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6-30-2010

Evaluation of Granular Subgrade Modulus fromField and Laboratory TestsBiqing ShengFlorida State University

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Recommended CitationSheng, Biqing, "Evaluation of Granular Subgrade Modulus from Field and Laboratory Tests" (2010). Electronic Theses, Treatises andDissertations. Paper 1773.

THE FLORIDA STATE UNIVERSITY

FAMU-FSU COLLEGE OF ENGINEERING

EVALUATION OF GRANULAR SUBGRADE MODULUS FROM FIELD AND

LABORATORY TESTS

By

BIQING SHENG

A Thesis submitted to the Department of Civil & Environmental Engineering

in partial fulfillment of the requirements for the degree of

Master of Science

Degree Awarded: Summer Semester, 2010

ii

The members of the committee approve the thesis of Biqing Sheng defended on June 30, 2010.

__________________________________

Wei-Chou Virgil Ping Professor Directing Thesis

__________________________________

Tarek Abichou Committee Member

__________________________________

Ren Moses Committee Member

Approved:

____________________________________________________________

Kamal Tawfiq, Chair, Department of Civil & Environmental Engineering

____________________________________________________________

Ching-Jen Cheng, Dean, College of Engineering

The Graduate School has verified and approved the above-named committee members.

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ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to my advisor, Dr. Wei-Chou

Virgil Ping, for his instruction in my research work. Without him, this thesis would not

have been possible.

I would like to thank Dr. Tarek Abichou and Dr. Ren Moses, my thesis Committee

member, for reading this thesis and offering constructive comments.

Finally, I thank my family and friends for encouraging me move along.

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TABLE OF CONTENTS

List of Tables ................................................................................................................ vii

List of Figures .............................................................................................................. viii

Abstract ......................................................................................................................... x

CHAPTER 1. INTRODUCTION ..................................................................................... 1

1.1 Background ...................................................................................................... 1

1.2 Scope of Study ................................................................................................. 3

1.3 Report Organization ......................................................................................... 3

CHAPTER 2. LITERATURE REVIEW ........................................................................... 4

2.1 General ............................................................................................................ 4

2.2 Basic Concepts ................................................................................................ 4

2.2.1 Soil Resilient Modulus .............................................................................. 4

2.2.2 Modulus of Subgrade Reaction ................................................................ 5

2.3 Resilient Modulus Tests ................................................................................... 6

2.4 Factors Affecting Resilient Modulus ................................................................. 9

2.5 Empirical Resilient Modulus Models ............................................................... 11

2.6 Modulus of Subgrade Reaction ...................................................................... 12

CHAPTER 3. EXPERIMENTAL PROGRAMS ............................................................. 18

3.1 General .......................................................................................................... 18

v

3.2 Field Experimental Program ........................................................................... 18

3.3 Test-pit Experimental Program ....................................................................... 21

3.3.1 Test Materials ........................................................................................ 21

3.3.2 Test-pit Setup......................................................................................... 22

3.3.3 Test Sequence ....................................................................................... 22

3.4 Laboratory Experimental Program .................................................................. 24

3.4.1 Test Materials ........................................................................................ 24

3.4.2 Resilient Modulus Testing Program ....................................................... 24

3.4.3 Test Procedure ...................................................................................... 25

3.4.4 Determination of Resilient Modulus ....................................................... 26

CHAPTER 4. SUMMARY OF EXPERIMENTAL RESULTS ........................................ 37

4.1 Field Experimental Results ............................................................................. 37

4.1.1 Load-Deformation Curve Regression Model .......................................... 37

4.1.2 Plate Bearing Load Test Results............................................................ 40

4.2 Test-pit Test Results ....................................................................................... 40

4.2.1 Equivalent Resilient Modulus ................................................................. 40

4.2.2 Equivalent Resilient Modulus Results .................................................... 41

4.3 Laboratory Test Results .................................................................................. 42

4.3.1 Regression Analysis .............................................................................. 42

4.1.3 Moisture Effect on Resilient Modulus ..................................................... 43

CHAPTER 5. ANALYSIS OF EXPERIMENTAL RESULTS ......................................... 52

vi

5.1 Comparison of Laboratory and Test-pit Test Results ...................................... 52

5.1.1 Layered System Simulation for Test-pit Study ....................................... 52

5.1.2 Comparison of Resilient Modulus from Laboratory

Test and Test-pit Test ............................................................................ 53

5.1.3 Effect of Groundwater Level on Resilient Modulus ................................ 56

5.2 Comparison of Laboratory and Field Test Results .......................................... 57

5.2.1 Comparison of Secant Modulus and Laboratory

Resilient Modulus ................................................................................... 57

5.2.2 Comparison of Resilient Modulus and Modulus of

Subgrade Reaction ................................................................................. 58

CHAPTER 6. SUMMARY AND CONCLUSIONS ........................................................ 73

6.1 Summary ........................................................................................................ 73

6.2 Conclusions .................................................................................................... 74

REFERENCES .......................................................................................................... 76

BIOGRAPHICAL SKETCH ........................................................................................... 82

vii

LIST OF TABLES

Table 2.1 Summary of Resilient Modulus Prediction Models ....................................... 14

Table 3.1 Summary of Test Sites for Field Plate Load Test ......................................... 27

Table 3.2 Engineering Properties of Subgrade Material for Test-pit Test ..................... 28

Table 3.3 Summary of Material Properties for Laboratory Test .................................... 29

Table 3.4 Comparison of Test Procedures for Granular Soils ...................................... 30

Table 4.1 Secant Modulus and k Value from Plate Bearing Load Tests....................... 44

Table 4.2 Plate Load Equivalent Modulus Test Results ............................................... 45

Table 4.3 Laboratory Resilient Modulus Test Results of Subgrade (Field Plate Bearing Load Test Project) ....................................................... 46

Table 4.4 Laboratory Average Resilient Modulus at Different Moisture Conditions (Test-pit Plate Load Test Project) ............................................... 47

Table 5.1 Comparison of Layer Modulus from Test-pit Plate Load Test and Laboratory Resilient Modulus ................................................................ 62

Table 5.2 Comparison of Modulus from Field Load Test and Laboratory Resilient Modulus ......................................................................................... 63

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LIST OF FIGURES

Figure 2.1 Illustration of Soils Behavior under Repeated Loads ................................... 16

Figure 2.2 Field Plate Bearing Load Test Schematic ................................................... 17

Figure 3.1 Field Falling Weight Deflectometer Test Setup ........................................... 31

Figure 3.2 Field Plate Bearing Load Test ..................................................................... 32

Figure 3.3 Test-pit System: (a) Overview of Test Pit; (b) Schematic Diagram of Loading System & Cross Sectional View .................................. 33

Figure 3.4 Test-pit Test Sequence for Different Water Level Adjustment..................... 34

Figure 3.5 Schematic of Laboratory Resilient Modulus Test Program ......................... 35

Figure 3.6 Schematic of Triaxial Test for Resilient Modulus Measurement .................. 36

Figure 4.1 Rectangular Hyperbolic Representation of Load-Deformation Curve ........................................................................................................... 48

Figure 4.2 Load-Deformation Curves of the Tested Soils ............................................. 49

Figure 4.3 Laboratory Resilient Modulus vs. Moisture Content for Phase I and II Soils ................................................................................................. 51

Figure 5.1 Comparison of Resilient Modulus from Laboratory Triaxial Test with Equivalent Layer Modulus from Test-pit Test ...................................... 64

Figure 5.2 Comparison of Resilient Modulus from Laboratory Test using Middle-Half LVDT with Resilient Modulus from Test-pit Test....................... 65

Figure 5.3 Comparison of Resilient Modulus from Laboratory Tests using Middle-Half LVDT and Full-Length LVDT .................................................... 66

Figure 5.4 Comparison of Resilient Modulus from Laboratory Test using Full-Length LVDT with Resilient Modulus from Test-pit Test ...................... 67

Figure 5.5 Equivalent Resilient Modulus at Different Groundwater Levels ................... 68

Figure 5.6 Percent Reductions of Resilient Modulus at Different Groundwater levels . 69

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Figure 5.7 Comparison of Laboratory Resilient Modulus with Secant Modulus at Deflection of 1.27 mm ............................................................... 70

Figure 5.8 Comparison of Laboratory Resilient Modulus with Average Secant Modulus at Pult ............................................................................ 71

Figure 5.9 Comparison of Laboratory Resilient Modulus at Pult with Modulus of Subgrade Reaction .................................................................. 72

x

ABSTRACT

The Resilient Modulus of pavement subgrade materials is an essential parameter to determine the stress-stain characteristics of pavement structures subjected to traffic loadings for mechanistically based flexible pavement design procedure. The modulus of subgrade reaction is a required parameter for design of rigid pavements. The load-deformation characteristics of the granular subgrade soils were investigated using the laboratory triaxial test, test-pit plate load test, and field rigid plate load bearing test. Several typical subgrade soils used for pavement construction in Florida were obtained for evaluation. The resilient modulus of subgrade materials were evaluated by laboratory triaxial testing program. The resilient properties of subgrade materials were found to be strongly influenced by the moisture content and test procedure. Then, a full scale laboratory evaluation of the subgrade performance was conducted in a test-pit facility to simulate the actual field conditions. The subgrade materials were tested under various moisture conditions that simulated different field groundwater level. It was shown that the resilient modulus of subgrade materials increases with the decrease of groundwater level. In addition, the field plate load testing program was carried out to evaluate the bearing characteristics of pavement base, subgrade, and embankment soils. A hyperbolic model was used to represent the relationship of the load-deformation curve obtained from the field plate load bearing test.

