-2 (GEO6) 3,800 12,328 90,370 1.1 1.2 1.3
TI-3 (GEO3) 4,906 15,199 58,364 1.4 1.5 0.8
TI-4 (GEO1) 3,800 18,489 91,198 1.1 1.8 1.3
TI-5 (GEO7) 3,304 13,647 68,680 0.9 1.3 1.0
TI-6 (GEO8) 5,946 21,424 127,492 1.7 2.1 1.8
TI-7 (GEO1-MID) 2,976 8,453 39,110 0.8 0.8 0.6
TI-8 (GEO9) 3,513 14,330 45,353 1.0 1.4 0.6
TI-9 (GEO10) 6,327 17,786 180,000 1.8 1.7 2.6
TBR=traffic benefit ratio
5.5.2 Traffic Benefit Ratio
Traffic benefit ratio (TBR), which is defined by AASHTO R 50-09 (AASHTO 2018) as
the ratio of the number of load cycles of a reinforced pavement structure to reach a defined
failure state to the number of load cycles of an identical unreinforced pavement structure, were
calculated at 6.3, 12.5, and 25 mm of rutting (Table 5.9). Overall, an improvement in TBR (i.e.
TBR > 1.0) was observed in 71% of the cases. When a 25% improvement is considered (i.e.
TBR > 1.25), 54% of the cases exceeded this value. Only eight cases (33%) were found to
exhibit a 50% improvement.
Average TBR at 6.3 mm rutting was 1.2 and 62% of the test items had TBR > 1.0.
Average TBR at 12.5 mm rutting was 1.5 and 88% of the test items had TBR > 1.0. Average
TBR at 25 mm rutting was 1.3 and 50% of the test items had TBR > 1.0. The improvement in
average TBR from 6.3 to 12.5 mm rutting suggests that some level of deformation is required to
fully mobilize the benefit of a majority of the geosynthetics. Further, it is observed that
reductions in TBR and reduction in the percentage of items displaying a benefit as rutting
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increased to 25 mm suggests that, globally, the geosynthetics were not as successful in
improving performance as pavement damage increased.
TI-6 (GEO8) and TI-9 (GEO10) were found to be good performers at all rutting levels,
and a review of the Table 5.2 geosynthetic properties shows that GEO8 and GEO10 had tensile
strengths near the higher end of reported values. TI-7 (which had GEO1 installed at mid-depth)
was found to be the worst performer at all rutting levels. This result does not agree with some
literature (Al-Qadi, Dessouky, Kwon, and Tutumluer 2008; Chan, Barksdale, and Brown 1989;
Chen, Hanandeh, Abu-Farsakh, and Mohammad 2017) that suggests geosynthetic placement
higher in a base layer should improve performance. Construction data indicated that TI-7 was
weaker than TI-1, suggesting that geosynthetic placement at mid-depth of the base could not
overcome reduced structural properties of adjacent layers.
GEO1 (when installed at the base/subgrade interface) and GEO3 were observed to be
good performers at 6.3 and 12.5 mm rutting. At 25 mm rutting, GEO1 remained a positive
performer and GEO3 was observed to perform worse than the unreinforced item. At higher
rutting levels, GEO1 was found to outperform GEO3 which agrees with previous research
(Robinson, Mahaffay, Howard, and Norwood 2019). Similarities in performance trends
observed between the two inherently different studies (both pavement structure and loading
mechanisms were different) suggest that geosynthetic properties can influence pavement
performance. Due to the difference in aperture shape, no repeated laboratory data (i.e. tensile
strength, see Table 5.2) are published by the manufacturer, making determination of intrinsic
properties responsible for pavement performance difficult to define. Some studies (Dong, Han,
and Bai 2010; White, Vennapusa, Gieselman, Douglas, Zhang, and Wayne 2011) found that
triangular aperture geogrids improved performance when compared to biaxial aperture geogrids.
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Visual observation by the authors indicates that GEO1 appears to be a stiffer geosynthetic than
GEO3, suggesting that geosynthetic stiffness may affect performance in airfield pavements.
Similar observations have been made by others (Qian, Han, Pokharel, and Parson 2013; Sun,
Han, Wayne, Parsons, and Kwon 2014) under highway loading conditions.
5.5.3 Interpretation of Average Rut Depth Measurements
More rapid rutting under initial aircraft traffic was observed for all sections up to
approximately 5,000 passes, likely attributed to aggregate shakedown and initial densification,
with gradually decreasing rutting rates being observed thereafter. Improved rutting performance
was observed in TI-2, TI-4, and TI-6 after approximately 10,000 aircraft passes, and TI-3, TI-7,
and TI-8 displayed somewhat worse rutting performance. Recall TI-3, TI-7, and TI-8 had
slightly lower as-built thickness and/or density when compared to TI-1, suggesting that GEO3,
GEO1 (installed at mid-depth), and GEO9 could not overcome the reduced as-built layer
characteristics. TI-5 was found to have approximately equivalent performance to TI-1
throughout traffic duration. TI-4, TI-6, and TI-9 (the best performers) showed improved rutting
performance when compared to TI-1 throughout the remainder of trafficking, suggesting that
GEO1, GEO8, and GEO10 were effective in reinforcing the aggregate layer.
Direct comparison of TI-4 and TI-7 (both contained GEO1) indicate that rutting
performance was equivalent up to approximately 2,000 aircraft passes. After this point, it was
observed that TI-7 experienced a higher rutting rate than TI-4 indicating that GEO1 was more
effective in reinforcing the base layer when placed at the base/subgrade interface. It is
hypothesized that placement procedures may have affected the ability of the base aggregate to
effectively interlock with the geosynthetic at mid-depth. The initial lift of base aggregate was
compacted with a smooth drum vibratory roller resulting in a relatively firm, smooth surface
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after which the geosynthetic was unrolled and the final aggregate lift was placed. The
compacted, smooth surface may have prevented strikethrough of the aggregate particles, thereby
reducing aggregate interlock, and decreasing the effectiveness of GEO1 when installed mid-
depth. Abu-Farsakh and Chen (2011) found that construction procedures had a meaningful
effect on aggregate interlock based on smaller scale cyclic plate load tests and that improved
performance was observed when a geosynthetic was placed between two loose aggregate lifts
and compacted simultaneously.
To directly compare reinforced rutting performance with unreinforced rutting
performance the data were plotted as presented in Figure 5.2. The line of equality is plotted as a
solid line and is paralleled by dashed lines indicating the range of measurement resolution (± 1.6
mm). Values below the line of equality indicate improved performance. Rut depth severity level
zones (low, medium, and high) as defined in ASTM D5340 (ASTM 2018) were plotted to
understand practical performance implications. It was observed that generally equivalent
performance was observed at low (13 mm) or less of rutting when all products were collectively
considered. Within the medium zone (13 to 25 mm rutting), TI-6 and TI-9 showed some
improvement and as the transition was made to the high severity zone (greater than 25 mm
rutting), TI-4, TI-6, and TI-9 showed improvement. These data suggests that some level of
deformation is required to realize a performance benefit with some geosynthetics, which is
consistent with other work (Robinson, Mahaffay, Howard, and Norwood 2019).
Engineering Technical Letter (ETL) 14-3 (AFCEC 2014) contains the Air Forces’
recommended maintenance actions for asphalt pavements based on pavement distress severity.
It is noted that for medium and high severity rutting, the recommended repair action is deep
patching, which refers to replacing the surface, base, and subbase. It could be argued that
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although improvement was observed at larger rut depths, an operational DOD airfield pavement
would not typically be allowed to remain in service above 13 mm surface rutting so that the
pavement may not remain in-service to an extent needed to realize a geosynthetic benefit.
Figure 5.2 Reinforced vs unreinforced rutting
5.5.4 Interpretation of Subgrade Earth Pressure Cell (EPC) Measurements
Comparison of reinforced and unreinforced EPC measurements are presented in Figure
5.3. It was observed that overall subgrade pressures trended upward throughout loading
duration, with a more rapid increased observed early in trafficking. Pavement damage would
increase subgrade pressure, especially if pavement rutting was due to lateral spreading (i.e.
thinner pavement absent noticeably more dense materials whose modulus increase from
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densification). Increased pressure in the subgrade over time could also occur if confinement
reduced stress-softening tendencies of the fine-grained soils (for comparable deflections, higher
moduli lead to higher stress). With the data available, no specific statements can be made other
than subgrade pressure increase with simulated aircraft passes. Other studies (e.g. Rushing and
Howard 2011) have addressed problems of the nature relative to rutting and measured subgrade
pressure over time. TI-4, TI-5, and TI-6 had measured vertical subgrade pressures lower than
TI-1 while the other test sections had vertical subgrade pressures higher than TI-1. It is noted
that TI-4, TI-5, and TI-6 performed better than or equal to the control suggesting that the
geosynthetics were effective in reducing vertical stress on the subgrade layer. However, TI-9
(the best performer at 25 mm rutting) had higher observed vertical subgrade pressures than TI-1,
which was not expected considering the subgrade CBR was nearly identical. Therefore,
relationships between the influence of geosynthetics on measured vertical subgrade pressure for
the loading conditions described herein are inconclusive. In this paper, the authors placed more
value on rutting performance than changes in vertical subgrade pressure.
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Figure 5.3 Reinforced vs unreinforced subgrade pressure
5.5.5 Falling Weight Deflectometer Measurements
FWD measurements were collected at seven locations within each test item to monitor
structural deterioration with increased traffic loading. The configuration used included seven
sensors (D1 to D7) spaced at 300 mm intervals and impact loads of 98, 85, 58, and 40 kN. A
total of 4,018 test drops were made across the nine item test series and data from the 3rd drop (98
kN) were selected as the representative drop for this analysis. Data were averaged across seven
locations at each traffic level for each test section. Some common deflection-based parameters
were investigated to determine if trends in geosynthetic reinforced airfield performance could be
correlated to FWD measurements or if reductions in deterioration under traffic could be detected.
Base damage index (BDI) and modified base damage index (MBDI) were used to examine
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potential changes in base course performance. Base curvature index (BCI), modified base
curvature index (MBCI), and fourth area index (AI4) parameters were used to investigate
potential changes in subgrade performance. The area under pavement profile (AAUP) parameter
was used to explore performance of the entire pavement structure. Mathematical expressions for
each parameter are shown in Table 5.6 through Table 5.8.
