Variation of Resilient Modulus with Soil Suction for Compacted ...

Variation of Resilient Modulus with Soil Suction for Compacted Subgrade Soils Shu-Rong Yang, Wei-Hsing Huang, and Yu-Tsung Tai compacted to various levels of relative compaction (RC) and then saturated to a higher moisture content to test for soil suction and resilient modulus. Finally, the mathematical relationship between soil suction and the resilient modulus was established.

The variations of resilient modulus with the postconstruction moisture content and soil suction for cohesive subgrade soils were evaluated. In particular, the effects of relative compaction of the subgrade on the suction and resilient modulus were investigated. To simulate subgrade soils at in-service conditions, soil specimens were compacted at various relative compactions and optimum moisture content and then saturated to equilibrium moisture content to test for resilient modulus and soil suction. The filter paper method was used to measure the total and matric suctions of two cohesive soils. Test findings demonstrated that resilient modulus correlated better with the matric suction than with total suction. Matric suction was found to be a key parameter for predicting the resilient modulus of cohesive subgrade soils. A prediction model incorporating deviator stress and matric suction for subgrade soil resilient modulus was established.

SEASONAL EFFECT ON SUBGRADE SOILS Construction specifications often require that pavement subgrades be compacted to more than 90% of the maximum dry unit weight and at OMC. On sealing the ground surface, subgrade soils (with pavement on top) exhibit an increased average moisture content at the shallow part of the subgrade and a decreased fluctuation of water content over time (4). Uzan (5) found that the clayey material underneath the pavement increased its moisture content to approximately 20 to 30% higher than its plastic limit and reached equilibrium condition during the first 3 to 5 years of service. Numerous studies (4–6) indicated that the moisture content of the subgrade, after construction, would attain an equilibrium with the environment, and this equilibrium is referred to as the equilibrium moisture content (EMC). This equilibrium condition usually corresponds to the subgrade soil type, level of the water table, condition of the surface layer, and so forth. Quintus and Killingsworth (7) examined the subgrade of 137 LongTerm Pavement Performance test sites, including 59 sites with cohesive subgrade soils and 78 sites with granular subgrades. They reported that the in situ moisture content of the 59 cohesive subgrades was always at the wet side of OMC. This finding confirmed that the moisture content of cohesive subgrades increases after construction. According to Sauer and Monismith (8), resilient modulus and residual deformation can be correlated to soil suction. They observed that higher soil suction produced higher resilient modulus. Shackel (9) examined the effect of repetitive loading on soil suction in a kaolinite sand mixture. It was reported that soil suction decreased as the number of load cycles increased. Khoury et al. (1) further demonstrated that the resilient modulus increased when matric suction (ψm) increased but that significant changes in resilient modulus were not caused by osmotic suction (ψπ). They concluded that resilient modulus correlated better with soil suction than with moisture content. Likos and Lu (10) indicated that matric suction characteristics can be used to model and predict the coefficient of permeability for groundwater flow in unsaturated soils. Total suction (ψt) characteristics have been applied to assess the swelling potential of expansive clayey soils. In the integrated climatic model used in Guide for MechanisticEmpirical Design of New and Rehabilitated Pavement Structures (11), the subgrade modulus for frozen and unfrozen states can be determined for fine-grained soils with the following regression equations (12):

