Correlating Strength and Stiffness Data of the PENCEL

Correlating Strength and Stiffness Data of the PENCEL Pressuremeter and Triaxial
Compression Tests in Florida Sands
by
Florida Institute of Technology
for the degree of
All Rights Reserved
We the undersigned committee, having examined the attached thesis;
“Correlating Strength and Stiffness Data of the PENCEL Pressuremeter and
Triaxial Compression Tests in Florida Sands”
By
_________________________________________________
Professor, Civil Engineering
_________________________________________________
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Abstract
Correlating Strength and Stiffness Data of the PENCEL Pressuremeter and Triaxial
Compression Tests in Florida Sands
By: Jacob William Jansen
The poorly graded sands found throughout Florida provide geotechnical engineers
with a difficult challenge when performing testing samples in laboratory tests.
These challenges have caused lab tests such as the triaxial compression test to be
overlooked. Since geotechnical engineers estimate strength and stiffness parameters
from basic field tests, they often produce overly conservative designs.
Understanding how the automated in-situ PENCEL Pressuremeter (PPMT) test
correlates with the triaxial compression test can reduce the time and costs
associated with laboratory triaxial testing. Results from triaxial tests yield a
Young’s Elastic Modulus, shear strength, and internal friction angle. Results from a
PPMT test yield a pressuremeter modulus, lift off pressure, and a limit pressure.
The different types of outputted data do not allow for direct comparisons to be
made between the triaxial compression test and the PPMT test. This research seeks
to correlate the outputted data.
This research involved twenty PPMT tests performed in poorly graded sands, with
loose to medium dense texture. PPMT results were compared with results from
twenty-one triaxial compression tests performed using soil removed from the test
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sites. The triaxial test density ranged from 20% to 65% of the soils relative density.
An equation from Baguelin (1978) was proven to correlate triaxial shear strength
with PPMT limit pressure.
Correlations indicate that triaxial elastic modulus and triaxial shear strength
correlate moderately well. The triaxial modulus is 93 times greater than the shear
strength. The PPMT modulus correlates well with the limit pressure, the PPMT
modulus is 8.3 times greater than the limit pressure in loose to medium dense sands
(R 2 =0.89). The triaxial elastic modulus and PPMT elastic modulus correlation
show PPMT moduli being on average 60% greater than triaxial moduli in similar
density and confining conditions. The correlations from this study indicate that data
from the triaxial compression test and the PPMT test can be correlated.
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Table of Figures .................................................................................................................... vii
List of Tables ......................................................................................................................... ix
List of Symbols ....................................................................................................................... x
2 Literature Review ........................................................................................................... 5
2.1 The Pressuremeter ................................................................................................ 5
2.1.4 Variations of strain-controlled PMT Tests ................................................... 15
2.1.5 Pressuremeter Theories............................................................................... 17
2.3 Methods of determining the at rest earth pressure ........................................... 30
2.3.1 Jaky Determination, 1944 ............................................................................ 30
2.3.2 Laboratory Methods .................................................................................... 31
3.1 Test Site Locations ............................................................................................... 38
3.1.1 Florida Tech Overflow lot ............................................................................ 39
3.1.2 Southgate Field ............................................................................................ 40
4 Test Methods ............................................................................................................... 42
4.1 In-Situ tests .......................................................................................................... 42
5.1.1 Grain Size ..................................................................................................... 51
5.1.2 Optimum Moisture ...................................................................................... 52
5.1.3 Relative Density ........................................................................................... 53
5.2 Triaxial Results ..................................................................................................... 55
5.3 Pressuremeter Results ......................................................................................... 57
5.4.2 Pressuremeter moduli and strength correlation ......................................... 60
5.4.3 Triaxial and pressuremeter stiffness correlation ......................................... 64