Deflection parameters were plotted with increasing passes on a logarithmic scale. Best-
fit trend lines were fitted and slope, intercept, and R2 values for each parameter are summarized
in Table 5.10. It is noted that initial (e.g. zero pass) data for TI-8 was not available, and as such
TI-8 was omitted from data analysis.
Slope values (m) for each deflection parameter were ranked in order of smallest to largest
and compared to performance rankings based on Table 8 measured rutting such that smaller
slopes indicated a reduction in deterioration with increased traffic. A review of rutting
performance indicates that TI-9, TI-6, and TI-4 (in order of decreasing performance) displayed
higher passes at 12.5 mm and 25 mm rutting than TI-1. None of the deflection parameters fully
matched the ranking based on rutting performance. BDI, MBDI, BCI, and AAUP ranked TI-1 as
the best performer. MBCI and AI4 were found to generally rank according to as-measured rut
depth as TI-4, TI-6, and TI-9 were found to rank higher than TI-1. MBCI ranked TI-2 as the best
performer followed by TI-6, TI-4, and TI-9, which with the exception of TI-2, was also the 12.5
mm rutting ranking. AI4 ranked TI-2 as the best performer followed by TI-4, TI-9, & TI-6,
which did not match the rutting ranking but did correctly group as better than TI-1. Interestingly,
both MBCI and AI4 ranked TI-3 and TI-7 as the worst performers, and MBCI correctly ranked all
test items that performed worse than TI-1 at 25 mm rutting. This data suggest that MBCI may be
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an effective deflection parameter in detecting effects of geosynthetic reinforcement in relatively
thick flexible airfield pavements.
Table 5.10 Regression parameters from FWD data analysis
Base Damage Index Modified Base Damage Index Fourth Area Index
Item m B R2 m b R2 m b R2
TI-1 7.4 206.2 0.50 12.5 298.1 0.60 -0.0030 0.2083 0.19
TI-2 15.1 148.5 0.92 21.6 232.4 0.91 -0.0070 0.2677 0.97
TI-3 15.2 156.4 0.89 21.8 236.6 0.92 0.0000 0.2171 0.43
TI-4 15.2 154.4 0.88 23.0 232.8 0.89 -0.0050 0.2347 0.84
TI-5 13.0 172.4 0.92 19.4 258.8 0.91 -0.0030 0.2261 0.68
TI-6 13.2 156.9 0.77 19.2 244.8 0.77 -0.0040 0.2634 0.66
TI-7 13.7 186.7 0.92 22.2 286.7 0.91 -0.0020 0.2212 0.39
TI-9 12.6 162.9 0.89 18.9 250.9 0.89 -0.0050 0.2634 0.81
Base Curvature Index Modified Base Curvature Index Area Under Pavement Profile
Item m B R2 m b R2 m b R2
TI-1 5.1 92.5 0.76 3.3 56.8 0.90 25.7 951.4 0.40
TI-2 6.5 83.9 0.85 2.4 71.2 0.72 58.0 700.6 0.89
TI-3 6.6 80.2 0.95 3.8 61.3 0.88 58.9 791.1 0.90
TI-4 7.8 78.4 0.90 3.2 64.2 0.87 57.6 743.4 0.86
TI-5 6.4 86.3 0.85 3.8 59.9 0.94 50.2 822.1 0.82
TI-6 6.0 87.9 0.76 3.1 71.2 0.71 55.3 694.4 0.70
TI-7 8.5 100.0 0.89 6.9 71.9 0.84 50.7 830.1 0.87
TI-9 6.3 88.0 0.87 3.3 72.2 0.78 54.9 742.2 0.88
Equation form: Deflection Parameter = m * ln (Passes) + b
Backcalculated layer moduli were determined using the procedure in the Pavement-
Transportation Computer Assisted Structural Engineering (PCASE) Version 2.09 software
package. Pre-traffic FWD measurements were utilized and the 1st drop from each test series was
considered to be a seating drop, thus was omitted from the calculation. The backcalculated layer
moduli were determined by allowing the software to backcalculate the moduli for all layers,
based on Table 5.3 as-built thicknesses, of the pavement structure. Results are summarized in
Table 5.11. A slight increase in backcalculated base course modulus was observed in some of
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the geosynthetic test items (TI-2, TI-3, TI-4, and TI-9), however TI-6 (one of the better rutting
performers) did not show an improvement in backcalculated base course modulus. In general, it
was observed that geosynthetic inclusion did not show a meaningful influence on backcalculated
subgrade modulus values, and that most geosynthetic test items had subgrade modulus values
slightly less than TI-1. This suggests that the FWD was generally not successful in detecting
changes in initial stiffness due to geosynthetic inclusion of the base or subgrade layers.
Table 5.11 Backcalculated layer modulus values
Backcalculated Layer Modulus (MPa)
Item Asphalt Base Subgrade Percent Error
TI-1 5,822 518 149 5.8
TI-2 8,352 651 149 5.2
TI-3 3,222 644 122 5.9
TI-4 5,240 567 150 4.8
TI-5 5,132 533 133 6.5
TI-6 5,266 514 121 5.4
TI-7 10,637 323 136 3.9
TI-8 9,577 214 102 4.6
TI-9 4,250 654 120 5.8
5.5.6 Statistical Analysis of Pavement Response Data
An analysis was performed to determine if observed differences in key performance
parameters (rutting, subgrade pressure, and FWD deflection parameter) between each test item
were statistically significant. Because data collection points were generally collected at similar
traffic intervals and construction techniques for each test item were similar, it was determined
that a paired-t test was appropriate for the dataset. Note that a paired t-test is based on the
assumptions that 1) the dependent variable (here the difference in observed performance) is
continuous, 2) the observations are independent, 3) the dependent variable is approximately
normally distributed, and 4) the dependent variable should not contain any outliers (See Devore
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2000 for more information). For this analysis a two-tailed rejection region was considered, i.e.
the mean difference between the observed values was zero, and all statistical measures were
evaluated at α=0.05.
Rutting and subgrade pressure data were analyzed in stages and from a rutting
perspective (e.g. low, medium, high) while FWD deflection parameters were analyzed from a
global perspective only. The analysis was intended to determine if geosynthetic inclusion
provided an overall statistically significant difference in performance and to investigate if
changes in geosynthetic inclusion could be detected at different levels of rutting. Results of the
t-test analysis are shown in Table 5.12 through Table 5.15. A two-tailed t-test was conducted
where the average mean difference (AMD) was calculated between observed values and a p-
value was calculated. The value in parenthesis interprets the AMD as being Better (B) or Worse
(W) for cases that were statistically significant. For this analysis Better was interpreted as less
rutting, reduced subgrade pressure, and improved FWD deflection parameter. Interpretations
should be made by selecting the test item of interest from the header row and moving down the
column to the subsequent item of interest. For example, in Table 5.12 in the Global Rutting
subsection, if one wants to determine did TI-4 perform better than TI-3, TI-4 is selected from the
header row and then the column is followed down to the intersection of row TI-3. This
intersection has a p-value of 0.012 and a (B) in parenthesis. These values are interpreted as the
AMD between TI-4 and TI-3 was statistically significant (i.e. global rutting performance was not
equal to zero) and TI-4 performed better (i.e. had less average rutting) than TI-3.
5.5.6.1 Rutting
The global rutting analysis found that TI-4, TI-6, TI-7, TI-8, and TI-9 were statistically
different from TI-1, and a review of the data indicates that mean rutting in TI-4 (GEO1), TI-6
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(GEO8), and TI-9 (GEO10) was less than mean rutting observed in TI-1. TI-4 mean rutting was
approximately 2 mm less than mean rutting in TI-1 that while statistically significant, is near the
accuracy of the measurement technique (1.6 mm), suggesting that TI-4 performance was
practically equivalent to TI-1. TI-6 was observed to have the lowest mean rutting of all test
items, followed by TI-9, and TI-4. A review of geosynthetic properties indicated that GEO8 had
the largest aperture size, followed by GEO10, and GEO1. Similar observations can be made
regarding tensile strength.
At low levels of rutting, it was observed that all test items except TI-5 (GEO7) were
statistically significant when compared to the control TI-1; some were better (B) while others
were worse (W). The data indicates that all statistically significant items sustained higher mean
passes than TI-1, except TI-7, which had approximately 25% less mean passes.
TI-4, TI-6, and TI-9 became statistically different from all other test items at the medium
rutting level, and TI-6 and TI-9 became statistically different from each other. TI-6 had 3 times
the average passes of TI-1, TI-9 had 2.2 times the average passes of TI-1, and TI-4 had 1.4 times
the average passes of TI-1.
Limited data were available to investigate performance at high rutting levels because the
targeted failure criteria for the test series was 25 mm of rutting. However, it can be observed that
of the available data, geosynthetic test items performed the same as or worse than TI-1.
Unfortunately, data were not available for TI-4, TI-6, and TI-9 (the best performers in the global,
low, and medium rutting analyses), but a practical review of the data suggests that these items
should continue to improve performance.
In general, performance was found to follow similar trends in both aperture size and
tensile strength which could suggest that at low rutting levels aggregate restraint (via aperture
123
size) dominates geosynthetic performance and that at higher rutting levels tensioned membrane
effect (higher tensile strength) supplements performance enhancement. It is noted that GEO8
(TI-6) was the best rutting performer in all cases while exceeding the FHWA recommended
maximum aperture size for the base course aggregate used in this study, suggesting that aperture
size/aggregate ratio recommendations require further investigation under aircraft loads.
A key takeaway from the rutting statistical analysis is that, overall, only two of the seven
geosynthetics (29%) evaluated in this study displayed a consistent improvement in rutting
performance. In a competitive bid environment, limiting geosynthetic selection to a small share
of the existing market may not be optimal. Further, aperture size appeared to be the driving
factor in performance, thus specifying a higher tensile strength geosynthetic (for performance
enhancement at higher rutting levels) may not be optimal due to the DOD’s limiting rut depth
criteria.
5.5.6.2 Subgrade Pressure
The statistical analysis indicated that geosynthetic inclusion was not generally successful
in significantly reducing subgrade pressure in an airfield pavement condition, particularly in
those items that displayed improved rutting performance, which does not agree with observations
made in some highway studies (such as Chen, Hanandeh, Abu-Farsakh, and Mohammad 2017;
Al-Qadi, Dessouky, Kwon, and Tutumluer 2008; Saghebfar, Hossain, and Lacina 2016).