Construction specifications generally require that subgrade soils be compacted in the field at or near optimum moisture content (OMC). As such, subgrade soils should be treated as unsaturated soils. Moisture content in the pavement components changes over time as a result of environmental and traffic factors, particularly when a drainage system is not adequately designed or functioning. In addition, subgrade moisture is sensitive to rising levels in the water table, infiltration of water, and evaporation (1). In recent years, interest in determining the soil suction of unsaturated subgrade soils beneath a pavement has increased markedly. This is because soil suction that defines the state of stress in unsaturated soils varies with the changes in moisture content (2). Resilient modulus (Mr) of subgrade soils was introduced as an important parameter in the 1986 AASHTO Guide for Design of Pavement Structures (3), because it is a more rational soil property than soil support value or modulus of subgrade reaction. It is analogous to the modulus of elasticity and mathematically defined by the ratio of deviatoric stress to recoverable strain. Resilient modulus is sensitive to the state of stress within a subgrade and is greatly influenced by the presence of water and the development of capillary suction above the water table. Because soil suction dictates the state of stress in unsaturated soils, it is important to understand the influence of soil suction on resilient modulus. This study examined the effect of soil suction on the resilient modulus of compacted subgrade soils. To simulate the moisture condition of in-service subgrade soils, a wetting procedure was developed to saturate (wet) soil specimens in the laboratory. Soil samples were Department of Civil Engineering, National Central University, Jungli, Taoyuan 32054, Taiwan. Transportation Research Record: Journal of the Transportation Research Board, No. 1913, Transportation Research Board of the National Academies, Washington, D.C., 2005, pp. 99–106.

Mr = 27.06 − 0.526θ 99

for γ d > 100 lb ft 3

(1)

100

Transportation Research Record 1913

Mr = 18.18 − 0.404θ

for γ d < 100 lb ft 3

(2)

where Mr = resilient modulus (kips/in.2), θ = volumetric water content (%), and γd = dry density (lb/ft3). On the basis of the laboratory resilient modulus tests, Johnson et al. (13) developed the following model for thawed sand subgrades: Mr = 1.35 × 10 6 (101.36 − ψ )−2.36 ( J1 )−3.25 ( γ d )−3.06

(3)

where Mr ψ J1 γd

= = = =

resilient modulus (MPa), moisture tension or suction (kPa), first stress invariant (kPa), and dry density (mg/m3).

To predict the effects of seasonal moisture variation on resilient modulus, it is evident that the influence of moisture variations on soil suction of compacted subgrade soils needs to be evaluated.

then wrapped in a geotextile sheet to prevent soils from running off. Two halves of an acrylic cylinder were placed on the geotextile and held together tightly by six screws to prevent lateral expansion. The acrylic cylinder was perforated to permit water penetration and wet the whole sample in a homogeneous fashion (Figure 1). A surcharge weight of 5 kg was placed on top of the upper spacer to simulate the overburden of a surface layer, base, and subbase. The entire setup was placed in a water tank and soaked for approximately 3 h to allow for water intake. After soaking, the sample (with the surcharge weight on top) was removed from the water and conditioned in moist air with 100% relative humidity. This conditioning is required for water to move from the sample surfaces to the inner portion of the sample and to obtain an acceptable distribution of moisture along the radial distance. To ensure longitudinal homogeneity in the sample, the sample was inverted (top to bottom) every day and weighed to measure moisture content. When the moisture content in the sample stopped increasing, the sample was considered to have reached its EMC. Finally, a 5-mm slice was cut off the top and bottom portions of the sample before resilient modulus testing.

Sample Testing SIMULATION OF FIELD MOISTURE CONTENT IN LABORATORY Sample Preparation Two locally available soils, a residual lateritic soil and a pulverized mudstone, were used to study the soil suction and resilient modulus of cohesive soils. The soils were classified as A-7-5 and A-7-6; both are cohesive according to AASHTO T 292-91. Their basic properties, including specific gravity (Gs), liquid limit (LL), plasticity index (PI), maximum dry density (MDD), and OMC were evaluated; the results are summarized in Table 1. Soil specimens were 70 mm in diameter and 150 mm in height and were prepared in accordance with the modified proctor test (AASHTO T180). Samples were compacted to three RC levels of 100%, 95%, and 88%, representing typical field construction situations. At each level of RC, the specimens were compacted at OMC, as determined by the AASHTO T180 standard.