6 Conclusions and Recommendations ............................................................................ 68
6.1 Conclusions .......................................................................................................... 68
6.2 Recommendations ............................................................................................... 69
Table of Figures
Figure 2-1: From right to left; Pencel, Ménard, and Texam Pressuremeter types (From
RocTest) ................................................................................................................................. 6
Figure 2-2: Typical Pressuremeter results curve (from Shaban 2016) .................................. 9
Figure 2-3: Determination of lift off pressure for a self-boring and pre-bored
pressuremeter (Mair and Wood 1987). ............................................................................... 10
Figure 2-4: PMT unload-reload cycle (From Shaban 2016) ................................................. 12
Figure 2-5 Typical Load/Unload PMT test data ................................................................... 16
Figure 2-6 Inflated probe shapes in an unconfined environment vs in a confined
environment (from Murat and Lemoigne 1988) ................................................................. 17
Figure 2-7 Durham Geo Load frame .................................................................................... 19
Figure 2-8 Durham Geo triaxial cell ..................................................................................... 20
Figure 2-9 Humboldt data aquisition unit ........................................................................... 21
Figure 2-10 Triaxial control panel ........................................................................................ 22
Figure 2-11 Idealized relation for dilation angle, Ψ, from triaxial results ........................... 24
Figure 2-13a: Typical Mohr's Circle for CD triaxial data (From Holtz and Kovacs 1981) ..... 28
Figure 2-13b: Typical Mohr's Circle for CU triaxial data (From Holtz and Kovacs 1981) ..... 28
Figure 2-14: Soft Oedometer Ring (Kolymbas, 1993) .......................................................... 32
Figure 2-15: Correlation between the SPT N values, normalized effective overburden, and
the triaxial compression phi value (DeMello, 1971) ............................................................ 33
Figure 2-16: Correlation between CPT data and the effective phi angle in sand soils
(Robertson and Campanella, 1983) ..................................................................................... 35
Figure 2-17: Chart developed by Mair and Wood (1987) to determine the phi value using
stress strain slope. ............................................................................................................... 37
Figure 3-1: General location of testing sites on the FIT campus shown by stars. ............... 39
Figure 3-2: Arial overview of the overflow test site. The transect on which tests were
performed is shown by the yellow line. .............................................................................. 40
Figure 3-3: Southgate field test site. The transect tested is shown by the yellow line. ...... 41
Figure 4-1 Pressuremeter control unit with added digital instrumentation (From Shaban,
2016) .................................................................................................................................... 43
Figure 4-2 Screenshot of the APMT user interface and data reduction .............................. 44
Figure 4-3 Typical membrane calibration curve for PPMT tests ......................................... 46
Figure 4-4 Typical volume calibration curve from a PPMT test ........................................... 47
Figure 4-5 Borehole driving guide, with thin wall driving tube (From Shaban 2016).......... 49
Figure 5-1 Grain size distributions for test sites in FIT campus ........................................... 52
Figure 5-2 Standard Proctor moisture density data from a mixed sample, optimum
moisture content was determined to be 12% ..................................................................... 53
Figure 5-4 Correlation between the triaxial initial moduli and the shear strength of the soil
at 5% strain .......................................................................................................................... 59
Figure 5-5 Comparison between the calculated limit pressure, measured initial modulus
and shear strength ............................................................................................................... 61
Figure 5-6 Correlation between PMT initial modulus and PMT limit pressure using all data
points ................................................................................................................................... 62
Figure 5-7 Limit pressure vs pressuremeter modulus for loose to compact sands ............ 63
Figure 5-8 Relationship between strength and stiffness data for both triaxial and PPMT
tests ..................................................................................................................................... 64
Figure 5-9 Predicted PMT modulus from measured Triaxial data ....................................... 65
Figure 5-10 Prediction of triaxial moduli using field measured PMT moduli ...................... 66
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List of Tables Table 2-1 Pressuremeter insertion recommendation table (Winter 1986) .......................... 8