However, it is noted that these studies had measured subgrade pressures ranging from
approximately 20 to 100 kPa, which are much lower than the range of approximately 140-400
kPa observed in this study.
TI-6 (GEO8) was the only geosynthetic item (in the global analysis) that displayed lower
average subgrade pressure than all other test items and had the largest aperture size and was
124
among the higher reported tensile strengths. These data agree with rutting performance where
TI-6 was identified as the best rutting performer. If reduced subgrade pressure was considered to
be a key performance metric, then the market for airfield geosynthetics would be severely
limited.
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Table 5.12 Paired t-test results for rutting
Item TI-1 TI-2 TI-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9
Global Rutting (all data)
TI-1 -- 0.852 0.397 0.000 (B) 0.147 0.000 (B) 0.000 (W) 0.005 (W) 0.000 (B)
TI-2 0.852 -- 0.445 0.000 (B) 0.401 0.000 (B) 0.002 (W) 0.012 (W) 0.000 (B)
TI-3 0.397 0.445 -- 0.012 (B) 0.722 0.000 (B) 0.040 (W) 0.000 (W) 0.007 (B)
TI-4 0.000 (W) 0.000 (W) 0.012 (W) -- 0.000 (W) 0.000 (B) 0.000 (W) 0.001 (W) 0.011 (B)
TI-5 0.147 0.401 0.722 0.000 (B) -- 0.000 (B) 0.000 (W) 0.003 (W) 0.001 (B)
TI-6 0.000 (W) 0.000 (Y) 0.000 (W) 0.000 (W) 0.000 (W) -- 0.000 (W) 0.000 (W) 0.000 (W)
TI-7 0.000 (B) 0.002 (Y) 0.040 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.415 0.000 (B)
TI-8 0.005 (B) 0.012 (Y) 0.000 (B) 0.001 (B) 0.003 (B) 0.000 (B) 0.415 -- 0.000 (B)
TI-9 0.000 (W) 0.000 (W) 0.007 (W) 0.011 (W) 0.001 (W) 0.000 (B) 0.000 (W) 0.000 (W) --
Low Rutting (6 to 13 mm)
TI-1 -- 0.004 (B) 0.000 (B) 0.008 (B) 0.146 0.024 (B) 0.003 (W) 0.006 (B) 0.000 (B)
TI-2 0.004 (W) -- 0.000 (B) 0.035 (B) 0.203 0.033 (B) 0.001 (W) 0.214 0.000 (B)
TI-3 0.000 (W) 0.000 (W) -- 0.447 0.000 (W) 0.045 (B) 0.000 (W) 0.000 (W) 0.000 (B)
TI-4 0.008 (W) 0.035 (W) 0.447 -- 0.002 (W) 0.033 (B) 0.005 (W) 0.016 (W) 0.020 (B)
TI-5 0.146 0.203 0.000 (B) 0.002 (B) -- 0.022 (B) 0.015 (W) 0.009 (B) 0.000 (B)
TI-6 0.024 (W) 0.033 (W) 0.045 (W) 0.033 (W) 0.022 (W) -- 0.021 (W) 0.030 (W) 0.070
TI-7 0.003 (B) 0.001 (B) 0.000 (B) 0.005 (B) 0.015 (B) 0.021 (B) -- 0.003 (B) 0.000 (B)
TI-8 0.006 (W) 0.214 0.000 (B) 0.016 (B) 0.009 (W) 0.030 (B) 0.003 (W) -- 0.000 (B)
TI-9 0.000 (W) 0.000 (W) 0.000 (W) 0.020 (W) 0.000 (W) 0.070 0.000 (W) 0.000 (W) --
Medium Rutting (14 to 25 mm)
TI-1 -- 0.366 0.053 0.003 (B) 0.095 0.000 (B) 0.003 (W) 0.010 (W) 0.015 (B)
TI-2 0.366 -- 0.207 0.002 (B) 0.226 0.000 (B) 0.048 (W) 0.070 0.006 (B)
TI-3 0.053 0.207 -- 0.005 (B) 0.152 0.000 (B) 0.000 (W) 0.004 (W) 0.017 (B)
TI-4 0.003 (W) 0.002 (W) 0.005 (W) -- 0.002 (W) 0.000 (B) 0.002 (W) 0.004 (W) 0.035 (B)
TI-5 0.095 0.226 0.152 0.002 (B) -- 0.000 (B) 0.004 (W) 0.012 (W) 0.015 (B)
TI-6 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W) -- 0.000 (W) 0.001 (W) 0.001 (W)
TI-7 0.003 (B) 0.048 (B) 0.000 (B) 0.002 (B) 0.004 (B) 0.000 (B) -- 0.078 0.011 (B)
TI-8 0.010 (B) 0.070 0.004 (B) 0.004 (B) 0.012 (B) 0.001 (B) 0.078 -- 0.014 (B)
TI-9 0.015 (W) 0.006 (W) 0.017 (W) 0.035 (W) 0.015 (W) 0.001 (B) 0.011 (W) 0.014 (W) --
High Rutting (26 to 32 mm)
TI-1 -- 0.117 0.081 ND 0.209 ND 0.007 (W) 0.039 (W) ND
TI-2 0.117 -- 0.000 (W) ND 0.000 (W) ND 0.002 (W) 0.000 (W) ND
TI-3 0.081 0.000 (B) -- ND 0.003 (B) ND 0.295 0.003 (W) ND
TI-4 ND ND ND -- ND ND ND ND ND
TI-5 0.209 0.000 (B) 0.003 (W) ND -- ND 0.089 0.000 (W) ND
TI-6 ND ND ND ND ND -- ND ND ND
TI-7 0.007 (B) 0.002 (B) 0.295 ND 0.089 ND -- 0.346 ND
TI-8 0.039 (B) 0.000 (B) 0.003 (B) ND 0.000 (B) ND 0.346 -- ND
TI-9 ND ND ND ND ND ND ND ND --
Results contained in this table should be interpreted as “Is header test item better than row test item?”; A detailed description is
available in the text; ND = no data available; p-value (α = 0.05).
126
Table 5.13 Paired t-test results for subgrade pressure
Item TI-1 TI-2 TI-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9
Global Pressure (all data)
TI-1 -- 0.000 (W) 0.000 (W) 0.155 0.154 0.000 (B) 0.000 (W) 0.000 (W) 0.002 (W)
TI-2 0.000 (B) -- 0.045 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.550 0.000 (B) 0.424
TI-3 0.000 (B) 0.045 (W) -- 0.007 (B) 0.002 (B) 0.000 (B) 0.043 (W) 0.772 0.478
TI-4 0.155 0.000 (W) 0.007 (W) -- 0.032 (B) 0.000 (B) 0.000 (W) 0.000 (W) 0.012 (W)
TI-5 0.154 0.000 (W) 0.002 (W) 0.032 (W) -- 0.004 (B) 0.000 (W) 0.001 (W) 0.000 (W)
TI-6 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W) 0.004 (W) -- 0.000 (W) 0.000 (W) 0.000 (W)
TI-7 0.000 (B) 0.550 0.043 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.000 (B) 0.550
TI-8 0.000 (B) 0.000 (W) 0.772 0.000 (B) 0.001 (B) 0.000 (B) 0.000 (W) -- 0.550
TI-9 0.002 (B) 0.424 0.478 0.012 (B) 0.000 (B) 0.000 (B) 0.550 0.550 --
Pressure at Low Rutting (6 to 13 mm)
TI-1 -- 0.000 (W) 0.001 (W) 0.002 (W) 0.000 (W) 0.066 0.000 (W) 0.000 (W) 0.000 (W)
TI-2 0.000 (B) -- 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B)
TI-3 0.001 (B) 0.000 (W) -- 0.930 0.000 (B) 0.000 (B) 0.000 (W) 0.011 (W) 0.316
TI-4 0.002 (B) 0.000 (W) 0.930 -- 0.000 (B) 0.001 (B) 0.000 (W) 0.010 (W) 0.353
TI-5 0.000 (B) 0.000 (W) 0.000 (W) 0.000 (W) -- 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W)
TI-6 0.066 0.000 (W) 0.000 (W) 0.001 (W) 0.000 (B) -- 0.000 (W) 0.000 (W) 0.002 (W)
TI-7 0.000 (B) 0.000 (W) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.002 (B) 0.017 (B)
TI-8 0.000 (B) 0.000 (W) 0.011 (B) 0.010 (B) 0.000 (B) 0.000 (B) 0.002 (W) -- 0.087
TI-9 0.000 (B) 0.000 (W) 0.316 0.353 0.000 (B) 0.002 (B) 0.017 (W) 0.087 --
Pressure at Medium Rutting (14 to 25 mm)
TI-1 -- 0.001 (W) 0.039 (W) 0.097 0.086 0.033 (W) 0.000 (W) 0.000 (W) 0.155
TI-2 0.001 (B) -- 0.000 (B) 0.001 (B) 0.003 (B) 0.001 (B) 0.877 0.008 (B) 0.818
TI-3 0.039 (B) 0.000 (W) -- 0.058 0.109 0.075 0.004 (W) 0.036 (W) 0.552
TI-4 0.097 0.001 (W) 0.058 -- 0.587 0.587 0.000 0.000 0.258
TI-5 0.086 0.003 (W) 0.109 0.587 -- 0.267 0.000 (W) 0.003 (W) 0.182
TI-6 0.033 (B) 0.001 (W) 0.075 0.587 0.267 -- 0.000 (W) 0.000 (W) 0.284
TI-7 0.000 (B) 0.877 0.004 (B) 0.000 0.000 (B) 0.000 (B) -- 0.001 (B) 0.849
TI-8 0.000 (B) 0.008 (W) 0.036 (B) 0.000 0.003 (B) 0.000 (B) 0.001 (W) -- 0.567
TI-9 0.155 0.818 0.552 0.258 0.182 0.284 0.849 0.567 --
Pressure at High Rutting (26 to 32 mm)
TI-1 -- 0.001 (W) 0.220 ID 0.049 (W) ID 0.003 (W) ID 0.048 (W)
TI-2 0.001 (B) -- 0.003 (B) ID 0.661 ID 0.010 (W) ID 0.187
TI-3 0.220 0.003 (W) -- ID 0.016 (W) ID 0.001 (W) ID 0.030 (W)
TI-4 ID ID ID -- ID ID ID ID ID
TI-5 0.049 (B) 0.661 0.016 (B) ID -- ID 0.024 (W) ID 0.047 (W)
TI-6 ID ID ID ID ID -- ID ID ID
TI-7 0.003 (B) 0.010 (B) 0.001 (B) ID 0.024 (B) ID -- ID 0.869
TI-8 ID ID ID ID ID ID ID -- ID
TI-9 0.048 (B) 0.187 0.030 (B) ID 0.047 (B) ID 0.869 ID --
Results contained in this table should be interpreted as “Is header test item better than row test item?”; A detailed
description is available in the text; ID=insufficient data for statistical analysis; p-value (α = 0.05).