Resilient Modulus The AASHTO T 292-91 test method was used to determine the resilient modulus of each specimen. The resilient modulus test consists of applying a cyclic load that has a square-shaped load pulse with a load duration of 0.1 s and a fixed cycle duration of 1.0 s. Specimens were loaded according to the T 292-91 test method. For each loading sequence, the load and the vertical displacement for the last five cycles were measured and used to determine the resilient modulus. The load was measured with an externally mounted load cell with a capacity of 10 kN. The resilient displacements were measured by using linear variable differential transformers (LVDTs) mounted externally. The LVDTs had a maximum stroke of 10 mm. The data acquisition

Surcharge Spacer

Sample Soaking and Conditioning

Perforated acrylic cylinder 140 mm

To simulate the in-service moisture content, the sample was compacted at OMC (simulation of construction phase) and then saturated (wetted) to EMC (simulation of in-service phase). In addition, a sample with a transition moisture content (TMC) in between OMC and EMC also was prepared. After compaction, the sample was extracted from the mold, and its initial weight, height, and diameter were measured. The sample was

70 mm

Geotextile sheet Specimen

TABLE 1

Properties of Soils

Soil

Gs

LL

PI

MDD (g/cm3)

OMC (%)

1 2

2.71 2.67

54 50

20 23

1.76 1.80

18 17

AASHTO Classification Spacer A-7-5 A-7-6

FIGURE 1

Sketch of soaking and conditioning.

Yang, Huang, and Tai

101

system was programmed to take 200 readings per second for each channel monitoring the axial loading and deformation.

Soil Suction Devices and methods for measuring soil suction and soil–water characteristics are numerous, diverse, and well documented. The filter paper method, as specified in ASTM D 5298-94, is an inexpensive test method, and it is relatively simple. It is the only known method that covers the full range of suction. It is also the only way to measure total suction and matric suction simultaneously. The filter papers are calibrated by determining the relationship between equilibrium filter paper water content (wfp) and vapor-phase relative humidity (or total suction). A typical calibration curve for filter papers, as recommended by ASTM, consists of two parts, as shown in Figure 2a. The filter papers used in this study were Whatman No. 42, ashfree quantitative Type II with a diameter of 5.5 cm. Salt solutions of known concentrations were used to control relative humidity. Figure 2b shows the results of the calibration tests with Whatman No. 42 filter paper using a NaCl solution. The linear trendline based

on least-squares regression analysis is shown through the data points, with a determination coefficient (R2) of 0.941. The lower segment of the ASTM calibration curve was used in this study. To measure soil suction, the filter paper was placed in direct contact with the soil to determine the matric suction and placed on a disk above the soil (noncontact) to determine the total suction of soil. A 10-day period was allowed for the filter paper and soil to reach equilibrium. The moisture content of the filter paper was then determined, and the suction of the specimen was obtained from the calibration curve using the filter paper moisture content.

RESULTS AND DISCUSSION OF RESULTS Simulation of EMC in Laboratory Figures 3a and 3b present the relationship between the sample conditioning time and the moisture content for A-7-5 and A-7-6 soils, respectively. The moisture content of the samples increased rapidly during the initial soaking period (3 h). Thereafter, the increase in moisture content slowed significantly until the soils reached their EMC in approximately 7 to 10 days.

26

ASTM D 5928-94

5

ψ t = 5.327 − 0.0779wfp

4

24

Water Content, %

Total Suction, in log kPa

6

3

ψ t = 2.412 − 0.0135wfp

2 1

22

20 88% RC 95% RC 100% RC

18

0 0

20 40 60 80 Filter Paper Water Content wfp , % (a)

100 0

5

10

15

20

25

Time, Day (a)

5

ψ t = 5.426 − 0.0830wfp

30

R2 = 0.960

28

4

Water Content, %

Total Suction, in log kPa

6

3 2 1

26 24 22 20 88% RC 95% RC 100% RC

18

0 0

20 40 60 80 Filter Paper Water Content wfp , % (b)

100

FIGURE 2 Calibration curve on Whatman No. 42 filter paper: (a) typical calibration curve suggested by ASTM (ASTM D 5298-4) and (b) calibration curve used in this study.

16

0

2

4

6

8

10

12

14

16

Time, Day (b) FIGURE 3 Relationship between sample conditioning time and moisture content: (a) A-7-5 soil and (b) A-7-6 soil.