Table 5-1 Summary of moisture density results from mixed samples ................................ 53
Table 5-2 Summary of maximum density tests ................................................................... 54
Table 5-3 Summary of minimum density tests .................................................................... 55
Table 5-4 Summary of triaxial test results ........................................................................... 56
Table 5-5 Averages of triaxial data, based off of density .................................................... 57
Table 5-6 Summary of PPMT test results ............................................................................ 58
Table 5-7 Averages of PPMT data based off of site ............................................................. 58
Table 5-8 Relationships and correlations between the strength and stiffness for PPMT,
triaxial, and combined data ................................................................................................. 64
Table 5-9 Correlation summary ........................................................................................... 67
DPMT Pressuremeter Diameter Po Lift off pressure
G Elastic shear modulus
Vm Mean volume of the cylindrical cavity
V Change in volume over the corresponding change in pressure, P
Possion’s ratio Pl Limit Pressure Ei Initial elastic modulus
Er Reload elasticmodulus
σ’ Effective stress
σtotal Total stress
Ψ Angle of dilation
Shear stress
Pressuremeter shear strength
Triaxial shear strength
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Acknowledgements
I would firstly like to acknowledge the faculty and staff of the Civil Engineering
Department at Florida Tech for their support in my graduate studies. I would like to thank
my committee chair, Dr. Paul Cosentino for his guidance and helping my academic
growth. Additionally I would like to Thank Dr. Alaa Shaban for his help performing the
Pressuremeter tests and helping guide my research process. Finally, I would like to thank
my parents and family for the support they provided through my time here at Florida
Tech.
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1 Introduction
1.1 Background In the state of Florida, the most commonly found soil is a poorly graded sand,
referred to here as Florida sands. Geotechnical investigations used to determine the soil
properties of these sands can range from simple observational tests with no sampling, to
more involved field testing and/or sampling. These more involved tests can provide the
strength and stiffness characteristics of the soil at the site can either be performed in a
geotechnical laboratory or in-situ.
A commonly used laboratory test to determine the soil strength and stiffness is the
triaxial compression test. This test uses samples transported from the site to a laboratory,
where the sample is prepared, and then tested in compression with confinement to
obtain the axial strength data. This process of sampling, drying, remolding, and testing
take between a hour and a day per test. This test has been used by engineers since the
late 1930’s, and provides a wide range of engineering data, under various soil drainage
conditions. The data from a triaxial compression test can be used to determine the
Young’s elastic modulus (E), soil shear stress (τ), and angle of internal friction (φ) as a
function of unit weight. These engineering parameters are integral parts of many design
equations used in geotechnical applications. However, instead of attempting to mimic the
conditions of an in-situ soil in a laboratory engineers have designed a device to directly
measure the soil’s strength and stiffness characteristics in-situ.
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The pressuremeter (PMT), first successfully developed in 1956 (Ménard, 1956),
has allowed geotechnical engineers to examine soil strength and stiffness in-situ. This
device allows the radial strength data of the soil to be measured, without having to
transport soil samples and attempt to replicate site conditions in a laboratory. Cosentino
et al. (2006) automated the PENCEL pressuremeter unit, leading to a significant increase
in its use. The data from the automated PMT can be used to determine the PMT modulus
(E), rebound modulus (Er), lift off pressure (Po), and limit pressure (Pl). These data
parameters can take between 30 minutes and an hour per test. These engineering
parameters are used in some geotechnical design equations; however they are not as
commonly used. The time and cost savings to perform an automated PPMT could lead to
large savings in engineering design and construction if the relationship between PPMT
data and other more common tests can be determined.
1.2 Objective The objective of this research is to correlate in-situ soil strength and stiffness parameters
obtained from PMT tests in Florida sands with the engineering properties obtained from
the triaxial compression test.
1.3 Approach This research will use the PENCEL Pressuremeter (PPMT) in pre-bored holes to determine
the strength and stiffness characteristics of the soil. Previously determined relationships
found in literature will be used to relate the PMT data with the soil shear strength. Finally
triaxial tests of samples will be performed to determine the soils’ laboratory shear
strength and stiffness. Correlations between the PPMT and the triaxial engineering
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parameters of the Florida sands will be determined. The steps required in this process
are outlined below.