127
5.5.6.3 FWD Deflection Parameters
Modified base course index (MBCI) was the only deflection parameter that identified all
test items as statistically significant when compared to TI-1. However, mean MBCI values were
higher than TI-1 values in all reinforced test items, indicating that MCBI did not adequately
describe a performance improvement from geosynthetic inclusion.
TI-6 and TI-9 were found to be statistically equivalent to TI-1 for BDI. TI-8 was the only
item found to be statistically significant from all other test items and had the highest mean BDI
value. For fourth area index, all items were statistically different than TI-1, except TI-4 that was
nearly significant (p=0.051). Also, AI4 showed that TI-6 and TI-9 were statistically different
from all other test items and from each other. It was observed that for MBDI, TI-6 and TI-9
were the only test items found be statistically insignificant when compared to TI-1.
A generally observation from the statistical analysis of FWD deflection parameters is that
the FWD did not identify impacts of including geosynthetics in thicker airfield pavements.
Further, some parameters (i.e. BDI, BCI, MBDI, and AAUP) were found to be statistically
equivalent to the unreinforced item for those items (TI-6 and TI-9, specifically) that were found
to be statistically significant in terms of rutting.
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Table 5.14 Paired t-test results for BDI, BCI, MBDI, and MBCI
Item TI-1 TI-2 TI-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9
Base Damage Index
TI-1 -- 0.032 (W) 0.016 (W) 0.019 (W) 0.002 (W) 0.912 0.000 (W) 0.000 (W) 0.119
TI-2 0.032 (B) -- 0.196 0.310 0.318 0.076 0.000 (W) 0.000 (W) 0.397
TI-3 0.016 (B) 0.196 -- 0.666 0.368 0.000 (B) 0.006 (W) 0.007 (W) 0.025 (B)
TI-4 0.019 (B) 0.310 0.666 -- 0.631 0.000 (B) 0.002 (W) 0.005 (W) 0.039 (W)
TI-5 0.002 (B) 0.318 0.368 0.631 -- 0.012 (B) 0.000 (W) 0.000 (W) 0.050
TI-6 0.912 0.076 0.000 (Y) 0.000 (W) 0.012 (W) -- 0.000 (W) 0.001 (W) 0.120
TI-7 0.000 (B) 0.000 (B) 0.006 (Y) 0.002 (B) 0.000 (B) 0.000 (B) -- 0.046 (W) 0.000 (B)
TI-8 0.000 (B) 0.000 (B) 0.007 (Y) 0.005 (B) 0.000 (B) 0.001 (B) 0.046 (B) -- 0.000 (B)
TI-9 0.119 0.397 0.025 (Y) 0.039 (B) 0.050 0.120 0.000 (W) 0.000 (W) --
Base Curvature Index
TI-1 -- 0.016 (W) 0.441 0.005 (W) 0.076 0.587 0.000 (W) 0.000 (W) 0.015 (W)
TI-2 0.016 (B) -- 0.189 0.157 0.939 0.438 0.000 (W) 0.000 (W) 0.477
TI-3 0.441 0.189 -- 0.000 (W) 0.025 (W) 0.867 0.000 (W) 0.000 (W) 0.012 (W)
TI-4 0.005 (B) 0.157 0.000 (B) -- 0.029 (B) 0.005 (B) 0.000 (W) 0.001 (W) 0.157
TI-5 0.076 0.939 0.025 (B) 0.029 (W) -- 0.422 0.000 (W) 0.000 (W) 0.399
TI-6 0.587 0.438 0.867 0.005 (W) 0.422 -- 0.000 (W) 0.000 (W) 0.045 (W)
TI-7 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.006 (B) 0.000 (B)
TI-8 0.000 (B) 0.000 (B) 0.000 (B) 0.001 (B) 0.000 (B) 0.000 (B) 0.006 (W) -- 0.000 (B)
TI-9 0.015 (B) 0.477 0.012 (B) 0.157 0.399 0.045 (B) 0.000 (W) 0.000 (W) --
Modified Base Damage Index
TI-1 -- 0.017 (W) 0.036 (W) 0.013 (W) 0.006 (W) 0.812 0.000 (W) 0.000 (W) 0.065
TI-2 0.017 (B) -- 0.648 0.267 0.632 0.127 0.000 (W) 0.000 (W) 0.703
TI-3 0.036 (B) 0.648 -- 0.245 0.903 0.012 (B) 0.000 (W) 0.001 (W) 0.326
TI-4 0.013 (B) 0.267 0.245 -- 0.245 0.001 (B) 0.000 (W) 0.003 (W) 0.059
TI-5 0.006 (B) 0.632 0.903 0.245 -- 0.040 (B) 0.000 (W) 0.000 (W) 0.338
TI-6 0.812 0.127 0.012 (W) 0.001 (W) 0.040 (W) -- 0.000 (W) 0.000 (W) 0.091
TI-7 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.635 0.000 (B)
TI-8 0.000 (B) 0.000 (B) 0.001 (B) 0.003 (B) 0.000 (B) 0.000 (B) 0.635 -- 0.000 (B)
TI-9 0.065 0.703 0.326 0.059 0.338 0.091 0.000 (W) 0.000 (W) --
Modified Base Curvature Index
TI-1 -- 0.006 (W) 0.000 (W) 0.001 (W) 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W)
TI-2 0.006 (B) -- 0.393 0.957 0.692 0.030 (W) 0.000 (W) 0.000 (W) 0.000 (W)
TI-3 0.000 (B) 0.393 -- 0.213 0.228 0.071 0.000 (W) 0.000 (W) 0.000 (W)
TI-4 0.001 (B) 0.957 0.213 -- 0.484 0.000 (W) 0.000 (W) 0.000 (W) 0.000 (W)
TI-5 0.000 (B) 0.692 0.228 0.484 -- 0.015 (W) 0.000 (W) 0.000 (W) 0.000 (W)
TI-6 0.000 (B) 0.030 (B) 0.071 0.000 (B) 0.015 (B) -- 0.000 (W) 0.000 (W) 0.004 (W)
TI-7 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.000 (B) 0.000 (B)
TI-8 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (W) -- 0.000 (B)
TI-9 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.004 (B) 0.000 (W) 0.000 (W) --
Results contained in this table should be interpreted as “Is header test item better than row test item?”; A detailed description is
available in the text; p-value (α = 0.05).
129
Table 5.15 Paired t-test results for AI4 and AAUP
Item TI-1 TI-2 TI-3 TI-4 TI-5 TI-6 TI-7 TI-8 TI-9
Fourth Area Index
TI-1 -- 0.000 (W) 0.000 (W) 0.051 0.008 (W) 0.000 (W) 0.000 (W) 0.001 (W) 0.000 (W)
TI-2 0.000 (B) -- 0.516 0.000 (B) 0.011 (B) 0.000 (W) 0.991 0.953 0.003 (W)
TI-3 0.000 (B) 0.516 -- 0.000 (B) 0.000 (B) 0.000 (W) 0.416 0.143 0.002 (W)
TI-4 0.051 0.000 (W) 0.000 (W) -- 0.030 0.000 (W) 0.000 (W) 0.008 (W) 0.000 (W)
TI-5 0.008 (B) 0.011 (W) 0.000 (W) 0.030 -- 0.000 (W) 0.000 (W) 0.093 0.000 (W)
TI-6 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) 0.000 (B) -- 0.000 (B) 0.000 (B) 0.001 (B)
TI-7 0.000 (B) 0.991 0.416 0.000 (B) 0.000 (B) 0.000 (W) -- 0.077 0.018 (W)
TI-8 0.001 (B) 0.953 0.143 0.008 (B) 0.093 0.000 (W) 0.077 -- 0.014 (W)
TI-9 0.000 (B) 0.003 (B) 0.002 (B) 0.000 (B) 0.000 (B) 0.001 (W) 0.018 (B) 0.014 (B) --
Area Under Pavement Profile
TI-1 -- 0.329 0.004 (W) 0.095 0.061 0.709 0.000 (W) 0.000 (W) 0.153
TI-2 0.329 -- 0.002 (W) 0.281 0.184 0.188 0.001 (W) 0.000 (W) 0.413
TI-3 0.004 (B) 0.002 (B) -- 0.001 (B) 0.008 (B) 0.000 (B) 0.841 0.023 (W) 0.003 (B)
TI-4 0.095 0.281 0.001 (W) -- 0.919 0.002 (B) 0.078 0.002 (W) 0.729
TI-5 0.061 0.184 0.008 (W) 0.919 -- 0.003 (B) 0.068 0.000 (W) 0.591
TI-6 0.709 0.188 0.000 (W) 0.002 (W) 0.003 (W) -- 0.006 (W) 0.001 (W) 0.018 (W)
TI-7 0.000 (B) 0.001 (B) 0.841 0.078 0.068 0.006 (B) -- 0.013 (W) 0.026 (B)
TI-8 0.000 (B) 0.000 (B) 0.023 (B) 0.002 (B) 0.000 (B) 0.001 (B) 0.013 (B) -- 0.002 (B)
TI-9 0.153 0.413 0.003 (W) 0.729 0.591 0.018 (B) 0.026 (W) 0.002 (W) --
Results contained in this table should be interpreted as “Is header test item better than row test item?”; A detailed description is
available in the text; p-value (α = 0.05).