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Transportation Research Record 1913

C

E

H

D

F

I

G

J

B

Location

Moisture Content, %

A

22.07

B

20.83

C

19.41

D

19.10

E

19.87

F

19.17

G

19.74

H

19.82

I

19.11

J

19.82 (b)

(a)

FIGURE 4 Moisture uniformity in sample after soaking and conditioning: (a) location in sample and (b) moisture measurements.

To evaluate the moisture distribution in the laboratory-saturated samples, the water content of soils at various locations in the samples was determined; the results are shown in Figure 4. The difference in moisture content between the outside and the center of the tested samples was less than 0.5%, except at the very top (Point A) and the bottom (Point B) of the samples (Figure 4a). These parts were cut from the samples before the resilient modulus test. Table 2 presents the results of moisture conditioning of the two soils. The EMC for the soils increased with decreasing RC. For instance, the EMC of A-7-5 soil compacted at 100% compaction was 23.1%, whereas that of soils compacted at 95% and 88% compaction was 24.4% and 27.2%, respectively. Similarly, A-7-6 soil sample with the lowest RC (88%) attained the highest EMC of 29.3% during moisture conditioning. A straightforward explanation of this finding is that more pores exist in soils with lower degree of compaction; hence, the soil is capable of absorbing more water. Soil suction, to be discussed in the following sections, provides a further insight into this finding.

400

Resilient Modulus, MPa

A

simulating long-term in-service condition, and the TMC in between OMC and EMC. Figure 5 presents the relationship between the resilient modulus and the deviator stress for A-7-5 soil at various RCs. The resilient modulus was found to be insensitive to the deviator stress at OMC. However, at TMC and EMC, the resilient modulus decreased with increasing deviator stress. Notably, the resilient modulus showed a

350 300 OMC TMC EMC

250 200 150 100 50 0 0

20

40

60

80

100

120

Deviator Stress, kPa (a) 250

Resilient Modulus, MPa

CL

200 OMC TMC EMC

150 100 50 0

0

20

40

60

80

100

120

Deviator Stress, kPa (b)

Resilient Modulus

TABLE 2

Soil A-7-5

A-7-6

Results of Moisture Conditioning of Soils Relative Compaction (%)

Optimum Moisture Content, OMC (%)

Equilibrium Moisture Content, EMC (%)

100 95 88

18.1 18.1 18.2

23.1 24.4 27.2

100 95 88

17.2 17.1 16.9

22.6 24.7 29.3

140

Resilient Modulus, MPa

To evaluate the effect of increased moisture content on the resilient modulus of soil, samples were prepared with three different moisture contents by the aforementioned soaking and conditioning procedures. These are the OMC simulating as-compacted condition, the EMC

120 100 OMC TMC EMC

80 60 40 20 0

0

20

40

60

80

100

120

Deviator Stress, kPa (c) FIGURE 5 Variation of resilient modulus at various RCs for A-7-5 soil: (a) 100% RC, (b) 95% RC, and (c) 88% RC.

Yang, Huang, and Tai

103

drastic decrease, representing reductions ranging from 50 to 85%, as the moisture content increased from OMC to TMC. Further increases in moisture content from TMC to EMC exhibited only a slight decrease in resilient modulus. Figure 6 shows the relationship between the resilient modulus and the deviator stress for A-7-6 soil at various RCs. In general, the resilient modulus declined as the applied deviator stress increased. Again, sharp decreases in resilient modulus were observed with an

Resilient Modulus, MPa

250 OMC TMC EMC

200 150 100 50

increasing moisture content. Reductions in resilient modulus varied from 60 to 90%, depending on the moisture content of the sample. The A-7-5 soil, a residual lateritic soil, was observed to be more sensitive to moisture changes than A-7-6, although the two soils have very close plasticity indexes. Test results presented in Figures 5 and 6 indicate that the moisture content of the soil had greater influence on the resilient modulus of different soils than the deviator stress. To illustrate the effects of RC on resilient modulus of compacted soils, Figure 7 is plotted for three levels of RC for the two soils at OMC. Obviously, the resilient modulus declined with decreasing RC of the subgrade. Resilient modulus test results presented so far revealed that, among the three factors influencing the variations in the resilient modulus of cohesive subgrades, the moisture content and the RC played vital roles and that deviator stress had a less significant effect. It is evident the resilient modulus of subgrade soils declined rapidly after wetting or an increase in moisture content. Furthermore, insufficient compaction of the subgrades during construction was also likely to induce a sharp reduction in resilient modulus. Hence, it is very important that pavement drainage systems be adequately designed and functioning, and that the subgrade be densely compacted.