1.3.1 Literature Review
A full review of the pressuremeter history and test procedure was performed; this
provided the background on the aspects of the pressuremeter being used, as well as prior
methods used for testing of different soil parameters. Additionally, a review of triaxial
testing and the behavior of granular material in shear was conducted to gain an
understanding of the mechanical behaviors exhibited by the soil during testing. Finally a
review of the development of methods for determining soil properties from both the
PPMT and triaxial shear test was presented.
1.3.2 Site Selections
The sites selected for field testing were selected based upon their location, uniformity,
and soil type. The two main sites that were selected contain a loose to medium dense,
poorly graded fine sandy soils (SP).
1.3.3 Laboratory Testing
Laboratory testing was used to measure data in a controlled and reproducible
environment. The soil properties and strengths were determined using the Consolidated
Drained Triaxial test (ASTM D7181). The soil gradation and USCS (Unified Soil
Classification System) classification were determined from ASTM D6913 (soil gradation)
and ASTM D2487 (USCS soil classification). In order to determine relative density a
minimum and maximum density test was performed (ASTM D4253, ASTM D4254). The
optimum moisture content was determined from the standard Proctor test (ASTM D698).
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1.3.4 Field Testing
The sites selected in task 1.3.2 were used for field testing. A transect was set up at both
sites, with test points every 25ft along the transect. PPMT tests were performed at each
point to gather data over a very contained and uniform sample. A total of 20 test points
were accumulated in the field testing procedures.
1.3.5 Results
Results from the PPMT tests and triaxial tests were compiled into tables to compare the
data from each test. These tables included the site, modulus of elasticity, moisture
content, densities, as well as parameters specific to each type of test.
1.3.6 Analysis
Test data from both laboratory and field tests were reduced to standard and usable
engineering parameters. The data were analyzed using a Excel and R-studio, and multiple
linear regression analyses were performed with the data. Additionally, correlations to
previous methods discussed in literature were examined.
1.3.7 Conclusions and Recommendations
Using the correlations and their corresponding statistical significance, as well as the
theoretical results conclusions were made. Recommendations for further studies were
developed based on the process and findings from this study.
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2 Literature Review This section seeks to discuss the background, development, uses and interpretation of the
pressuremeter tests and the triaxial tests. Additionally, uses and implications of previous
studies will be examined in this section.
2.1 The Pressuremeter
2.1.1 Pressuremeter Development
Geotechnical engineers have used laboratory test methods to determine the stress-strain
relationships of soils for decades. However, many problems arose from trying to extract,
transport, and test undisturbed samples in the lab. The problems associated with
transporting and testing caused geotechnical researchers to develop in-situ devices in
order to determine the stress-stain relationships of soils. By testing these relationships at
the site, researches figured the least amount of disturbance would be applied to the soil,
with the main disturbance being due to the testing process itself.
The Pressuremeter was initially developed by Kögler in 1933; however, was not
successfully deployed until Louis Ménard in 1956. This device is defined as a “cylindrical
device designed to apply uniform pressure to the walls of a borehole by means of a
flexible membrane” (Mair and Wood, 1987). Ménard’s Pressuremeter used a three cell
system, with all cells being inflated to the same pressure. The cells are rubber membranes
fixed around a metal core, and bound by two metal endplates (Mair and Wood, 1987).
The middle cell was used to take measurements, while the two cells on each side, known
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as guard cells, were used to reduce end effects. The guard cells make the probe function
as an infinite cylinder, allowing for the assumption of plane strain to be used for analysis
(Baguelin et al, 1978).
Many different types of pressuremeter’s have been developed since Ménard’s initial
Pressuremeter. The tri-celled Pressuremeter was adapted into a mono-cell version. The
two main type of mono-celled Pressuremeter’s are the TEXAM Pressuremeter and the
PENCEL Pressuremeter. The TEXAM is approximately 2.75 inches in diameter by 18 inches
long, while the PENCEL is approximately 1.3 inches in diameter by 9 inches long. Both
Pressuremeters are shown in Figure 2-1.