Correlation relationships were established between the laboratory resilient modulus and the resilient modulus measured using test-pit facility. It was shown that the resilient modulus measured from laboratory test could be used to predict the resilient deformation of the pavement subgrade layers if an appropriate calculation method was used. Correlation relationship between the subgrade soil resilient modulus and the modulus of subgrade reaction was also established, which was found to be close to the theoretical relationship from the AASHTO design guide. These correlation relationships could be utilized in the Florida pavement design guide to better predict the resilient deformation of pavement subgrades.

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CHAPTER 1

INTRODUCTION

1.1 Background When design pavements, the characteristic of the subgrade upon which the

pavement is placed is an essential design parameter need to be considered. Subgrade materials are typically characterized by their resistance to deformation under load, which can be either a measure of their strength or stiffness. In general, the more resistant to deformation a subgrade is the more loads it can support before reaching a critical deformation value. A basic subgrade stiffness/strength characterization is resilient modulus (MR). Both the AASHTO 1993 Design Guide [AASHTO, 1993] and the mechanistic based design methods [ARA, 2004] use the resilient modulus of each layer in design process.

The modulus of subgrade reaction (k) is a required parameter for the design of rigid pavements [AASHTO, 1986 & 1993]. It estimates the support the layers below a rigid pavement surface course. The modulus of subgrade reaction is determined from the field plate bearing load test [Huang, 1993]. However, the field plate bearing load test is elaborate and time consuming. Recently, resilient modulus is commonly applied for both flexible and rigid pavement in the design guide [AASHTO, 1993]. Therefore, it was necessary to develop a relationship between the modulus of subgrade reaction (k) and the roadbed soil resilient modulus. For instance, a theoretical relationship was developed in the AASHTO Guide for Design of Pavement Structures [AASHTO, 1993, Vol. II]. Some other relationships based on the Long Term Pavement Performance (LTPP) database were established in the Mechanistic-Empirical Pavement Design Guide (MEPDG) [ARA, 2004] and elsewhere [Setiadji and Fwa, 2009, ASCE]. However, there has been no research on using field measured k values from actual pavement sites to evaluate and calibrate the established theoretical relationship between the modulus of subgrade reaction (k) and the soil resilient modulus obtained from laboratory resilient modulus test.

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In Florida, several research projects in the past years have been conducted to study the resilient modulus characteristics of Florida pavement soils [Ping et al., 2000; Ping et al., 2001; Ping and Ling, 2007; Ping and Ling 2008]. Typical subgrade soils used for pavement construction in Florida were excavated and obtained from actual field sites for evaluation. A laboratory triaxial testing program was carried out to evaluate the resilient modulus of subgrade materials. The effects of moisture and soil properties on the resilient properties of subgrade materials were evaluated. In conjunction with the laboratory triaxial testing program, a full scale laboratory evaluation of the subgrade performance was conducted in a test-pit facility, which simulates the actual field conditions. The subgrade and base layer profile of a full-scale flexible pavement system was simulated in the test-pit facility. The subgrade materials were tested in the test-pit under various moisture conditions simulating different field pavement moisture conditions. In addition, and extensive field static plate bearing load testing program was also conducted to evaluate the bearing characteristics of pavement base, subbase, and subgrade soils [Ping, Yang, and Gao, 2002, ASCE]. Comparative studies were conducted to evaluate the resilient modulus from laboratory cyclic triaxial test and field experimental studies such as field plate bearing load test, falling weight deflectometer (FWD) test, and test-pit cyclic plate load test.

Conducting the soil resilient modulus test in laboratory and selecting an appropriate resilient modulus value for pavement design are very complex processes. The processes are even much more time consuming, labor intensive, and costly on conducting in-situ field plate bearing load test and obtaining field measured k values. The field plate bearing load test cannot be conducted at various moisture contents and densities to simulate the different service conditions or the worst possible condition during the design life. Thus, it was necessary to develop a correlation between the modulus of subgrade reaction (k) and the soil resilient modulus [AASHTO, 1993]. In view of the past experimental studies conducted in Florida relating the laboratory resilient modulus with the field plate bearing load test results, there seems to be a need in re-examining the field experimental studies to evaluate and calibrate the established theoretical relationship between the modulus of subgrade reaction and soil resilient modulus.

3

1.2 Scope of Study The objectives of this research study were to evaluate the load-deformation and

resilient modulus characteristics of the granular subgrade soils based on the past field and laboratory experimental studies and to further correlate the field test results with the laboratory resilient modulus measurements. The targeted goal is two-fold: a) to establish the correlation relationships between the measured soil resilient modulus using laboratory triaxial test and the resilient modulus measured using plate load test in test-pit facility; b) to re-evaluate the correlation relationships between the subgrade soil resilient modulus and the modulus of subgrade reaction (k) using field measured experimental results. The theoretical relationship from AASHTO [AASHTO, 1993] will be evaluated based on the experimental studies. The calibrated correlation relationships could be utilized in the Florida pavement design guide for obtaining the realistic resilient modulus values from laboratory resilient modulus measurements.

1.3 Report Organization This report summarizes the experimental programs, test results, and analyses of

the research study to evaluate the subgrade soil modulus. As an introduction, the background and objectives of this research study are presented in this chapter. A literature review of the research related to the study of subgrade soil modulus is summarized in Chapter 2. The past experimental programs, including a description of test equipment, test setup, and test procedure for the laboratory and field tests, are presented in Chapter 3. Chapter 4 summarizes and analyzes the experimental results of laboratory and field tests. After that, Chapter 5 presents the correlation relationships between laboratory test results and field test results. Finally, conclusions and recommendations of this research study are presented in Chapter 6.

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CHAPTER 2

LITERATURE REVIEW

2.1 General The 1993 AASHTO Guide for the Design of Pavement Structures has

incorporated the resilient modulus of component materials into the design process [AASHTO, 1993]. Considerable attention has also been given to the development of mechanistic-empirical approaches for the design and evaluation of pavements [ARA, 2004]. The literature survey will proceed in five sections. In the first section, the basic concepts of resilient modulus and modulus of subgrade reaction will be reviewed. Then, the experimental evaluation of resilient modulus using laboratory and field tests will be discussed. This is followed by a review of evaluation of factors affecting the resilient modulus of subgrade soils. After that, empirical resilient modulus models used to predict the resilient modulus values are reviewed. Finally, the research studies on the modulus of subgrade reaction will be discussed.

2.2 Basic Concepts 2.2.1 Soil Resilient Modulus

The resilient modulus is the elastic modulus to be used with the elastic theory. Most pavement materials, especially soils, are not pure elastic material, but exhibit elastic-plastic behavior. That means they act partly elastic under a static load but experience some permanent deformation. However, under repeated loads, they express other important properties. At the beginning, they perform just like they would under a static load. But after certain repetitions, the permanent deformation under each load repetition is almost completely recoverable. By this point, it can almost be considered elastic, if the repeat load is small enough compared to its strength, otherwise the soil structure would be damaged. Figure 2.1 illustrates the behavior of unbounded material under a sequence of repeating loads. Resilient modulus is a measurement of the elastic property of soil recognizing certain nonlinear characteristics, which is defined as the

5

ratio of the axial deviator stress to the recoverable axial strain, and is presented in the following equation:

r

dRM

= (2.1)

Where d = axial deviator stress r = axial recoverable strain This concept is derived from the fact that the major component of deformation induced into a pavement structure under the traffic loading is not associated with plastic deformation or permanent deformation, but with elastic or resilient deformation. Thus, the resilient modulus is considered to be a required variable for determining the stress-strain characteristics of pavement structures subjected to a traffic loading. Many factors influence the resilient modulus of soils. Moisture is one of the factors affecting the modulus of soils. The factors that influence the resilient modulus of soils also include the following: soil type, soil properties, dry unit weight, strain level, and test procedures. Thus, the resilient modulus test procedure and the selection of an appropriate design resilient modulus value for pavement subgrades have been significantly complicated. The lack of meaningful correlations between the field and laboratory resilient modulus values further complicates the issue.