0 0

20

40

60

80

100

120

Deviator Stress, kPa

400

(a) Resilient Modulus, MPa

350

Resilient Modulus, MPa

200 OMC TMC EMC

150

100

300

100% RC 95% RC 88% RC

250 200 150 100

50

50 0

0

0

20

40

60

80

100

20

40

60

80

100

120

Deviator Stress, kPa (a)

120

Deviator Stress, kPa (b) 240

100% RC 95% RC 88% RC

220

140

Resilient Modulus, MPa

Resilient Modulus, MPa

160 OMC TMC EMC

120 100 80 60 40 20

200 180 160 140 120 100

0

80

0

20

40

60

80

100

120

Deviator Stress, kPa (c) FIGURE 6 Variation of resilient modulus at various RCs for A-7-6 soil: (a) 100% RC, (b) 95% RC, and (c) 88% RC.

0

20

40

60

80

100

120

Deviator Stress, kPa (b) FIGURE 7 Effects of RC on resilient modulus of soils at OMC: (a) A-7-5 soil and (b) A-7-6 soil.

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Transportation Research Record 1913

The deviator stress model was widely used in the past to represent the resilient modulus of cohesive subgrade soils. The following form is suggested by AASHTO T 292-91: ( 4)

where k1 and k2 are regression parameters and σd is the deviator stress. Table 3 summarizes the results of regression analysis on the laboratory resilient modulus test data using the deviator stress model. This model has typically been used to simulate the behavior of cohesive subgrade soils under traffic loading. A limitation of the deviator stress model is that it does not reflect the effect of moisture content on resilient modulus. Therefore, it is necessary to derive separate regression parameters for different moisture conditions, as shown in Table 3.

Matric Suction, kPa

Mr = k1 (σ d )k2

8,000

A-7-5

16

18

20

22

24

26

28

Water Content, % (a)

88% RC 95% RC 100% RC

Matric Suction, kPa

10,000 8,000 6,000 4,000 2,000 0

16

18

20

22

24

26

28

30

Water Content, % (b) FIGURE 8 Matric suction versus soil water content relations: (a) A-7-5 soil at various RCs and (b) A-7-6 soil at various RCs.

k 1 and k 2 Constants for Deviator Stress Model Relative Compaction (%) 100

95

88

A-7-6

2,000

12,000

Figure 8 presents the matric suction data obtained from soil specimens prepared with different moisture contents and RCs. The matric suction of the A-7-5 and A-7-6 soils varied from 10 to 11,000 kPa in the moisture content range of 17% to 30%. High matric suctions imply high capillary stresses in soils and cause movement or flow of water in unsaturated soils, which results in increases in the moisture content of cohesive subgrades after construction. Figure 8 also indicates that matric suction increased as the degree of compaction decreased. This finding helps explain why the EMC increased with decreasing RC. Soil specimens compacted to low degrees exhibited high matric suction and, thus, reached a high EMC. In addition, Figure 8 shows that matric suctions were low and remained almost constant at moisture content levels of TMC and EMC. This finding indicated that the effect of moisture content on matric suction was not significant at the moisture content levels of TMC and EMC.