5.6 Conclusions
Nine full-scale airfield pavement sections were constructed and trafficked with the
primary goal of determining if the behavior of a variety of geosynthetic products provided a
global improvement in key response properties for thicker airfield pavements. The test sections
were constructed in a manner consistent with typical airfield construction practices and
tolerances, thus meaningful conclusions can be made. The rutting analysis indicated that only
three of the seven geosynthetic products provided a consistent rutting improvement, with two of
the three providing a more meaningful performance improvement. Only one geosynthetic was
found to provide a statistically significant vertical subgrade pressure reduction (which has been
suggested to be a key performance improvement factor in some highway studies), and no
130
improvement attributed to the geosynthetic could be identified in FWD response. Therefore, if
combined rutting improvement and vertical subgrade pressure reduction are considered to be the
key performance improvement metrics, a limited geosynthetic market (in the case of this study,
one product) exists for inclusion in thicker airfield pavements. From an agency perspective,
having only one to three available products that provide a noticeable performance improvement
would likely stifle widespread implementation.
A secondary objective was to compare the different geogrid products. The analyses
yielded the following conclusions:
1. GEO8 and GEO10 displayed the most performance benefit at 25 mm of rutting,
allowing for nearly 2 and 3 times the aircraft passes of the control section.
2. Comparison of geosynthetic placement location in TI-4 (base/subgrade interface)
and TI-7 (mid-depth base) indicates that, under the loading conditions of this
experiment, preferred placement location was at the base/subgrade interface over
mid-depth in the base.
3. TI-3, TI-7, and TI-8 had slightly lower as-built thickness and/or density when
compared to TI-1, suggesting that GEO3, GEO1 (installed at mid-depth), and
GEO9 could not overcome the reduced as-built layer characteristics.
4. A statistical analysis indicated that GEO1, GEO8, and GEO10 reduced average
rutting and that increasing aperture size and tensile strength improved
performance.
5. GEO8 was the only product found to provide a statistically significant reduction
in subgrade pressure.
131
CHAPTER VI
IMPLICATIONS OF INCORPORATING GEOSYNTHETICS IN AIRFIELD PAVEMENTS
This chapter has been submitted as a paper for consideration in a peer-reviewed journal.
The draft paper has been reformatted and reproduced herein with minor modifications to meet
the formatting requirements of this dissertation.
A majority of literature indicates that geosynthetic inclusion in flexible pavement bases,
subjected to highway loading, improves performance by reducing rutting or vertical pressure on
weak subgrade layers. Instances where geosynthetics were less successful in highway
pavements included strong subgrade soils and/or thick pavement layers. Thus, understanding the
improvement that can be expected from geosynthetic inclusion in airfield pavements, that are
often more substantial than highway pavements, requires an evaluation of existing airfield
pavement assets and design methodology. To achieve this objective, a number of tasks were
performed: 1) review of in-service pavement thickness and subgrade strength to quantify military
airfield pavement characteristics, 2) review of current Department of Defense (DOD) design
methodologies to determine if geosynthetic inclusion can be adequately characterized in the
design procedure, and 3) cost/benefit evaluation to determine if an expected performance
improvement is financially viable. Results indicated that airfield pavements were generally
thicker and stronger than highway pavements, and that in-service airfield pavements exceeded
the pavement characteristics where geosynthetics have been identified to provide a meaningful
performance improvement. A review of the existing DOD design methodology indicated that
132
any improvement from geosynthetic inclusion in thicker pavements was hidden within the
variability of the data used to formulate the existing design methodology. A cost/benefit analysis
indicated that design life extension should be the primary means of quantifying geosynthetic
improvement and that the reduction in aggregate thickness attributed to geosynthetic inclusion
did not result in a financial benefit for military airfields.
6.1 Introduction
Key metrics for quantifying the improvement gained from including geosynthetics in
flexible pavement bases are rutting and vertical subgrade pressure. The Chapter 2 literature
review that summarized forty-five references subjected to highway loading conditions found that
89% of accelerated pavement test cases showed an improvement in rutting performance. Where
instrumentation were installed to monitor vertical pressure at the top of subgrade, it was found
that measured vertical pressure (when compared to an equivalent unreinforced section) was
reduced anywhere from 8% to 46%. In cases where performance improvement reduction
(although still an improvement) was observed, the primary factors contributing to a reduction in
performance improvement included strong subgrade soils, thick base course layers, and thick
asphalt layers.
Recent evaluations (Robinson, Mahaffey, Howard, and Norwood 2019; Robinson,
Howard, Tingle, and Norwood 2020) that studied thicker airfield pavements did not observe the
level of improvement identified by the literature under highway loading. Robinson, Howard,
Tingle, and Norwood (2020) found that of seven different geosynthetics evaluated in a full-scale
pavement test section, only three (43%) displayed a statistically significant rutting improvement
and only two of those three (29% overall) displayed a meaningful rutting improvement. Further,
it was observed that relatively high levels of rutting (higher than would likely be allowed on an
133
operational airfield based on current airfield thresholds) were required to realize a meaningful
rutting improvement (Robinson, Mahaffey, Howard, and Norwood 2019; Robinson, Howard,
Tingle, and Norwood 2020).
Similar observations regarding subgrade pressure reductions were made. Robinson,
Howard, Tingle, and Norwood (2020) found that only one geosynthetic provided a statistically
significant reduction in vertical subgrade pressure when subjected to simulated aircraft loading.
Additionally, Robinson, Mahaffey, Howard, and Norwood (2019) found that measured subgrade
pressures were generally higher in geosynthetic reinforced test items when compared to an
unreinforced test item for thicker sections representing military airfields.
The limited number of geosynthetics that provided a performance improvement suggest
that widespread implementation in airfield pavements may not be optimal. However, there may
be cases where local airfield pavement designers desire to include geosynthetics, and as such
understanding the potential performance improvement and associated cost becomes important.
This paper investigates the current Department of Defense (DOD) design methodology and
provides an assessment of methods to include geosynthetics in airfield pavements.
6.2 Assessment of Existing Airfield Pavement Thickness
The U.S. Army Corps of Engineers (USACE) Engineer Research and Development
Center (ERDC) performs pavement condition inspections, structural evaluations, and maintains
record of these evaluations for all major U.S. Army airfields. Internal reports were reviewed to
summarize as-constructed pavement thicknesses as a means to quantify the physical properties of
an airfield pavement when compared to a typical highway pavement. These data were used to
understand as-built characteristics of existing airfield pavements and to anticipate the
134
performance expectations of geosynthetic inclusion in airfield pavements to those observed in
the literature from highway pavements.
Data were gathered from 163 different runway pavement sections that comprised twenty-
four different airfields. A series of relative frequency histograms (Figure 6.1) were plotted for
total asphalt thickness (which included original construction and any asphalt overlays) and
aggregate thickness (Figure 6.2) (which was comprised of both base and subbase layers, where
applicable). Average total asphalt thickness was approximately 213 mm, and average aggregate
thickness was approximately 376 mm. It was found that the average original construction
asphalt thickness was 102 mm, which agreed well with the minimum asphalt thickness
recommendations outlined in UFC 3-260-02 Pavement Design for Airfields (USACE 2001).
Thus, a majority of existing pavements have received additional asphalt overlays that can likely
be attributed to anticipated changes in mission traffic that required additional pavement structure.
A review of total pavement thickness above the subgrade (i.e. asphalt plus aggregate thickness)
indicated that twenty-five pavement sections were in the 406 to 457 mm range; forty-eight were
less than 406 mm total thickness while ninety pavement sections exceeded 457 mm total
thickness. These data can be compared to highway pavement studies summarized in Chapter 2
where asphalt thicknesses generally ranged from 76 to 152 mm and base thicknesses generally
ranged from 152 to 305 mm. Thus, when in-service airfield pavements are considered, it can be
concluded that they are typically substantially thicker than that observed in highway pavements.
Increased base course thickness (Chan, Barksdale, and Brown 1989; Collin, Kinney, and Fu
1996; Fannin and Sigurdsson 1996; Kinney, Abbot, and Schuler 1998) and increased asphalt
thickness (Henry, Clapp, Davids, and Barna 2009; Al-Qadi, Dessouky, Kwon, and Tutumluer
135
2012) have been noted as potential justifications for reduced geosynthetic performance benefit in
highway pavements.
A summary of observed subgrade modulus values are presented in Figure 6.3. These
airfield data were retrieved from the results of back-calculated falling weight deflectometer
(FWD) data that were gathered at approximate 30-m intervals in each runway section. The
results from highway pavement studies were based on reported design CBR values that were
converted to estimated modulus values using the relationship: modulus (MPa) = 10.3*CBR
(Heukelom and Klomp 1962), which is generally considered acceptable for CBR values less than
10. It was observed that the average in-service airfield subgrade modulus value was
approximately 188 MPa, and that the minimum subgrade modulus value was 83 MPa. Using the
approximate correlation previously mentioned, these values are near 18 and 8 CBR, respectively.
Other correlations were considered (Powell, Potter, Mayhew, and Nunn 1984; Putri, Kameswara,
and Mannan 2012) that resulted in CBR values ranging from a minimum of 10 to a maximum of
over 100, thus the correlation by (Heukelom and Klomp 1962) yielded conservative values that
were deemed appropriate in the context of this evaluation. If the existing geosynthetic airfield
guidance (USACE 2003) is consulted, it can be observed that subgrade modulus values of in-
service airfield pavements exceeded the maximum CBR where geosynthetics are considered
advantageous. Conversely, the literature indicated that research in highway pavements has been
focused on lower modulus values, and that 75% of the studies were below 69 MPa
(approximately 8 CBR). It has been found that increases in subgrade CBR resulted in
meaningful decreases in observed geosynthetic improvement. Cancelli, Montanelli, Rimoldi,
and Zhao (1996) and Montanelli, Zhao, and Rimoldi (1997) reported that traffic improvements
from geosynthetic inclusion decreased approximately 67% as subgrade CBR increased from 1 to
136
18. Strong subgrade soils were also associated with a lack of geosynthetic improvement in other
highway studies (Howard 2006; Aran 2006).
The findings from the summarized data highlighted the differences of in-service airfield
pavements and highway pavements. The data indicated that airfield pavements typically have
thicker asphalt layers, thicker base layers, and stronger subgrade layers than that observed in
highway pavements. These factors were identified as contributors to a lack of performance
improvement in the literature and supports the observations made by Robinson, Mahaffay,
Howard, and Norwood (2019) and Robinson, Howard, Tingle, and Norwood (2020).