2.2.2 Modulus of Subgrade Reaction The modulus of subgrade reaction (k) is a required parameter for the design of rigid pavements. It estimates the support the layer below a rigid pavement surface course. The modulus of subgrade reaction is determined from a field plate bearing load test with a circular plate (Figure 2.2). The load is applied at a predetermined rate until a pressure of 10 psi (69 kPa) is reached. The pressure is held constant until the deflection increase not more than 0.001 in. (0.025 mm) per minute for three consecutive minutes. The average of the three dial readings is used to determine the deflection. The modulus of subgrade reaction (k) is given by

6

=

pkmeasured (2.2)

Where p = pressure on the plate, or 10 psi = deflection of plate in inch The k-value is determined from the field tests. Since the plate loading test is time-consuming and expensive, it cannot be conducted at various moisture contents and densities to simulate the different service conditions or the worst possible condition during the design life. It is necessary to develop a relationship between the modulus of subgrade reaction (k) and the subgrade soil resilient modulus (MR). This allows the designer to treat the seasonal variation of the subgrade soil k-value by simply converting the same seasonal resilient modulus that would be used for flexible pavement design.

2.3 Resilient Modulus Tests It is well known that subgrade soils are nonlinear with an elastic modulus varying with the level of stress. The elastic modulus to be used with the layered systems is the resilient modulus obtained from repeated unconfined or triaxial compression tests [3]. The resilient modulus of unstabilized subgrade soils is highly dependent upon the stress state to which the material is subjected within the pavement in addition to other variables. As a result, constitutive models including the effect of stress state must be used to present laboratory resilient modulus test results, in a form suitable for use in pavement design. The resilient modulus depends on deviator stress and confining stress. Two popular and simple regression models are presented as follows:

1. when modulus is dependent on bulk stress:

21

kR kM = (2.3)

2. when modulus is dependent on confining pressure:

433

kR kM = (2.4)

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Where = bulk stress, sum of the principal stresses, (1+ 2+ 3) 3 = confining pressure or minor principal stress k1, k2, k3, k4 = regression constants The U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) conducted resilient modulus tests on materials from the MN/ROAD test site for the Minnesota Department of Transportation [Berg et al., 1996]. Laboratory resilient modulus tests were conducted on pavement materials to characterize their behavior under seasonal frost conditions, and to provide input necessary for modeling the materials with the Mechanistic Pavement Design and Evaluation Procedure. It was found that the modulus of all of the materials was stress dependent and increased as the degree of saturation decreased. Maher et al. [Maher et al., 2000] developed a laboratory testing program to determine the resilient modulus of typical New Jersey subgrade soils. A total of eight soils were tested at different levels of molded water content to determine their sensitivity to moisture content and cyclic stress ratio. Laboratory results were used to calibrate a statistical model for effectively predicting the resilient modulus of subgrade soils at various moisture content and stress ratios. Kim and Kim [Kim and Kim, 2007] developed a simplified repeated triaxial test procedure to investigate the typical sandy-silty-clay and silty-clay subgrade soils encountered in Indiana. It was shown that the simplified procedure was feasible and effective for design purpose.

Recently, several field tests were involved to study the resilient behavior of subgrade soils. These field tests include, among others, falling weight deflectometer (FWD), dynamic cone penetrometer (DCP), plate load test, and so on. The field tests results were found to be comparable with the laboratory measured modulus. Newcomb et al. [Newcomb et al., 1995] conducted tests at MN/ROAD in 1994 including both FWD and DCP examinations to compare with Long Term Pavement Performance (LTPP) laboratory resilient modulus values. FWD results showed high variability due to varying surface conditions, soil moisture content, modulus values. DCP results compared well with laboratory values, but no correlations were identified. George and Uddin [George and Uddin, 2000] studied the resilient modulus of subgrade using dynamic cone

8

penetrometer (DCP) and falling weight deflectometer (FWD) tests. Twelve as-built test sections reflecting typical subgrade soil materials of Mississippi were selected and tested with DCP and FWD before and after pavement construction. Undisturbed samples were tested in repeated triaxial machine for resilient modulus. It was found that the laboratory as well as backcalculated subgrade modulus calculations were usable in the field.

Andrew et al. [Andrew et al., 1998] used falling weight deflectometer test to determine the seasonal variation in subgrade resilient modulus, and to develop a rational approach for the selection of a unique design season resilient modulus. The seasonal variation in FWD response was also compared to the measured variation in subgrade moisture, and the resilient modulus was predicted from existing correlations between index properties and soil moisture content. Bandara and Rowe [Bandara and Rowe, 2002] carried out a study to determine the relationships between laboratory determined subgrade resilient modulus and the results of Limerock Bearing Ratio (LBR) and FWD tests for certain Florida subgrade soils. FWD tests were conducted along the selected roadways and LBR tests were conducted on bulk subgrade soil samples. Preliminary relationships from FWD and LBR tests were developed for considered typical pavement sections. Flintsch et al. [Flintsch et al., 2003] found strong correlations between laboratory resilient modulus tests and backcalculated resilient modulus values from in-situ FWD measurements for unbound granular materials from 12 sites in Virginia. George et al. [George et al., 2004] explored a method for correlating FWD moduli with triaxial test laboratory moduli in a selected pattern on subgrade sections to ensure accuracy even in conditions of nonhomogenous subgrades. Mohammad et al. [Mohammad et al. 1999] evaluated the resilient modulus of subgrade soils by cone penetration test. Field and laboratory testing programs were carried out on two types of cohesive soils in Louisiana. A model was proposed to estimate the resilient modulus from the CPT data and basic soil properties. Predicted values of the resilient modulus were consistent with laboratory measurements. Tests on soil samples and at 12 field sites in Mississippi found that DCP index could be correlated in different ways for fine grained and coarse-grained soils, and that regression models could be improved by testing for other physical properties [Rahim and George, 2002].

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2.4 Factors Affecting Resilient Modulus As mentioned earlier, many factors influence the resilient modulus of soils.

Tremendous amount of research work was developed to investigate the effects of those factors. Hicks and Monismith [Hicks and Monismith, 1971] analyzed the factors that may affect the resilient modulus of granular material. They found that the following factors may have a significant influence on the stress-deformation characteristics under short-duration repeated loads: 1) stress level (confining pressure), 2) degree of saturation, 3) dry density, 4) fine content, and 5) load frequency and duration. Burczyk et al. [Burczyk et al., 1994] conducted laboratory testing on subgrade cores obtained from 9 test sites in Wyoming. Several fundamental soil properties of these cores were determined and deflection data were used to determine resilient modulus values with backcalculation programs. The data analysis resulted in several important conclusions about factors that influencing the selection of a design subgrade resilient modulus value.

The effect of moisture content is considered a major factor which may change the value of resilient modulus, which has been noticed a long time ago. Seed et al. [Seed et al., 1962] noted a rapid increase in resilient deformations for specimens of the AASHO road test subgrade soils compacted with water content above the optimum level. For specimens compacted below optimum water content, resilient deformations were characteristically low. Thompson and Robnett [Thompson and Robnett, 1976] summarized the effect of an AASHO road test on subgrade soil. It was found that the resilient modulus decreases as moisture increases. Barksdale et al. [Barksdale et al., 1989] prepared a report about the laboratory determination of resilient modulus for flexible pavement design due to the moisture sensitivity of resilient modulus. Drumm et al. [Drumm et al., 1997] summarized their tests of the effect of saturation on resilient modulus. All soils exhibited a decrease in resilient modulus with an increase in saturation, but the magnitude of the decrease in resilient modulus was found to depend on the soil type. More experimental studies have been carried out in recent years to evaluate the effect of moisture content on the resilient modulus of subgrade soils [Mohammad et al., 2002; Li and Qubain, 2003; Khoury and Zaman, 2004; Stolle et al., 2006; and Cortez, 2007]. The effects of subgrade soil moisture content on subsurface

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strain and stress distributions were explored. It was found that the resilient modulus decreases as moisture increases. The resilient modulus of fine-grained soil is dependent on moisture content and the degree of saturation has a significant influence on the stress-deformation characteristics of subgrade materials.

The resilient modulus is also significantly influenced by the type of pavement soils. Chen et al. [Chen et al., 1994] investigated the variability of resilient moduli due to aggregate type. It was shown that for a given gradation, the difference in MR values due to aggregate sources were between 20 to 50%. Thompson and Robnett [Thompson and Robnett, 1976] concluded that soil properties that tend to contribute to low resilient modulus values are low plasticity, high silt content, low clay content and low specific gravity. From their study, regression equations were developed for predicting MR based on soil properties. The effect of dry density on the resilient response of subgrade soils was investigated by Trollope et al. [Trollope et al., 1962]. They reported that the resilient modulus of dense sand might be 50% higher than that of loose sand. The strain level also had an important effect on the resilient modulus. As the strain amplitude increased, the modulus of the soil decreased [Kim, 1991].

AASHTO T292-91I and T294-92 were two of the most extensively used test procedures. A new standard specification AASHTO T307-99 based on the SHRP Protocol P46 was adopted in 2000. The major improvement includes higher accuracy of the measurement devices, different measurement position, different confining and loading stress, and the specimen preparation method. Zaman et al. [Zaman et al., 1994] found that the T294-92 test procedure gave higher resilient modulus values than those obtained by using the T292-91I test procedure. Ping and Hoang [Ping and Hoang, 1996] had similar results. This phenomenon was attributed to the stress sequence, which had a stiffening and strengthening effect on the specimen structure as the stress level increased. Ping and Xiong [Ping and Xiong, 2003] investigated the influence of the LVDT positions on the resilient modulus test results. The investigation showed that the internal LVDT position leads to a better test result than that with an external position, and the internal full-length LVDT position has the most reliable test data.