Soil

4,000

0

Soil Suction

TABLE 3

88% RC 95% RC 100% RC

6,000

100

95

88

Moisture Content

k1

k2

R2

OMC TMC EMC OMC TMC EMC OMC TMC EMC

306.20 219.79 181.55 165.20 264.14 232.27 81.66 231.21 157.04

0.021 −0.278 −0.300 0.041 −0.348 −0.395 0.074 −0.435 −0.434

0.038 0.940 0.615 0.285 0.927 0.925 0.104 0.995 0.863

OMC TMC EMC OMC TMC EMC OMC TMC EMC

381.07 433.51 1132.40 165.58 178.23 242.10 255.27 515.22 126.47

−0.171 −0.418 −0.975 −0.183 −0.318 −0.661 −0.223 −0.643 −0.644

0.880 0.923 0.888 0.744 0.961 0.898 0.885 0.982 0.956

Resilient Modulus Model Incorporating Soil Suction To improve the deviator stress model (Equation 4) for broader applications, numerous correlative equations have been developed to predict the resilient modulus of subgrade soils by incorporating more predictor variables into the model, such as confining stress, bulk stress, or dry density. The prediction model proposed in this study uses soil suction as the additional variable. Figure 9 shows the variations of resilient modulus with total and matric soil suctions obtained at different moisture contents and RCs for A-7-5 and A-7-6 soils. The resilient modulus increased with increasing matric suction and total suction. The increase in resilient modulus can be attributed to the fact that high soil suction produced a stiffening effect on the specimens because of increased rigidity of the soil skeletons, and, thus, resulted in higher resilient modulus. On the basis of the effective stress concept, the effective stress of unsaturated soils presented by Bishop (14) took the following form: σ ′ = σ − ua + χ ( ua − uw )

(5)

Yang, Huang, and Tai

105

TABLE 4 k 5 and k 6 Constants for Deviator Stress–Matric Suction Model

5,000 Total Suction Matric Suction

Soil Suction, kPa

4,000

Soil

3,000 2,000

Relative Compaction (%)

k5

k6

R2

A-7-5

100 95 88

5.83 3.29 13.55

0.467 0.479 0.238

0.918 0.879 0.700

A-7-6

100 95 88

0.99 3.04 2.57

0.709 0.392 0.424

0.757 0.794 0.753

1,000 0 0

20

40

60

80

Resilient Modulus, MPa FIGURE 9 Relationship between resilient modulus and soil suction for A-7-5 and A-7-6 soils (
d  103 kPa,
3  21 kPa).

where σ′ σ ua uw χ

= = = = =

effective normal stress, total normal stress, pore air pressure, pore water pressure, parameter thought to be a function of degree of saturation (χ = 0 for dry soils, χ = 1 for saturated soils), and (ua − uw) = matric suction (i.e., ψm). Because the deviator stress model does not take the moisture condition of soils into account, the effective stress is to be used in the model, instead. Hence, Equation 4 can be rewritten as follows: Mr = k3 (σ d − ua + χψ m )k4

(6)

The validity of Equation 6 is manifested by the experimental data shown in Figure 9. It is illustrated that the resilient modulus correlated better with the matric suction than with the total suction. If ua = 0 is assumed, the effective stress model can, then, be simplified as the following equation for predicting the resilient modulus of unsaturated soils.

effect of high matric suction such that the resilient modulus will not be overestimated. Table 4 presents the deviator stress–matric suction model parameters (k5 and k6, with Mr in MPa) derived from A-7-5 and A-7-6 soils at various RCs. Notably, the deviator stress–matric suction model exhibited a good correlation fit (R2 > 0.7). More importantly, the introduction of matric suction into the resilient modulus model enhances its predictive capability, because the effects of seasonal variation of moisture content on the resilient modulus of subgrade soils are reflected by the matric suction. Figure 10 plots the predicted resilient modulus calculated by applying the deviator stress–matric suction model against the laboratory data obtained from A-7-5 and A-7-6 soils. The deviator stress–matric suction model predicted the resilient modulus of unsaturated soils effectively. The proposed prediction model also reflects the influence of seasonal moisture change on resilient modulus and allows improved characterization of the variations in resilient modulus with moisture content.