Figure 6.1 Relative frequency of asphalt thickness
137
Figure 6.2 Relative frequency of aggregate thickness
Figure 6.3 Relative frequency of subgrade modulus values
138
6.3 DOD Pavement Design Methodology
Four distinct datasets are discussed in the following paragraphs. Dataset 1 (DS1) refers
to the existing DOD design methodology that is based on the work by Gonzalez (2015) that does
not contain data related to geosynthetic inclusion. Work completed by Robinson, Tingle,
Norwood, and Howard (2018) that is based on an equivalent thickness concept for geosynthetic
inclusion in lightly loaded aircraft and highway pavements is identified as DS2. The results of
cyclic plate load test completed on geosynthetic-reinforced thick airfield pavements subjected to
high tire pressure (Robinson, Mahaffey, Howard and Norwood 2019) are referred to as DS3, and
the results of full-scale geosynthetic reinforced airfield pavements (Robinson, Howard, Tingle,
and Norwood 2020) evaluated with a Heavy Vehicle Simulator (HVS) are identified as DS4.
The current DOD flexible pavement design methodology (Gonzalez, Barker, and
Bianchini 2012; Gonzalez 2015) limits vertical stress on the subgrade as a function of subgrade
CBR. The procedure aims to ensure that there is adequate structural thickness to protect the
subgrade by distributing surface stress through sufficiently strong but progressively lower quality
layers. The procedure makes use of a parameter, beta or β, that is the ratio of vertical stress on
the subgrade divided by the subgrade CBR. Thus, mechanisms that have shown evidence of
altering stress distribution (in this case geosynthetics) could be numerically accounted for in
current DOD pavement design methodology. However, the work presented by Robinson,
Mahaffay, Howard, and Norwood (2019) and Robinson, Howard, Tingle, and Norwood (2020)
did not conclude that geosynthetics were capable of reducing vertical pressure on the subgrade,
which would be the most direct method of accounting for the inclusion of geosynthetics in
pavement design. Robinson, Tingle, Norwood, and Howard (2018) summarized a review of an
existing DOD geosynthetic design methodology that was primarily focused on geosynthetic
139
inclusion in light aircraft and highway pavements. The results updated an equivalent thickness
chart, where the benefit of a geosynthetic was assigned a value in terms of equivalent base
course aggregate thickness that was empirically derived from historical test section results. This
design chart represents a simplistic approach, and to extend this concept to existing DOD design
methodologies it is necessary to interpret the equivalent thickness concept in terms of a stress-
based design criterion (hereinafter referred to as Beta-geosynthetic).
To convert the equivalent thickness design chart to the DOD stress-based design
methodology, a flexible pavement section with 102 mm of asphalt was used, which is the
minimum asphalt thickness specified in UFC 3-260-02 Pavement Design for Airfields (USACE
2001). The equivalent thickness chart was used to assign base course thickness to both a
reinforced and unreinforced pavement section over subgrade CBR values of 3, 6, and 8, which
encompasses the applicable guidance contained in ETL-1110-1-189 Use of Geogrids in
Pavement Construction (USACE 2003). Vertical subgrade pressure values were calculated at
reinforced base course thicknesses ranging from approximately 152 to 559 mm. A single-wheel
C-17 loading condition (200 kN total load and 979 kPa tire pressure) was applied to match the
loading conditions in Robinson, Howard, Tingle, and Norwood (2020).
A stress concentration factor (n) was determined as a function of the assigned CBR value
(Equation 6.1). Vertical stress on top the subgrade (Equation 6.2) was then calculated using the
Frolich stress equation (Gonzalez 2015). Finally, beta (β) values were determined using the
calculated stress and the subgrade CBR (Equation 6.3). A summary of calculated values is
presented in Table 6.1. Unreinforced base thicknesses (UBT) and reinforced base thicknesses
(RBT) were determined from the equivalent thickness chart (Robinson, Tingle, Norwood, and
Howard 2018) that was developed from a review of historical test data collected over a period of
140
approximately 20 years at ERDC. Calculated beta values (BR) were based on the reinforced
base thickness (RBT) and coverages (CU) were determined from the existing DOD design
methodology (see equation form in Figure 6.4) that were based on the unreinforced base
thickness (UBT). Simply, the BR represents a response ratio of a thinner reinforced pavement
section that has the performance (i.e. coverages) of a thicker unreinforced pavement section.
Data were then plotted as β for the reduced thickness section (due to geosynthetic inclusion) on
the y-axis and coverages determined for the corresponding (thicker) control section on the x-axis.
Plotting the data in this manner should be interpreted such that the β-value for the reduced
thickness geosynthetic section yields performance (i.e. design coverages) of a thicker equivalent
unreinforced pavement section. A best-fit trend line was fitted through the plotted data by
maximizing R2 using a generalized reduced gradient (GRG) technique that was first introduced
by Abadie and Carpenter (1969) and Abadie (1970) and later implemented into a Fortran
program by Ladson, Waren, Jain, and Ratner (1978). The proposed trend line and fitted
coefficients for this approach (referred to as Beta-Geosynthetic) and the existing beta criteria
curve are shown in Figure 6.4.
An example of using the Beta-geosynthetic design methodology is provided. If one
assumes a design coverage level, for example, 100 coverages, then the design Beta-value for an
unreinforced pavement section would be 14.1 and the design Beta-value for a geosynthetic
reinforced pavement section would be 16.3. The difference in Beta-values would be interpreted
as a reduction in pavement thickness (i.e. base course) to yield similar performance. Recall the
Beta-value is the calculated pressure divided by the CBR, thus assuming a constant CBR for both
pavement sections, an increase in Beta would be a function of an increase in calculated pressure,
i.e. an increase in the numerator only. Conversely, if the chart is entered from the y-axis with an
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assumed Beta-value (i.e. constant thickness), then the offset in the trend lines would be
considered an extension of traffic. For example, if we assume a Beta-value of 20, then the
design coverages of an unreinforced pavement section would be 10, and the design coverages of
the same thickness reinforced pavement section would be 30.
𝑛 = 2 ∗ (𝐶𝐵𝑅
6)0.1912
(6.1)
In Equation 6.1, n = stress concentration factor and CBR = California bearing ratio.
𝜎𝑡 = 𝜎𝑜
[
1 −1
(√1 + (𝑟𝑡)2)
𝑛
]
(6.2)
In Equation 6.2, σt = vertical stress at depth t, σo = applied stress over the loaded area, r =
radius of the loaded area, and t = depth to location of computed stress.
𝛽 = 𝜎𝑡 ∗ 𝜋
𝐶𝐵𝑅 (6.3)
In Equation 6.3, β = stress-based capacity criteria, σt = vertical stress at depth t, and CBR
= California bearing ratio.
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Table 6.1 Calculated Beta-values based on equivalent thickness methodology (DS2)
Base Thickness 3 CBR 6 CBR 8 CBR
RBT
(in.)
UBT
(in.)
PRB
(psi) BR CU
PRB
(psi) BR CU
PRB
(psi) BR CU
6.1 9.7 64.2 67.2 0 70.6 37.0 3 73.3 29 10
7.1 10.4 57.9 60.6 0 63.9 33.5 4 66.5 26 14
9.0 12.0 47.8 50.1 1 53.1 27.8 9 55.4 22 34
11.1 13.7 39.0 40.8 1 43.6 22.8 19 45.6 18 92
13.2 15.3 32.2 33.7 2 36.1 18.9 44 37.8 15 262
15.2 17.0 27.1 28.4 4 30.5 16.0 116 32 13 897
17.3 18.7 22.9 24.0 7 25.8 13.5 332 27.1 11 3,634
19.5 20.5 19.4 20.3 15 21.9 11.5 1,116 23.1 9 18,049
21.0 21.7 17.4 18.2 24 19.7 10.3 2,670 20.8 8 60,317
22.1 22.5 16.2 17.0 35 18.3 9.6 5,044 19.3 8 139,490
Calculations are presented in Imperial units to match Existing Beta Criteria; RBT = reinforced base thickness; UBT
= unreinforced base thickness, PRB = calculated subgrade pressure based on reinforced base thickness; BR = Beta
factor calculated from PRB and CBR; CU = coverages based on existing Beta criteria and unreinforced base
thickness
Figure 6.4 Proposed geosynthetic modification to existing beta methodology
143
When examining potential changes to design criteria it is important to understand the
historical context of the original derivation. The Beta-methodology is based on data gathered
from studies conducted over an approximately 70-year time period that consisted of various
loading conditions and pavement thicknesses. The methodology is based on the assumption of
subgrade failure, is generally considered valid for subgrade CBR values less than 12, and is
based on data up to approximately 10,000 coverages. Data used to derive the existing Beta
formulation (i.e. DS1) were extracted from Gonzalez (2015) and reproduced as shown in Figure
6.5. In addition, performance data from Robinson, Mahaffey, Howard, and Norwood (2019),
Robinson, Howard, Tingle, and Norwood (2020), and the proposed Beta-geosynthetic trend line
were plotted. A number of observations regarding the plotted data can be made.
Figure 6.5 Data points comprising beta methodology
144
First, there is a meaningful amount of variability in the original dataset, and the fitted
trend line explains approximately 58% of the variability in the dataset (i.e. R2 = 0.58). When the
unreinforced data points were added to the original dataset, a slight increase in fit was observed
(R2 = 0.65). Limited data were originally available beyond 10,000 coverages, with the
unreinforced data from Robinson, Howard, Tingle, and Norwood (2020) providing an additional
data point beyond 10,000 coverages.
The unreinforced control data from the two studies (circled in Figure 6.5) fall above and
to the right of the existing beta criteria, with the exception of one data point (Robinson,
Mahaffey, Howard, and Norwood 2019) that falls nearly on the existing beta curve. For
reference, this data point is termed TI-8 in the original reference. It is noted that Robinson,
Mahaffey, Howard, and Norwood (2019) concluded that TI-8 was substantially weaker than the
other test items evaluated, suggesting that the existing beta criteria at higher coverage levels
yields conservative design recommendations. In fact, a review of the data indicates that if a
pavement were designed based on the coverages-to-failure observed in the test items (Robinson,
Howard, Tingle, and Norwood 2020) an additional 152 to 178 mm of pavement thickness would
be required.