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2.5 Empirical Resilient Modulus Models A very promising approach for obtaining resilient modulus for use in design, for at

least most agencies, is to determine values of resilient modulus using generalized empirical relationships with statistically relevant, easy to measure physical properties of the material. Considering the large variation in resilient modulus along the route and important design changes in moisture with time, the use in design of empirical resilient modulus relationships is considered to be justified. A number of states have already developed generalized resilient modulus relationships for use in design, particularly for cohesive subgrade soils. Statistically based equations, graphs or chars would then be developed for each class of materials for the range of properties routinely used in design within the region of interest.

Seed et al. [Seed et al., 1962] evaluated the influence on the undisturbed samples of the fine grained materials and found a relationship between the resilient modulus and the volumetric water content. The results from this model did not work well since the resilient modulus is based on only one parameter in this model, which contributes to inaccurate results. Carmichael and Stuart [Carmichael and Stuart, 1978] produced more than 3300 test data on 250 different types of fine-grained and granular materials to build the resilient modulus models. Two models were developed, one for fine-grained soils and the other for coarse-grained soils. Using the power model to express resilient modulus is a practical alternative to the slightly more accurate bilinear model. The bilinear resilient modulus model for fine-grained soils has a distinct breakpoint. The resilient modulus at the breakpoint was estimated by Thompson [Thompson, 1992]. Yau and Von Quintus [Yau and Von Quintus, 2002] studied the methods of choosing the right data for building the resilient modulus prediction models. They found that one equation did not fit all situations for all the soils. For greater accuracy, they tried to establish the model according to different material types. The prediction models for the subgrade soils were developed based on the constitutive equation with the regression constants k1, k2, and k3, which are material-specific. Zhang [Zhang, 2004] proposed a resilient modulus prediction model based on five types of granular subgrade soil commonly available in Florida. Several other prediction models, which correlate the laboratory resilient modulus data with the field data, were developed

12

in the different states in the recent days [Herath et al., 2005; Malla and Joshi, 2008; Rahim and George, 2005; Rahim, 2005; and Han et al., 2006]. These methods allow designers and engineers to reduce their reliance on the expensive, time-consuming and difficult process of testing resilient modulus. A summary of these resilient modulus prediction models are listed in Table 2.1; wherein, MR = Resilient Modulus, MC = moisture content (%), = bulk stress (1+ 2+ 3), 3 = confining pressure, d = deviator stress, PI = plastic index (%), LL = liquid index (%), P200 = percentage passing #200 sieve (%), %clay = % particles finer than 2 micron size, Pa = normalizing stress, oct = octahedral shear stress, Cu = uniformity coefficient, Cc = coefficient of curvature, k1, k2, and k3 = regression parameters.

2.6 Modulus of Subgrade Reaction The term subgrade reaction indicates the pressure between a loaded beam or

slab and the subgrade on which it rests and on to which it transfers loads. The factors which determine the value of the coefficient of subgrade reaction was evaluated by Terzaghi [Terzaghi, 1955]. The concept of modulus of subgrade reaction was introduced to account for the stress-dependent behavior of typical subgrade soil by Fischer et al. [Fischer et al., 1984]. It is usually impractical to conduct plate bearing load tests in the field on representative subgrade soils for design projects. Thus, a theoretical relationship between the k value and resilient modulus was developed in the Appendix HH of the AASHTO design guide [AASHTO, 1993], which is as follows:

4.19)psi()pci( Rmeasured

Mk = (2.5)

It should be noted that this theoretical relationship was developed based on the assumption that the roadbed material is linear elastic. Elastic layer theory and equations provide the basis for establishing the relationship.

Recently, Kim et al. [Kim et al., 2007] adopted the portable falling weight deflectrometer to evaluate material characteristics of well-compacted subgrades. In addition, the static plate-bearing load test was used to evaluate the modulus of

13

subgrade reaction. The test results show that there is a reasonable linear correlation between the dynamic deflection modulus and the modulus of subgrade reaction of well-compacted subgrades. A comparative study of methods of determination of modulus of subgrade reaction was discussed by Sadrekarimi and Akbarzad [Sadrekarimi and Akbarzad, 2009]. Khazanovich et al. [Khazanovich et al., 2001] performed backcalculation analysis on the deflection data for the rigid pavements from LTPP database. A theoretical pavement system of an infinite pavement slab supported on a dense-liquid foundation was used to estimate the k-values. The following relationship was obtained

(MPa)296.0(MPa/m) Ek = (2.6)

The k-E relationships generated by the MEPDG software can be closely represented by Equation (2.6). It should be noted that the k-value in Equation (2.6) is for full scale pavement slab but not the measured k value with 30 in. diameter plate. Setiadji and Fwa [Setiadji and Fwa, 2009] proposed a procedure to estimate modulus of subgrade reaction (k) from elastic modulus (E) by establishing an equivalency between two theoretical pavement models, a model of pavement slab supported by an elastic solid foundation and a model of pavement slab supported by a dense liquid foundation. Using the deflection test data of the LTPP database, it was found that there exists a relationship between the radii of relative stiffness of the two theoretical systems, which can be used to estimate k from given E value. This model was also compared to the k-E relationships derived previously by other researchers.

14

Table 2.1 Summary of Resilient Modulus Prediction Models Reference Prediction Model

Seed [Seed et al., 1962] pcfforM

pcfforMd

MCR

dMC

R

100004.018.18100006.006.27

=

Carmichael and Stuart [Carmichael and Stuart, 1978]

Coarse-Grained Soil:

GRSMMCM R

197.0173.0log544.0025.0523.0log +++=

Fine-Grained Soil:

MHCHPMCPIM

d

R

097.17722.363248.0179.01424.06179.04566.0341.37

3

200

++

+=

Thompson [Thompson, 1992]

PIclayM R 119.0%098.046.4 ++=

Yau and Von Quintus [Yau and Von Quintus, 2002]

Malla and Joshi [Malla and Joshi, 2008]

32

1

k

a

oct

k

a

aR PPPkM

=

Zhang [Zhang, 2004]

cu

dR

CCMCM

119.00243.00108.0041.001.2ln max,

++=

Rahim and George [Rahim and George, 2005] Coarse-Grained Soil:

2

111

k

daR PkM

++=

Fine-Grained Soil: 2

111

k

c

daR PkM

++=

Rahim [Rahim, 2005]

Coarse-Grained Soil: 4652.0

2008998.0

log114.324

+=

u

dR C

PMC

M

Fine-Grained Soil:

+

+=

609.0200

18.2

max, 100129.17 P

MCLLM dR

15

Table 2.1 Summary of Resilient Modulus Prediction Models - Continued where, MR = Resilient Modulus,

MC = moisture content (%), = bulk stress (1+ 2+ 3), 3 = confining pressure, d = deviator stress, PI = plastic index (%), LL = liquid index (%), P200 = percentage passing #200 sieve (%), %clay = % particles finer than 2 micron size, Pa = normalizing stress, toct = octahedral shear stress, Cu = uniformity coefficient, Cc = coefficient of curvature, k1, k2, and k3 = regression parameters.

16

Figure 2.1 Illustration of Soil Behavior under Repeated Loads.

17

Figure 2.2 Field Plate Bearing Load Test Schematic

18

CHAPTER 3

EXPERIMENTAL PROGRAMS

3.1 General Several field and laboratory experimental studies were conducted in Florida to evaluate the resilient modulus characteristics of Florida pavement soils [Ping et al., 2000; Ping et al., 2001; Ping and Ling, 2007; Ping and Ling, 2008]. An extensive field static plate bearing load testing program was carried out to evaluate the in-situ bearing characteristics of pavement base, subbase, and subgrade soils [Ping, Yang, and Gao, 2002]. Typical subgrade soils were excavated from the field test sites and obtained for laboratory resilient modulus evaluation. A full scale laboratory evaluation of the subgrade performance was also conducted in a test-pit facility, which simulates the actual field conditions. The subgrade and base layer profile of a full-scale flexible pavement system was simulated in the test-pit facility. The subgrade materials were tested in the test-pit using cyclic plate bearing load test under various moisture conditions simulating different field pavement moisture conditions [Ping and Ling, 2008]. In conjunction with the field and full-scale laboratory experimental programs, a laboratory triaxial testing program was performed to evaluate the resilient modulus characteristics of the subgrade materials [Ping et al., 2002; Ping and Ling, 2008]. Subsequently, comparative studies were conducted to evaluate the resilient modulus from laboratory cyclic triaxial tests and field experimental studies. The field and laboratory experimental programs are presented as follows.