Figure 2-1: From right to left; Pencel, Ménard, and Texam Pressuremeter types (From RocTest)
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2.1.2 Pressuremeter Insertion
Results from the Pressuremeter test will vary depending on how the probe is inserted into
the soil. This insertion process will affect the stress-strain characteristics of the soil at the
site. The soil type and relative density should be considered when selecting an insertion
method.
Insertion of a pre-bored Pressuremeter (PBPM) entails lowering the Pressuremeter into a
hole slightly larger than the diameter of the probe (between 1.03DPMT and 1.2DPMT). This
method is the most common for probe insertion. PBPM works best for shallow depth
testing, due to the higher probability of borehole collapse in deeper boreholes. The two
main methods of borehole preparation for the PBPM are either, drilled or pushed thin
wall sampler. Drilled methods include rotary drilling, continuous flight auger, and hand
auguring. These methods are not recommended in granular strata due to the large soil
disturbance associated with drilling. A pushed or driven thin wall sampler is best used in
granular soils. In cohesive soils, the interaction between the soil and the thin walled
sampler may cause large disturbance in the layer. In deep test sites a prepared drillers
mud is recommended to support the borehole wall (Winter 1986).
2.1.2.2 Insertion of the Self-Boring Pressuremeter
The self-boring Pressuremeter (SBPM) has an internal rotary bit leading the
Pressuremeter during insertion. The shavings are flushed with drilling mud up an internal
flushing tube, where they are collected in a settling tank. This method of insertion
requires the most effort and materials and is primarily used when inserting the probe
through cemented layers, or weathered rock (Winter 1986).
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2.1.2.3 Insertion recommendation chart
The method of insertion of the pressuremeter should be carefully considered, as the
insertion method can alter the overall test results. The following table from (Winter,
1986) can be used as a guide for deciding the insertion method. However, additional
factors, such as the cohesiveness, depth, grain size, aggregate size for road base course,
water table, and permeability are important factors to consider.
Table 2-1 Pressuremeter insertion recommendation table (Winter 1986)
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2.1.3 Pressuremeter data interpretation
The Pressuremeter test method produces a stress-strain graph, similar to that of many
materials tests. There are three distinctive sections that make up this graph, these are
listed and explained below.
Initial phase: is a re-establishing curve portion at which the membrane becomes into a
full contact with the walls of a borehole (from point A to point B),
Elastic phase: is a straight-line portion during which the change in volumetric-strains of
the membrane are assumed to be constant (from point B to point C),
Plastic phase: is a nonlinear curve portion at which the stressed soil cavity increases
significantly with a little increase in applied pressure (from point C to Point D).
Figure 2-2: Typical Pressuremeter results curve (from Shaban 2016)
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2.1.3.1 Lift off pressure
To estimate the lift off pressure (Po) is estimated from the curve where the tangent line
from the initial phase intersects the tangent line from the elastic phase on a pre-bored
PMT test, while it is the displacement of the graph above the strain axis on a self-boring
PMT test. The data reduction process is typically done by hand, drawing the two tangent
lines on a scale graph and visually determining this stress point. A hand analysis method is
highly variable, and may be best represented as a range instead of a single point. The lift
off pressure can be approximately related to the in-situ horizontal at rest earth pressure
for pre-bored tests. It is however not practical to predict the horizontal earth pressures
due to the relaxation and disturbance of the surrounding soil during the boring process.
The following figures show the methods of determining the lift off pressure (Mair and
Wood, 1987).
(a): SBPMT Curve (b): PBPMT
Curve Figure 2-3: Determination of lift off pressure for a self-boring and pre-bored pressuremeter (Mair and Wood 1987).
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2.1.3.2 Initial Elastic Modulus
The straight portion of the curve between the lift off pressure and the plastic zone (figure
2-2 section BC) is the soils elastic response region of the sample. Soil is considered elastic
in this region due to the straight line nature of this curve. Lamè (1852) proposed that the
radial expansion of a cylindrical cavity in an infinite elastic medium is shown with the
following equation:
where:
G…