The geosynthetic-reinforced data points, represented by solid triangles (Robinson,
Mahaffey, Howard, and Norwood 2019) and solid squares (Robinson, Howard, Tingle, and
Norwood 2020), also plot above and to the right of both the existing design curve and the
proposed Beta-geosynthetic design curve that is based on the equivalent thickness concept. It
was observed that there was not a meaningful difference in calculated Beta-values for the
unreinforced items and reinforced items, although coverages to failure were, in some cases much
higher than the unreinforced items, and lower in others.
145
A visual inspection of the Figure 6.5 data suggests that there is not sufficient evidence to
support changes to the existing design criteria, particularly at higher coverage levels. At higher
coverage levels (i.e. >10,000 coverages), any performance improvement that could be gained
from geosynthetic inclusion appears be concealed by the variability of the design method. For
instance, it can be observed that at Beta-values near 15 (i.e. where the geosynthetic datasets, DS3
and DS4, plot), the number of coverages could range anywhere from 10 all the way up to 70,000,
approximately three orders of magnitude. At lower coverage levels (i.e. <10,000 coverages), the
proposed beta-geosynthetic criteria could be used provisionally as a means to estimate
improvement gained from geosynthetic inclusion. At beta-values of approximately 20, it was
observed that coverages ranged from 10 to 200, or approximately one order of magnitude. Since
beta-geosynthetic criteria at lower coverages (i.e. thinner pavements) are generally founded on
data gathered from highway loading studies, additional data are needed under aircraft loading to
substantiate this claim.
It is noted that the pavements represented by DS3 and DS4 could be inherently stronger
than those represented by DS1 (i.e. some pavements were tested nearly 70 years ago).
Advancements in material properties (such as asphalt binder) and/or improvements in material
specifications (such as base course gradation) could explain some of the observed variability in
the datasets.
6.4 Cost/Value of Geosynthetics
Synovec, Howard, and Priddy (2019) presented a case study that investigated a holistic
approach to military asset management and highlighted funding challenges in DOD pavement
management. It was noted that the worst condition active duty airfield in the United States Air
Force (USAF) (Minot Air Force Base) required reconstruction of a failing runway at an
146
estimated cost of $56.7 million (as of 2014) and that $115 million over a ten-year period was
needed to increase the pavement condition to only slightly above average. Further, Synovec,
Howard, and Priddy (2019) noted that funding would likely be diverted to fund other base-wide
facilities and maintenance costs. Thus, it is important to understand the potential cost
implications of incorporating geosynthetics and the value that could be derived in terms of
precious facility dollars.
To determine a value of including geosynthetics, a simple cost/benefit analysis was
performed. A number of assumptions were made and it is noted that local markets and material
availability should be considered; however, this provides a relatively simple illustration for
making an allowance for geosynthetic value. It was assumed that the total runway value was
$56.7 million (from Synovec, Howard, and Priddy 2019) over a 20-year design life, thus the
annual construction value of the runway was considered to be $2.8 million. Traffic was assumed
to be applied annually and evenly over the 20-year design life, and airfield dimensions were
assumed to be 3,000-m-long and 46-m-wide. A brief internet search found that publicly
advertised geogrid cost ranged from approximately $3.80 (low) to $5.50 (high) per square meter.
Extensions in service life (in years) were computed from laboratory-scale (Robinson, Mahaffey,
Howard, and Norwood 2019) and full-scale airfield performance test results (Robinson, Howard,
Tingle, and Norwood 2020). The life extension was multiplied by the annualized value of the
pavement to give an indication of geosynthetic value (minus the initial geosynthetic cost), and
these data are summarized in Table 6.2. It was found that a positive return on investment could
be achieved in some cases, and that pavement design life extensions ranging from 5 to 16 years
in some cases could be realized. Table 6.2 uses terminology from previous studies; TI refers to
test item and the numbers denote different geosynthetic reinforced experiments. It is noted that
147
TI-9 (Robinson, Howard, Tingle, and Norwood 2020) and TI-4 (Robinson, Mahaffay, Howard,
and Norwood 2019) had a calculated 30+ year life extension that should be considered
unrealistic; the pavement would likely fail from environmental factors (i.e. cracking and
weathering) long before a benefit of this magnitude would be realized. However, these data
could be interpreted as a relatively low cost/high benefit safety net in the case of increased traffic
due to unplanned mission changes or overloaded aircraft conditions. Essentially, the inclusion of
geosynthetic could be considered a risk management tool for unknown future loading conditions
to preserve future dollars. A similar observation has been made in roadway and railway
applications (Correia, Winter, and Puppala 2016). An example of this philosophy was presented
by Synovec, Howard, and Priddy (2019) in the case of an overseas contingency airfield that was
structurally inadequate for the required traffic. Rutting upwards of 75 mm was noted but due to
mission requirements the airfield had to remain open, therefore the traffic lanes were offset to
meet mission requirements. Conditions such as these could be a useful application for
geosynthetics, particularly in the case of extreme loading where rutting develops rapidly, as
improvement has been observed at higher levels of rutting (Robinson, Mahaffay, Howard, and
Norwood 2019).
148
Table 6.2 Geosynthetic value in terms of extended life
Ref. Item
Annual
Traffic Extra Years
Value of
Extended
Life
Geosynthetic
Value (Low)
(Millions)
Geosynthetic
Value (High)
(Millions)
Robinson,
Howard,
Tingle, and
Norwood
2020
TI-1 3,516 0.0 --- --- ---
TI-2 4,519 5.7 $ 16.2 $ 15.6 $ 15.4
TI-3 2,918 (3.4) $ (9.6) $ (10.2) $ (10.4)
TI-4 4,560 5.9 $ 16.8 $ 16.2 $ 16.0
TI-5 3,434 (0.5) $ (1.3) $ (1.9) $ (2.1)
TI-6 6,375 16.3 $ 46.1 $ 45.5 $ 45.3
TI-7 1,956 (8.9) $ (25.2) $ (25.8) $ (26.0)
TI-8 2,268 (7.1) $ (20.1) $ (20.7) $ (20.9)
TI-9 9,000 31.2 $ 88.4 $ 87.8 $ 87.6
Robinson,
Mahaffey,
Howard, and
Norwood
2019
TI-2 185 0.0 --- --- ---
TI-4 486 32.5 $ 92.3 $ 91.7 $ 91.5
TI-5 73 (12.1) $ (34.3) $ (34.9) $ (35.1)
TI-6 205 2.2 $ 6.1 $ 5.5 $5.3
TI-7 129 (6.1) $ (17.2) $ (17.8) $ (18.0)
Note: Parenthesis indicate negative values; Negative values indicate no cost benefit from geosynthetic; TI = test
item
An alternative approach was investigated by determining the amount of base/subbase
aggregate that could be replaced by including geosynthetics, which gives an indication of cost
savings in immediate dollars. Geosynthetics have been noted as a sustainable construction
solution in highway and railway applications (Indraratna, Nimbalkar, and Rujikiatkamjorn 2014;
Hussaini, Indraratna, and Vinod 2015; Yonezawa, Yamazaki, Tateyama, and Tatsuoka 2014),
thus it is logical to evaluate their impact as a cost savings solution for airfield construction.
Actual passes to failure and subgrade CBR were used to determine the required pavement
thickness above the subgrade using PCASE design software. PCASE is an ERDC software
package that deploys the current design methodology in a Windows-based user interface.
Calculated differences in overall pavement structure were attributed to reductions in aggregate
thickness that could be achieved through the inclusion of a geosynthetic. Airfield dimensions
were assumed the same as previously described, 3,000 m long by 46 m wide. A large aggregate
supplier was contacted, and it was found that the cost of airfield quality aggregate generally
ranged from $18 to $20 per ton. It is noted these costs are material costs only and do not include
149
trucking. These costs should be considered representative of the market in the southeastern
region of the U.S. as of the fall of 2019. Geosynthetic value was taken to be the cost savings
derived from aggregate reduction (at an aggregate cost of $20 per ton) minus the cost of a
geosynthetic at the low and high geosynthetic price points noted earlier. These data are
presented in Table 6.3. It was found that the maximum aggregate reduction was approximately
33 mm and that the average aggregate reduction was on the order of 12 mm. None of the
geosynthetic sections were found to produce a cost savings in terms of aggregate reduction.
Practically, the calculated aggregate reductions, in most cases, are not sufficiently large enough
to overcome the construction variability expected in aggregate placement. If the aggregate
reductions were sufficiently large enough to reduce the aggregate thickness by an entire lift (i.e.
approximately 152 mm), then it could be argued that additional cost savings could be gained not
only from a material savings standpoint, but also a reduction in costs associated with haul
equipment, compaction equipment, and placement time. However, none of the aggregate
reductions were sufficiently large to justify these additional cost savings. If a breakeven analysis
is conducted (i.e. what would the aggregate cost need to be such that the aggregate savings is
equal to the cost of the geosynthetic) is was found that aggregate cost generally would need to be
anywhere from $50 to $200 per ton, representing over double to ten times the cost provided by a
representative aggregate producer. A doubling of the cost may be possible in some markets,
where airfield quality aggregates tend to be less plentiful (or if geosynthetic purchase prices are
lower than those used herein); however, a ten-fold increase in aggregate cost would be unlikely.
These data indicate that including a geosynthetic in a thicker airfield pavement for the primary
goal of aggregate reduction is not likely a cost-effective approach.
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The results of the cost analysis indicated that extended pavement life (in terms of rutting
performance) should be the primary means to rationalize geosynthetic inclusion. Geosynthetic
inclusion could be considered as a risk reduction tool where mission changes lead to a drastic
increase in airfield traffic or an increase in aircraft payload. Reductions in aggregate thickness
did not seem to be cost effective.