3.2 Field Experimental Program The primary objective of the field experimental program was to characterize the in-situ bearing behavior of pavement layers on selected types of pavement soils in Florida. To achieve this objective, a series of tests were conducted on several flexible pavement sites around Florida. The sites were evenly scattered within the state to better

19

represent different soil conditions in Florida (Table 3.1). The selection of sites took into account the following considerations: a) soil type and history, b) pavement layer homogeneity, c) layer thickness, d) field operational considerations. Granular materials (A-3 and A-2-4 soils) were most commonly encountered as roadbed in Florida. Thus, only the granular soils were analyzed in the field study [Ping et al., 2000]. Many different types of tests have been devised for measuring the characteristics of pavement structures in place. In Florida, plate bearing and Falling Weight Deflectometer (FWD) tests are commonly used for evaluating pavement structures at the FDOT. While the field plate bearing test is a destructive test, FWD is a non-destructive test. For this field study, these tests were used to determine the modulus of elasticity of pavement layers at the tested site under its natural moisture content and density. A Dynatest 8000 FWD system was used for the field testing program. The system consists of a Dynatest 8002E FWD trailer, a table top computer with a printer, and a Dynatest 8600 system processor interfaced with the FWD trailer as well as with the computer. A certain weight is mounted on a vertical shaft and housed in the trailer. The weight is hydraulically lifted to a predeterminated height and then dropped onto a rubber buffer system resulting in a load pulse of 25 to 30 msec. The load is applied to the surface of pavement through a load cell and a circular plate. The standard load plate has a 300 mm (11.8 in.) diameter. By use of different drop weights and heights, it can vary the impulse load to the pavement structure from about 6.7 to 120 kN (1,500 to 27,000 lb.) For flexible pavement, drops result in loads of approximately 27 kN (6,000 lb.), 40 kN (9,000 lb.), 53 kN (12,000 lb.), and 71 kN (16,000 lb.). The Dynatest 8000 has seven geophones spaced at radial distances of 0, 8, 12, 18, 24, and 36 inches apart from the center of the loading plate. The first sensor is always mounted at the center of the foot plate. Peak values of surface deflections are collected by the sensors and stored in or printed by the computer system. The FWD test setup is shown in Figure 3.1. After completion of the FWD test at each site, the in-service pavements were trenched. The asphalt concrete structural layer was cut, approximately 1.6783.355 m (5.511 ft.), and removed. For each layer of the pavement beneath the asphalt concrete,

20

including the base and stabilized subgrade (subbase), the in situ moisture content and density were measured using a nuclear gauge device. The speedy moisture content test was also conducted to check the nuclear moisture content data. In Florida, the speedy moisture content test is designated as FM 5-507 in the Manual of Florida sampling and testing methods [FDOT, 2001]. The layer thickness was measured to determine the vertical pavement profile, which was used later in the FWD backcalculation program. Representative bag samples of each layer were taken for future testing of the resilient modulus in the laboratory.

The plate bearing load test was conducted on the subgrade (embankment) layer (Figure 3.2). The test procedures employed may vary somewhat, depending on the adoptive agencies, but the method is generally in close agreement with ASTM D 1196 [ASTM, 2004]. In Florida, the plate load test is designated as FM 5-527 in the Manual of Florida sampling and testing methods [FDOT, 2000]. The test apparatus consisted of a water tanker with a total capacity of 27,240 kg (60,000 lb.) and a hydraulic jack with a spherical bearing attachment that was capable of applying and releasing the load increments. The hydraulic jack had sufficient capacity for applying the maximum load required and was equipped with an accurately calibrated gauge, which indicates the magnitude of the applied load. A 305 mm (12 in.) diameter circular steel plate was used for applying the load [ACI, 2006]. A schematic illustration of the test setup is shown in Figure 3.2.

An aluminum alloy deflection beam was used to mount two graduated (in units of 0.001 inch) dial gauges for measuring deflections. Prior to applying the incremental testing loads, three seating loads were applied to seat the loading system and bearing plate. Each seating load was to produce a total deflection of about 0.762 m (0.03 in.). Each of the three seating loads was applied in four or five equal increments. After each increment of test load had been applied, the deflection was allowed to continue until a rate of no more than 0.0254 mm/min (0.001 in./min). The load and deflections were then recorded. This procedure continued until the average total deflection of 1.27 mm (0.05 in.) plus the average rebound deflection from the third seating load had been reached. The moisture and density of the subgrade layer were measured before the test and the soil samples were taken after the plate bearing load test. After completion of the plate

21

bearing load test program, the subgrade soil layer was excavated up to more than 1 m below the tested stratum to check the layer homogeneity.

After completion of the field tests, representative samples of the subgrade materials were then obtained and transported to the laboratory. The basic properties of each soil were evaluated in the laboratory prior to the resilient modulus test. All soil materials were reconstituted in the laboratory to the in situ moisture and density conditions, and the optimum conditions for the resilient modulus test. Two replicate resilient modulus tests were conducted on each type of soil. This was done to investigate the repeatability of the test and to ensure the validity of the resilient modulus test results.

3.3 Test-pit Experimental Program The purpose of test-pit experimental program was to evaluate the resilient modulus of the subgrade materials with changing groundwater levels. The test-pit evaluation of subgrade soils served the following advantages:

(1) The test-pit can be used to simulate the different material components of a pavement system on a full-scale basis.

(2) The test-pit can facilitate the change of water level so as to simulate the different moisture conditions in a practical situation.

(3) Together with a loading system, the test can be used to investigate the deformation characteristics of subgrade materials under the influence of static and dynamic loads.

3.3.1 Test Materials The subgrade materials under evaluation in the test-pit test were the typical A-3 (fine sand with %fines < 10% ) and A-2-4 (silty or clayey sand with %fines < 35%) commonly in use in Florida. A total of ten types of subgrade soil representing the percent of fines passing No. 200 sieve, which range from 4% to 30%, were evaluated in three stages of testing (Phase I, Phase II, and Phase III). The pertinent characteristics of the subgrade soils are presented in Table 3.2. The test materials were tested using

22

both the test-pit facility and the laboratory triaxial test under the same density and moisture conditions for the future comparison.

3.3.2 Test-pit Setup The complete setup of test-pit experiment is mainly comprised of two parts: full-scale test-pit and loading system. The schematic is shown in Figure 3.3. The FDOT test-pit is shaped like a rectangular reinforced concrete vessel that is 7.31 m (24 ft.) long, 2.44 m (8 ft.) wide, and 2.13 m (7 ft.) deep. Below the subgrade material was the standard embankment that was composed of three layers of different materials. The bottom layer was composed of a bed of 305 mm (12 in.) river gravel that facilitated the upward percolation of ground water. A builders sand layer that was 305 mm (12 in.) thick rested upon the river gravel and was kept separated with gravel by a permeable filter fabric. The third layer was a 305 mm (12 in.) depth of standard A-3 soil that was used as the top layer of simulated embankment. The test-pit was surrounded by a sump with an interconnecting channel system for controlling the water table. A hydraulic loading device was attached to an over-hanging 24 WF Beam which facilitated the transverse movement of the loading device, while the 24 WF beam itself traveled longitudinally above the test-pit, thus providing a two-dimensional selection of loading location. A standard 305 mm (12 in.) diameter rigid plate was used to simulate the single wheel load upon the tested soil. Vertical deformations of the soil were measured through linear variable displacement transducer (LVDT). To best simulate the dynamic impact of moving vehicles on the subgrade, the plate loads were conducted in a cyclic manner, on second per cycle with loading periods of 0.1 and 0.9 seconds for the rebound of tested materials. This was consistent with the loading frequency used in laboratory triaxial resilient modulus test. In order to achieve a certain deformation curve with respect to the number of load cycles, 30,000 load cycles were conducted.

3.3.3 Test Sequence The subgrade layer was prepared by achieving the maximum dry unit weight with

a vibratory compactor for each layer. The water content and unit weight of each layer was recorded upon compaction. After the water level was set to the desired height, a

23

TDR probe was deployed for measuring the moisture content within each layer of subgrade material in the test pit. Sufficient time was allowed to ensure that the moisture equilibrium was completed through capillary action at each water level. The base clearance, which is defined as the clearance or separation between the groundwater level and pavement base layer within a pavement system, is also introduced to evaluate the water level.

The groundwater level simulation was arranged as follows and is illustrated in Figure 3.4:

1. Drained condition water level was at 610 mm (24 in.) below the top of the embankment (i.e., 1.5 m/5.0 ft. base clearance)

2. Optimum condition water level was at the top of the embankment (i.e., 0.9 m/3.0 ft. base clearance)

3. Wet condition 1 water level was at 305 mm (12 in.) above the embankment (i.e., 0.6 m/2.0 ft. base clearance)

4. Wet condition 2 water level was at 610 mm (24 in.) above the embankment (i.e., 0.3 m/1.0 ft. base clearance)

5. Soaked condition water level was at the top of the subgrade test material (i.e., 0.0 m/0.0 ft. base clearance)

A standard 305 mm (12 in.) diameter rigid plate was used to simulate the single wheel load upon the tested soil. Two load types were applied: 137.8 kPa (20 psi) without a base layer and 344.5 kPa (50 psi) with a 127 mm (5 in.) limerock base layer in place. The soils were tested under 137.8 kPa (20 psi) loading first; then the base layer was added and the 344.5 kPa (50 psi) loading was applied. The test sequence was arranged as follows and is illustrated in Figure 3.4:

Phase I soils (A-3): at water levels A, B, C, F, and H Phase I soils (A-3 and A-2-4): at water levels B, C, F, and H Phase II soils: at water levels A, B, C, F, and H Phase III soils: at water levels B, C, D, E, F, G, H, I, and J

24

3.4 Laboratory Experimental Program The primary objective of the laboratory experimental program was to evaluate the

resilient modulus characteristics of the Florida subgrade soils. To achieve the objective, samples of soil material were collected from the field test sites and from the test-pit test program. The subgrade soils were transported to the laboratory in Tallahassee for evaluation. The laboratory experimental program includes the test materials, the resilient modulus testing program, and the engineering property analysis [Ping et al., 2000; Ping and Ling, 2008].

3.4.1 Test Materials The soil materials consisted of A-3 soils, A-2-4 soils, and A-2-6 soils. The basic

properties of each soil were evaluated in the laboratory prior to the resilient modulus test. A summary of the soil materials that were tested for basic properties characterization and classification is presented in Table 3.2 and Table 3.3 for information.

3.4.2 Resilient Modulus Testing Program For the resilient modulus test, all of the test protocols require the use of a triaxial chamber, in which confining pressure and deviator stress can be controlled. The test method for determining the resilient response of pavement materials is basically a triaxial compression test, in which a cyclic axial load is applied to a cylindrical test specimen. The load is measured by a load cell, while the resilient strain is measured. The test is usually conducted by applying a number of stress repetitions over a range of deviator stress levels and confining pressure levels representing variations in depth or location from the applied load. An MTS model 810 closed-loop servo-hydraulic testing system and a resilient modulus triaxial testing system were used in this study. The schematic of configuration of the test system is shown in Figure 3.5. The major components of this system include the loading system, digital controller, workstation computer, triaxial cell, and linear variable differential transducer (LVDT) deformation measurements system.

25

The schematic of triaxial cell setup is shown in Figure 3.6. In addition to two vertical LVDTs measuring the vertical displacements, a fixture attached with four horizontal LVDTs was designed to measure the horizontal displacements for the determination of Poissons ratio. This device was positioned in the middle half length of the specimen with four horizontal LVDTs attached at 90-degree intervals. The horizontal displacement can be obtained by averaging the measurements from the four horizontal LVDTs. The tests were performed using the AASHTO T292-91I [AASHTO, 1991] test standard for the Phase I and II soils, with both middle-half and full-length LVDT position measurements. The AASHTO T307-99 [AASHTO, 2003] test standard was used for the Phase III soils with full-length LVDT position measurement. The AASHTO T307-99 method was considered an improvement to the AASHTO T292-91I method. The T307-99 method covers procedures for preparing and testing untreated subgrade and untreated base/subbase materials for determination of resilient modulus under conditions representing a simulation of the physical conditions and stress states of materials beneath the flexible pavements subjected to moving wheel loads. The original setup of LVDTs in Designation T307-99 was at an external position outside of the triaxial cell, but the location was changed to an internal position inside the cell for a better test results. As for the compaction effort, the 100% Modified Proctor was used for the Phase I and Phase II soils in accordance with AASHTO Designation T 180, while the 100% Standard Proctor was used for the Phase III soils in accordance with AASHTO Designation T 99. An updated controller with its advanced software was used to control the test processes and acquire the test data. The raw data was acquired using the mode of the Peak/Valley, which recorded the test data at its peak and valley levels of each cycle. The output of the data acquisition system includes a graphic display of sampled dynamic load and displacement waveforms and a data file. The data in data file format was selected for further data deduction and analyses.

3.4.3 Test Procedure The resilient modulus test procedures were basically followed from AASHTO T292-91I and AASHTO T307-99. The comparison of resilient modulus test procedures

26

for granular soils is shown in Table 3.4. Since the laboratory resilient modulus simulates the conditions in the pavement subgrade, the stress-state should be selected to cover the expected in-service range. Resilient properties of granular specimens should be tested over the range of confining pressures expected within the subgrade layer. A template was created to monitor the test sequences. In the test sequences, the confining pressures decrease while the deviator stresses increase during each confining pressure stage. The procedures are described as follows:

Open a template and apply 50 repetitions (T292-91I) or 100 repetitions (T397-99) of the smallest deviator stress at the highest confining pressure. The average recoverable deformation of each repetition is recorded automatically.

Apply the same repetitions of each of the remaining deviator stresses to be used at the present confining pressure.

Decrease the confining pressure to the next desired level and adjust the deviator stress to the smallest value to be applied at this confining pressure.

Increase the deviator stress to the next desired level and continue the process until testing has been completed for all desired stress states.

Disassemble the triaxial chamber and remove all apparatus from the specimen.

3.4.4 Determination of Resilient Modulus During the resilient modulus test, after finishing the specimen conditioning stage, a series of tests with different deviator stresses at different confining pressure were performed and the data were recorded for every cycle of each test. However, only the last five cycles of each test were used for analyses following the AASHTO procedures. The resilient modulus was calculated from the load and deformation using the following equation:

r

dRM

= (3.1)

Where d = axial deviator stress r = axial recoverable strain

27

Table 3.1 Summary of Test Sites for Field Plate Load Test Embankment Location Soil

Type Lee County, Site A A-3 Lee County, Site B A-3 Polk County, Site A A-3 Polk County, Site B A-3 Clay County, Site A A-3 Clay County, Site B A-3

Martin County, Site A A-3 Martin County, Site B A-3

Osceola County, Site A A-3 Osceola County, Site B A-3 Seminole County, Site B A-3 Gadsden County, Site A A-2-4 Seminole County, Site A A-2-4 Jefferson County, Site B A-2-4

28

Table 3.2 Engineering Properties of Subgrade Material for Test-Pit Test

Soil

Phase

Soil Classification

Passing No. 200 Sieve (%)

Clay Content (%)*

Optimum Moisture Content (%)

Maximum Dry Unit Weight

LBR

Permeability (cm/sec) AASHTO USCS pcf kN/m3

A3-1 I A-3 SW-SP 4 - 10.0 106.5 16.7 22 5.5E-03 A3-2 I A-3 SW-SP 8 6 11.5 112.0 17.6 45 2.1E-03 A24-1 II A-2-4 SC-SM 12 3 12.1 110.6 17.4 30 3.1E-04 A24-2 I A-2-4 SC-SM 14 10 10.5 122.0 19.2 124 2.5E-04 A24-3 III A-2-4 SC-SM 15 4 9.2 118.2 18.6 83 2.8E-04 A24-4 II A-2-4 SC-SM 20 8 10.0 124.4 19.5 146 1.0E-04 A24-5 III A-2-4 SC-SM 23 6 8.8 128.4 20.2 132 7.4E-07 A24-6 II A-2-4 SC-SM 24 5 10.7 116.3 18.3 69 6.5E-05 A24-7 II A-2-4 SC-SM 30 - 12.0 116.0 18.2 72 2.0E-05 A24-8 III A-2-4 SC-SM 30 16 10.3 123.3 19.4 127 5.6E-07

* Particles finer than 0.002 mm AASHTO T 180 and T 90 LBR: Limerock Bearing Ratio, LBR = 1.25*CBR (California Bearing Ratio) AASHTO D5084-90

29

Table 3.3 Summary of Material Properties for Laboratory Test Embankment Location Soil

Type Dry

Density (pcf)

Moisture Content

(%) Lee County, Site A A-3 107.9 10.8 Lee County, Site B A-3 105.0 13.3 Polk County, Site A A-3 108.7 12.4 Polk County, Site B A-3 110.2 10.8 Clay County, Site A A-3 104.7 12.8 Clay County, Site B A-3 101.7 12.4

Martin County, Site A A-3 106.3 9.6 Martin County, Site B A-3 113.4 9.5

Osceola County, Site A A-3 107.3 12.7 Osceola County, Site B A-3 103.4 12.8 Seminole County, Site B A-3 107.3 12.7 Gadsden County, Site A A-2-4 116.6 15.1 Seminole County, Site A A-2-4 108.5 8.4 Jefferson County, Site B A-2-4 131.1 8.3

30

Table 3.4 Comparison of Test Procedures for Granular Soils Test method AASHTO T292-91I AASHTO T307-99 Procedure Confining

Pressure Deviator Stress

Load Number

Confining Pressure

Deviator Stress

Load Number

Unit psi psi psi psi Conditioning 15 12 1000 6 4 500

1 15 7 50 6 2 100 2 15 10 50 6 4 100 3 15 15 50 6 6 100 4 10 5 50 6 8 100 5 10 7 50 6 10 100 6 10 10 50 4 2 100 7 10 15 50 4 4 100 8 5 3 50 4 6 100 9 5 5 50 4 8 100 10 5 7 50 4 10 100 11 5 10 50 2 2 100 12 2 3 50 2 4 100 13 2 5 50 2 6 100 14 2 7 50 2 8 100 15 2 10 100

31

Figure 3.1 Field Falling Weight Deflectometer Test Setup

32

Figure 3.2 Field Plate Bearing Load Test

33

(a)

(b) Figure 3.3 Test-pit System: (a) Overview of Test Pit; (b) Schematic Diagram of

Loading System & Cross Sectional View

Nuclear Gauge

TDR

34

River Gravel

Builder's Sand

Embankment Soil

Subgrade TestMaterial

TDR

Prob

e

137.8 kPa (20 psi ) Load

WT@-24 in.