Table 6.3 Geosynthetic value in terms of reduced aggregate thickness
Ref. Item
Passes to
failure
Subgrade
CBR
Calculated
Required
Thickness
(mm)
Aggregate
Thickness
Difference
(mm)
Geosynthetic
Value (Low)
(Thousands)
Geosynthetic
Value (High)
(Thousands)
Robinson,
Howard,
Tingle,
and
Norwood
2020
TI-1 70,318 7.6 635 0 --- ---
TI-2 90,370 7.1 663 28 $ (382.5) $ (632.5)
TI-3 58,364 7.2 646 12 $ (499.4) $ (749.4)
TI-4 91,198 7.9 630 (5) $ (619.6) $ (869.6)
TI-5 68,680 8.2 611 (24) $ (753.7) $ (1,003.7)
TI-6 127,492 7.4 659 24 $ (411.3) $ (661.3)
TI-7 39,110 7.5 623 (12) $ (669.2) $ (919.2)
TI-8 45,353 7.6 623 (12) $ (668.5) $ (918.5)
TI-9 180,000 7.4 668 33 $ (348.3) $ (598.3)
Robinson,
Mahaffey,
Howard,
and
Norwood
2019
TI-2 3,700 2.9 749 0 --- ---
TI-4 9,720 2.9 780 31 $ (360.8) $ (610.8)
TI-5 1,460 3.0 707 (42) $ (885.3) $ (1,135.3)
TI-6 4,100 2.9 752 3 $ (559.0) $ (809.0)
TI-7 2,575 2.8 748 (1) $ (592.4) $ (842.4)
Note: Parenthesis indicate negative values; Negative values indicate no cost benefit from geosynthetic; TI = test
item
6.5 Other Uses of Geosynthetics in Airfield Pavements
Previous research efforts have focused on the premise of including geosynthetics in
newly constructed or reconstructed airfield pavements. However, limited new construction or
reconstruction is taking place that would require an airfield closure; more focus has been recently
placed on localized repairs. Synovec, Howard, and Priddy (2019) identified two reasons for
limiting airfield closures: 1) closing a runway essentially eliminates the airfield’s capability to
function as a weapons system, and 2) the relocation of aircraft and support personnel to a
different location is nearly impossible to accomplish. Further, Synovec, Howard, and Priddy
151
(2019) noted that a typical US Air Force (USAF) airfield has one primary runway, one parallel
taxiway, and one or two primary parking aprons. Therefore, reconstruction of any one of these
airfield components would effectively shut down the entire airfield suggesting that the likelihood
of shutting down an airfield for complete reconstruction activities is very small. Thus, localized
repairs, when funds are available, should be expected to be the primary maintenance technique.
New construction is advantageous in that geosynthetics can be overlapped and extended
into non-traffic areas creating an embedment depth that assists in anchoring the geosynthetic. In
contrast, a pavement repair may likely have a vertical cut thus limiting the embedment depth.
Some research within USACE-ERDC has investigated the use of geosynthetics for airfield
damage repair. The extension of geosynthetic inclusion in airfield repair (either within the U.S.
or in a contingency environment) is a logical progression to further evaluate geosynthetics in
airfield pavements. A geosynthetic is generally lightweight, easy to transport, easy to place, and
adaptable to any size or shape repair. To evaluate geosynthetics in airfield repair, a series of
simulated ordinance-induced craters were constructed and reinforced with a 6 oz. non-woven
needle punched geotextile. The test series consisted of a 152 mm thick rapid setting concrete
(commonly used in DOD contingency airfield repair scenarios) placed over a 305 mm thick sand
subbase. The geotextile was placed at the sand/subgrade interface, mid-depth (2 layers), and 1/3
depth (3 layers), and the simulated craters were trafficked with a single-wheel F-15 load cart
(156 kN total load at 2241 kPa tire pressure). It was found that the single layer geotextile was
the best performer, and increased passes to failure (1986 passes for the reinforced vs. 1344
passes for the unreinforced). Two layers of geotextile provided a slight performance
improvement (1456 passes), and three layers of geotextile was found to perform worse than the
152
control (1108 passes). Failure was generally identified as excessive slab settlement and extensive
cracking in the 152 mm thick rapid setting concrete surface slab.
Another implementation may be incorporating geosynthetics in airfield asphalt pavement
as a means to improve cracking resistance. An assessment of historical pavement condition data
(maintained at ERDC) was performed to identify predominant pavement distress types in flexible
pavements (Robinson 2019). It was found that longitudinal and transverse cracking were the
most common and recurring distress and were observed early in pavement life (typically 2 to 4
years). It was noted that load related distress (i.e. rutting) did not generally occur until much
later in the pavement design life (typically around 15 years). Thus, the inclusion of a
geosynthetic as a crack mitigation method could prove cost effective, though one should note a
geosynthetic near a pavement surface might limit future in-place recycling or milling options.
Offenbacker (2019) evaluated geogrid inclusion as a crack mitigation technique for military
pavements in a series of laboratory scale tests. The laboratory tests indicated that geogrid
inclusion was capable of extending the fatigue life of an airfield paving mixture and delaying
crack propagation. Others (Austin and Gilchrist 1996; Abdessemed, Kenai, and Bali 2015;
Buonsanti, Leonardi, and Scopelliti 2012; Chantachot, Kongkitkul, Youwai, and Jongpradist
2016; Correia and Zornberg 2014; Von Quintas, Mallela, and Lytton 2009) have made similar
observations.
6.6 Conclusions
An assessment of geosynthetic inclusion into military airfields (thicker airfield in
particular) was performed utilizing the results of laboratory-scale and full-scale evaluations.
Existing DOD design methodology was investigated, as well as, potential cost implications.
153
1. A review of in-service airfield pavement characteristics indicated that flexible
airfield pavements are considerably thicker and stronger than highway pavements
validating the lack of improvement observed in airfield pavement test sections
when compared to highway pavements.
2. Changes to existing DOD design methodology to account for geosynthetic
inclusion should be considered with caution. Inherent variability in the original
methodology derivations exceed the potential improvement observed in
laboratory-scale and full-scale test sections.
3. The beta-geosynthetic methodology could be utilized in light duty airfield
pavements, and seems better suited for lower traffic levels.
4. The cost/benefit analysis indicated that design life extension should be the
primary method to assign a monetary value to geosynthetic inclusion in thicker
airfield pavements; reductions in aggregate thickness did not suggest a likely cost
benefit.
5. Alternative uses of geosynthetics should be investigated such as repair
applications or cracking resistance. These alternatives may prove to be more
beneficial than aggregate base reinforcement for military airfields.
154
CHAPTER VII
CONCLUSIONS AND RECOMMENDATIONS
7.1 Summary
The work presented in this dissertation, while primarily focused on geosynthetic
inclusion in military airfields, could be used as a source of information for geosynthetic inclusion
in commercial aviation airfields. The research presented in Chapter 4 was leveraged by the FAA
as a selection tool to determine geosynthetic type, placement location, and representative
pavement cross-sections for a full-scale evaluation at the National Airport Pavement Test
Facility. As of Summer 2020, the full-scale test sections were under construction, and traffic
testing was expected to begin later in the year. Thus, the information presented in this
dissertation has already had an impact on shaping future research.
7.2 Conclusions
The overall goal of this dissertation was to investigate the unknown performance
implications of incorporating geosynthetics in thicker military airfield pavements and to
determine if similar performance improvement as that documented in thinner highway
pavements could be realized. Additionally, the practical implications of incorporating
geosynthetics in thicker airfield pavements were investigated to understand potential design
implementation, including cost-benefit in terms of extended service life or aggregate thickness
reduction. Overall findings from the assessment of geosynthetics in representative military
airfield pavements at two testing scales indicated that a limited number of geosynthetics
155
provided a performance benefit. Specific conclusions drawn throughout this dissertation are
summarized below.
• A review of light duty aircraft and highway studies conducted at ERDC, as
described in Chapter 3, supported the geosynthetic equivalent thickness design
methodology previously implemented in ETL 1110-1-189.
• The results of cyclic plate load tests, described in Chapter 4, found that the
inclusion of some geosynthetics (not all) increased cycles to failure.
• Changing subbase material CBR in cyclic plate load test from approximately 15-
18 to 55 improved pavement performance well beyond that observed from
geosynthetic inclusion.
• The results of a full-scale investigation found that only three of seven
geosynthetic products provided a consistent rutting improvement, and that only
two of the three provided a meaningful rutting improvement.
• The results of the cyclic plate load test and full-scale test found that some level of
permanent deformation was required to engage the reinforcing benefit of some
geosynthetics. The level of permanent deformation was generally higher than
would be allowed on an operational military airfield.
• The results of the cyclic plate load test and full-scale test concluded that most
geosynthetics did not result in a meaningful decrease in measured vertical
subgrade pressure, which was contrary to observations made in highway
pavement studies.
• A cost/benefit analysis, based on the Chapter 4 and Chapter 5 performance test
results, found that an extension of service life was the preferred valuation method
for geosynthetic inclusion rather than base course aggregate reduction. None of
the geosynthetics yielded a cost benefit in terms of base course aggregate
reduction.
• The overall conclusion of this dissertation is that geosynthetic inclusion in thicker
airfield pavements does not provide the same level of improvement as that
observed in thinner highway pavements, and that widespread implementation is
not likely to be practical as of the summer of 2020.
156
7.3 Recommendations
The work presented in this dissertation specifically addressed the evaluation of
geosynthetic inclusion in thick airfield pavements. Based on observations made during the
course of the research presented in this dissertation, the following recommendations were made.
• Considering the inclusion of geosynthetics is recommended in contingency
military airfields as it could be a useful approach to mitigate risk of failure from
unknown mission changes (i.e. rapid operational increase in aircraft traffic and/or
aircraft load).
• The beta-geosynthetic methodology (presented in Chapter 6) is recommended for
consideration with respect to utilization for light duty aircraft design
considerations; however, additional research is needed to quantify geosynthetic
inclusion at lower traffic levels.
• There was modest evidence to suggest that increasing geogrid aperture size and
tensile strength improved rutting performance. Additional research is
recommended to investigate specific geosynthetic characteristics that may
influence airfield pavement performance.
• Little documented research has been conducted to understand the performance
implications of including geosynthetics in airfield damage repair. Preliminary
research performed at ERDC indicated that airfield damage repair might be an
appropriate geosynthetic application. Additional work is recommended in this
arena.
• Some laboratory work has indicated that geosynthetic inclusion as an interlayer in
asphalt layers can improve cracking performance. Military airfield serviceability
failures are typically attributed to pavement cracking, therefore further evaluation
of geosynthetic interlayers in airfield pavements are needed. There are several
areas of research in the arena of rehabilitation of airfield pavements and the value
of geosynthetics in this arena that are recommended for future study.
157
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