WT@0 in.

WT@12 in.

WT@24 in.

WT@36 in.

A

B

C

D

E

F

G

H

I

J

137.8 kPa (20psi) Load w/oBase Layer

344.5 kPa (50psi) Load w/Limerock BaseLayer

344.5 kPa (50 psi) Load

River Gravel

Builder's Sand

Embankment Soil

Subgrade TestMaterial

Limerock Base Layer

Bas

e Cl

eara

nce

Figure 3.4 TestPit Test Sequence for Different Water Level Adjustments

35

Figure 3.5 Schematic of Laboratory Resilient Modulus Test Program

36

Figure 3.6 Schematic of Triaxial Test for Resilient Modulus Measurement

37

CHAPTER 4

SUMMARY OF EXPERIMENTAL RESULTS

4.1 Field Experimental Results The field experimental Program was conducted to evaluate the supporting characteristics of in situ pavement layers [Ping et al., 2000]. The experimental results from the field bearing plate load tests are presented in this section. The plate bearing load test results were calibrated by using secant modulus concept [Ping et al., 2000].

4.1.1 Load-Deformation Curve Regression Model In the field testing program the number of load applications, the angle of internal

friction, and the geometry of the bearing plate are constant. Base on this information, after analyzing the data obtained from the field plate load test, a two-constant hyperbolic model was proposed to represent the relation of the load-deformation as follows:

+

=

baP (4.1)

where P = load = deflection a, b = constants. The representation of the load-deflection curve is illustrated in Figure 4.1. In order to determine the modulus of materials in situ by means of plate load tests, some assumptions concerning material behavior must be made. With the assumption that the soil layer is homogeneous, isotropic, elastic, and infinite in depth, the modulus of the material can be determined from the Equation (4.2) and (4.3).

)1(2

2pi

=

prE (4.2)

38

Where, E =

the modulus of elasticity of the material = deflection of plate associated with the pressure p = pressure applied to the surface of the plate r = radius of the circular plate = Poissons ratio 0.35 was chosen for the Poissons ratio of granular subgrade materials [Ping et al.

2003]. Equation (4.2) can be further derived from the following equation:

=

prE 38.1 (4.3)

Then, Equation (4.3) can be rewritten as follows:

=

PE 23.288 (4.4)

While

2r

Pppi

= (4.5)

where E = modulus of elasticity of the material (MPa) P = load (kN) = deformation of the plate associated with the load P (10-5 m) Replacing Equation (4.1) into Equation (4.4) yields the following two equations:

)(123.288 Pfa

bPE =

= (4.6)

)(23.288 =+

= fba

E (4.7)

39

In the two-constant hyperbolic model, the strength parameter was assumed to be related to the shape of the load-deformation parameter curve. From Equation (4.6), the modulus was dependent on the load P so that various modulus values were obtained due to the different applied load. Therefore, the use of a secant modulus seemed to be a logical approach to the analysis. The modulus in Equation (4.4) can be considered as a secant modulus. But it is necessary to select a value for the load (P) or deformation (). In the Florida method FM5-527, the modulus of the tested soil layer is calculated using the deflection of 1.27 mm (0.05 in.). It may not be able to perfectly reflect the characteristics of the soils to determine the moduli for soils of various strengths at the same deflection value. The average secant modulus, which depends only on the parameters of the load-deflection curve, was defined as follows:

aP

EdPE

ult

Pult12.1440

==

(4.8)

aEPPE ultant

12.144)21(@sec === (4.9)

It was found that the average secant modulus will be equal to the secant modulus when the applied load is equal to half of the ultimate load of the soil [Ping et al. 2002]. Modulus of subgrade reaction k is an essential parameter for the rigid pavement design. Combining Equations (2.2) and (4.1), it can be determined by the following equation:

+=

=

=

=

baabPPP

rk 5.137015.13705.137012pi

(4.10)

where k = modulus of subgrade reaction (MPa/m) P = load (kN) = displacement (10-5 m)

40

In the AASHTO design guide, is the displacement of a 30 in. (762 mm) diameter rigid plate under a given static pressure, p = 10 psi (68.9 kPa).

4.1.2 Plate Bearing Load Test Results The two-constant hyperbolic model (Equation (4.1)) was proposed to represent the load-deformation relationship. The load-deformation curve and hyperbolic function of each test soil are shown in Figure 4.2. The R-squared values are above 0.99. The model fits the data very well. The load-deformation relationship parameters a and b can be considered as representative of the soil strength, and were used to calculate the modulus of elasticity. The secant modulus at the deflection of 1.27 mm and average secant modulus values calculated from Equation (4.6) and (4.7) are presented in Table 4.1. The modulus of subgrade reaction k, which were calculated using Equation (4.10) with 12 in. diameter plate, and are also listed in Table 4.1.

4.2 Test-Pit Test Results Ten types of soil representing typical Florida subgrade materials were tested in the test-pit program [Ping and Ling, 2008]. For each soil, static and cyclic (up to 30,000 cycles for simulation of the dynamic effect) plate load tests were conducted under different levels of groundwater table. Since the resilient behavior of subgrade soil under the dynamic loading was influenced by the soil properties as well as the moisture profile for various groundwater levels. The test-pit experimental results are presented in reference to the various level of groundwater table.

4.2.1 Equivalent Resilient Modulus The resilient modulus obtained from the plate load tests on subgrade is based on

Boussinesqs theory of deflections at the center of a circular plate. Burmister has extended this theory to a two-layer elastic system [Burmister, 1943]. The layers are assumed to be homogeneous, isotropic, and elastic solid with a continuous interface, and with the bottom layer being infinite in depth. In the test pit, more than two layers of different materials are under the rigid test plate. An equivalent resilient modulus is used to indicate that the calculated composite resilient is not exactly the modulus of the

41

tested subgrade material, but an equivalent value under test pit situation. In the plate load test, a major portion of the stress inference occurs, according the theory, within a depth of two times of diameter. Because the tested subgrade layer thickness is three times of rigid test plate diameter, this equivalent modulus is very close to the resilient modulus of the subgrade. Under these circumstances, the equivalent single-layer resilient modulus under the cyclic loading on a two-layer system (base and subgrade layers) can be derived from the theory of elasticity:

)1( 2pi

=

ReR

paE (4.11)

Where, EeR = Equivalent resilient modulus of a two-layer system R = Resilient deflection of the two-layer system at N (number of cyclic load) p = Surcharge pressure from the circular plate a = Radius of the circular plate = Poissons ratio 0.35 was chosen for the Poissons ratio of granular subgrade materials [Ping et al.

2003]. The equivalent resilient modulus can be further derived from the following equation:

ReR

paE

=

38.1 (4.12)

The static load test was cycled three times for a total of three static loads and followed with repeated rigid plate tests. The number of load cycles up to 30,000 was recorded. The equivalent resilient modulus was determined from the resilient deflections using Equation (4.12).

4.2.2 Equivalent Resilient Modulus Results The resilient modulus obtained from the plate load tests on subgrade is based on

Boussinesqs theory of deflections at the center of a circular plate. The equivalent

42

resilient modulus values were calculated using Equation (4.12). The plate load test results for the ten soils are summarized in Table 4.2, which records the average water content and plate load equivalent resilient modulus under different water level conditions.

4.3 Laboratory Experimental Results Both the AASHTO T292-91I and T307-99 test method were used to measure the resilient modulus of subgrade soils. During the resilient modulus test, specimen conditioning was conducted first. Then, a series of tests at different deviator stresses and confining pressures were performed, and the data were recorded for every cycle of the test. However, only the last five cycles were used for computation of resilient modulus. Of each condition, two replicate resilient modulus tests were conducted on each type of soil. The resilient modulus was calculated from the deviator stress and resilient strain using Equation (3.1).

4.3.1 Regression Analysis The resilient modulus test results were reported in a tabular form including the deviator stress, axial strain, confining pressure, and bulk stress. A regression model was used to get the regression equation of MR from the confining pressure and bulk stress as follows:

when modulus is dependent on bulk stress:

21

kR kM = (4.13)

when modulus is dependent on confining pressure:

433

kR kM = (4.14)

Where = bulk stress, sum of the principal stresses, (1+ 2+ 3) 3 = confining pressure or minor principal stress k1, k2, k3, k4 = regression constants

43

In actual field conditions, the confining pressure at subgrade layers was found to be approximately 13.8 kPa (2.0 psi). Because the laboratory resilient modulus is stress dependent, a constant stress level has to be determined in selecting the resilient modulus of roadbed soils for pavement design. The Asphalt Institute [AI, 1982] recommended using a confining pressure of 13.8 kPa (2.0 psi) and a deviator stress of 41.4 kPa (6.0 psi) for subgrade layers in determining the resilient modulus [Ping and Ge, 1997]. Daleiden et al. [Daleiden et al., 1994] believed that using a deviator stress of 13.8 kPa (2 psi) and a confining pressure of 13.8 kPa (2 psi) could represent the average stress and pressure values that occurred in the subgrade under traffic loading and surcharge. Chen [Chen, 1999] found that the deviatoric stresses and confining pressures for the pavement structure under a 40 kN load were approximately 20 to 35 kPa (3-5 psi) and 7 to 14 kPa (1-2 psi), respectively. In a laboratory resilient modulus test, the resilient modulus value obtained at a deviator stress of 34.5 kPa (5.0 psi) under the confining pressure 13.8 kPa (2.0 psi) was considered representative of the in-situ subgrade modulus. Therefore, the average resilient modulus values at 2 psi confining pressure as well as at 11 psi bulk stress were obtained for each soil material. To compare the laboratory resilient modulus with the resilient modulus from test-pit test and field test, the laboratory resilient modulus results from those two projects are listed in Table 4.3 and 4.4 for the further comparison.

4.3.2 Moisture Effect on Resilient Modulus The representative resilient modulus data obtained from the bulk stress of 75.8

kPa (11.0 psi) at different moisture conditions are presented in Table 4.4. In general, the resilient modulus decreases with an increase in moisture content. Two exceptions were the slightly increase of soil A3-1 and A24-7 from optimum condition to soaked condition. This situation may be caused by the soil cohesion due to the added moisture. The effect of moisture content on the resilient modulus of Phase I and II soils is shown in Figure 4.3. It is clearly shown that the resilient modulus of subgrade materials usually decreases with an increase in moisture content. Specifically, moisture content has a significant effect on the resilient modulus of A24-2 soil and A24-7 soil whereas its effect is not significant on the resilient modulus of other types of soils [Ping et al. 2010].

44

Table 4.1 Secant Modulus and k Value from Plate Bearing Load Tests Embankment Location Soil

Type Average Secant

Modulus (MPa)

Secant Modulus at =1.27 mm

(MPa)

Modulus of Subgrade Reaction k

(MPa/m) Lee County, Site A A-3 53.8 58.8 466.7 Lee County, Site B A-3 114.0 145.9 1054.7 Polk County, Site A A-3 71.5 114.8 667.3 Polk County, Site B A-3 156.0 244.8 1469.7 Clay County, Site A A-3 120.4 148.1 1214.9 Clay County, Site B A-3 121.7 166.6 1132.7

Martin County, Site A A-3 81.3 86.5 594.4 Martin County, Site B A-3 89.6 81.9 870.4

Osceola County, Site A A-3 114.8 117.7 1040.3 Osceola County, Site B A-3 78.1 92.8 581.1 Seminole County, Site B A-3 120.2 136.4 1102.4 Gadsden County, Site A A-2-4 123.1 152.1 1185.3 Seminole County, Site A A-2-4 121.5 158.2 1126.4 Jefferson County, Site B A-2-4 127.2 114.5 1144.2

45

Table 4.2 Plate Load Equivalent Modulus Test Results Water Level (in)

Test Sequence

A3-1 A3-2 A24-1 A24-

2 A24-

3 A24-

4 A24-

5 A24-

6 A24-

7 A24-

8

Equivalent Resilient Modulus (MPa) -24 A 178 - 174 - - 196 - 171 209 - 0 B 145 204 125 183 114 183 344 187 186 178 0 E - - - - 247 - 671 - - 591

12 C 132 174 125 154 109 175 336 135 183 195 12 F 264 300 241 227 196 250 607 214 260 542 12 J - - - - 182 - 414 - - 378 24 D - - - - 82 - 205 - - 119 24 G - - - - 144 - 413 - - 335 24 I - - - - 135 - 192 - - 220 36 H 196 225 174 106 108 208 157 169 99 112

46

Table 4.3 Laboratory Resilient Modulus Test Results of Subgrade (Field Plate Bearing Load Test Project)

Subgrade Soil Soil Type

LAB MR at the state of stress of plate bearing load test (MPa)

P(=1.27 mm) Pult Lee County, Site A A-3 105.48 107.67 Lee County, Site B A-3 131.34 146.91 Polk County, Site A A-3 177.08 282.92 Polk County, Site B A-3 205.19 267.52 Clay County, Site A A-3 113.11 123.17 Clay County, Site B A-3 129.46 150.97

Martin County, Site A A-3 132.62 135.03 Martin County, Site B A-3 163.02 159.45

Osceola County, Site A A-3 138.89 140.00 Osceola County, Site B A-3 123.88 131.87 Seminole County, Site B A-3 154.66 161.71 Gadsden County, Site A A-2-4 102.84 113.32 Seminole County, Site A A-2-4 173.28 194.59 Jefferson County, Site B A-2-4 82.09 79.62

47

Table 4.4 Laboratory Average Resilient Modulus at Different Moisture Conditions (Test-pit Plate Load Test Project)

Soil Number

of Specimen

Dry Condition Optimum Condition Soaked Condition Moisture Content

(%)

Mr (MPa) Moisture Content

(%)

Mr (MPa) Moisture Content

(%)

Mr (MPa) Middle

Half Full

Length Middle

Half Full

Length Middle

Half Full

Length A3-1 2 6.2 167 116.9 9.6 145.5 108.1 15.2 149.5 81.8 A3-2 2 5.4 208.2 120.2 11.4 157.7 118.8 13.6 137.7 91.9

A24-1 2 7.1 140.6 96.5 12.1 117.9 95.6 14.1 107.6 90.8 A24-2 2 8.4 540.6 305.7 10.6 246.6 166.6 11.5 182.6 97.2 A24-3 2 - - - 9.3 - 67.0 - - - A24-4 2 7.8 212.0 142.5 10.0 123.2 109.4 12.0 121.6 100.0 A24-5 2 - - - 8.8 - 186.3 - - - A24-6 2 7.7 125.7 109.4 10.7 116.5 92.1 11.7 94.8 68.9 A24-7 2 6.7 775.3 285.2 12.2 133.0 71.8 13.3 136.3 71.7 A24-8 2 - - - 10.4 - 63.8 - - -

48

Deformation,

Load

, P

Pultasymptote

Pult/21/b

Ei Esec(at P = Pult/2)

tan = 1/a

P = /(a+b)

Figure 4.1 Rectangular Hyperbolic Representation of Load-Deformation Curve

49

Lee County, Site A

0

5

10

15

20

25

30

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(2.679+0.018)R2 = 0.994

Lee County, Site B

01020304050607080

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.264+0.006)R2 = 0.998

Polk County, Site A

0

10

20

30

40

50

60

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(2.015+0.004)R2 = 0.996

Polk County, Site B

0

20

40

60

80

100

120

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)P = /(0.924+0.002)R2 = 0.998

Clay County, Site A

01020304050607080

0 50 100 150 200Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.197+0.006)R2 = 0.994

Clay County, Site B

01020304050607080

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.184+0.004)R2 = 0.995

Martin County, Site A

05

101520253035404550

0 50 100 150 200Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(2.203+0.009)R2 = 0.999

Martin County, Site B

05

10152025303540

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.44+0.017)R2 = 0.993

Figure 4.2 Load-Deformation Curves of the Tested Soils

50

Osceola County, Site A

0

10

20

30

40

50

60

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.255+0.009)R2 = 0.999

Osceola County, Site B

05

101520253035404550

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(2.291+0.006)R2 = 0.999

Seminole County, Site B

0

10

20

30

40

50

60

70

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.199+0.007)R2 = 0.997

Gadsden County, Site A

01020304050607080

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)P = /(1.171+0.006)R2 = 0.996

Seminole County, Site A

01020304050607080

0 50 100 150Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.187+0.005)R2 = 0.995

Jefferson County, Site B

0

10

20

30

40

50

60

0 50 100 150 200Deformation, (x10-5 m)

Loa

d, P

(x103

N

)

P = /(1.132+0.011)R2 = 0.996

Figure 4.2 Load-Deformation Curves of the Tested Soils continued

51

Resilient Modulus vs. Moisture Content(Confining Pressure 13.8 kPa, Deviator Stress 34.5 kPa)

A24-2y = 815392x-3.4375

A24-7y = 125793x-2.6846

A24-4y = 3141.5x-1.3427

A24-6y = 397.07x-0.5543

A24-1y = 294.13x-0.3744

A3-1y = 202.71x-0.1219

A3-2y = 429.2x-0.4252

0

100

200

300

400

500

600

700

800

4 6 8 10 12 14 16Moisture Content (%)

Res

ilien

t Modu

lus

(MPa

)A3-1

A3-2

A24-1

A24-2

A24-4

A24-6

A24-7

Figure 4.3 Laboratory Resilient Modulus vs. Moisture Content for Phase I and II soils

52

CHAPTER 5

ANALYSIS OF EXPERIMENTAL RESULTS

5.1 Comparison of Laboratory and Test-pit Test Results The comparison of resilient modulus from laboratory triaxial test and test-pit plate load test is presented in this section. To compare with the laboratory triaxial test results, a layered system was employed to calculate the resilient modulus of each subgrade layer instead of the equivalent modulus for all of the layers. The comparison of the resilient modulus from laboratory triaxial tes