CONCLUDING REMARKS Two cohesive soils were applied to study the resilient modulus and soil suction. The experimental results demonstrated that the stress state, moisture content, and degree of compaction influenced the resilient

400

( 7)

In this model, called the deviator stress–matric suction model, the resilient modulus (Mr) is expressed as a function of deviator stress (σd ) and matric suction (ψm). Both the matric suction and deviator stress act to increase the rigidity of soil skeletons. Therefore, the (σd + χψm) term is analogous to an effective stress. For soils with low moisture content, the matric suction tends to be so high that the resilient modulus is dominated by the matric suction. Conversely, the effect of the deviator stress on resilient modulus becomes more important than the matric suction for soils with high moisture content. In Equation 7, χ can be considered as the weight of matric suction; it represents the contribution of matric suction to the effective stress. Parameter χ is particularly important for soils compacted at low energy levels, which produce high matric suction and a low degree of saturation (low χ). In this case, the low χ values will limit the

A-7-5 soil A-7-6 soil

Predicted Mr , MPa

Mr = k5 (σ d + χψ m )k6

300

200

100

0 0

100

200 Measured Mr , MPa

300

400

FIGURE 10 Predicted versus measured resilient modulus using deviator stress–matric suction model.

106

modulus and soil suctions. Subgrade resilient modulus is highly dependent on the moisture condition of soil. Furthermore, the degree of compaction during construction has a great effect on the resilient modulus. The combination of high subgrade moisture content and poor compaction results in sharp decreases in subgrade resilient modulus and should always be avoided for pavement subgrades. Laboratory results showed that the soil suction increases as moisture content and RC of soil decreases. In addition, the variation of resilient modulus exhibited a better correlation with matric suction than with total suction. Based on the concept of effective stress of unsaturated soils, the matric suction of soil proved to be a good prediction variable for resilient modulus. In addition, the effects of seasonal variation of moisture content on the resilient modulus of subgrade soils are reflected in the deviator stress–matric suction model for the prediction of resilient modulus.

ACKNOWLEDGMENT This study was supported by the National Science Council of Taiwan.

REFERENCES 1. Khoury, N. N., M. Musharraf, J. B. Nevels, and J. Mann. Effect of Soil Suction on Resilient Modulus of Subgrade Soil Using Filter Paper Technique. Presented at 82nd Annual Meeting of the Transportation Research Board, Washington, D.C., 2003. 2. Fredlund, D. G., and H. Rahardjo. Soil Mechanics for Unsaturated Soils. John Wiley & Sons, Inc., New York, 1993. 3. AASHTO Guide for Design of Pavement Structures. AASHTO, Washington, D.C., 1986. 4. Thadkamalla, G. B., and K. P. George. Characterization of Subgrade Soils at Simulated Field Moisture. In Transportation Research Record 1481, TRB, National Research Council, Washington, D.C., 1995, pp. 21–27.

Transportation Research Record 1913

5. Uzan, J. Characterization of Clayey Subgrade Materials for Mechanistic Design of Flexible Pavements. In Transportation Research Record 1629, TRB, National Research Council, Washington, D.C., 1998, pp. 189–196. 6. Elfino, M. K., and J. L. Davidson. Modeling Field Moisture in Resilient Modulus Testing. Geotechnical Special Publication. No. 24. ASCE, Reston, Va., 1989, pp. 31–51. 7. Quintus, H. V., and B. Killingsworth. Analyses Relating to Pavement Material Characterizations and Their Effects on Pavement Performance. Publication FHWA-RD-97-085. FHWA, U.S. Department of Transportation, 1998. 8. Sauer, E. K., and C. L. Monismith. Influence of Soil Suction on Behavior of a Glacial Till Subjected to Repeated Loading. In Highway Research Record 215, HRB, National Research Council, Washington, D.C., 1968, pp. 8–23. 9. Shackel, B. Changes in Soil Suction in a Sand-Clay Subjected to Repeated Triaxial Loading. In Highway Research Record 429, HRB, National Research Council, Washington, D.C., 1973, pp. 29–39. 10. Likos, W. J., and N. Lu. Automated Measurement of Total Suction Characteristics in the High-Suction Range: Application to Assessment of Swelling Potential. In Transportation Research Record: Journal of the Transportation Research Board, No. 1755, TRB, National Research Council, Washington, D.C., 2001, pp. 119–128. 11. Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures. Final Report, 2004. NCHRP Project 1-37A. www.trb. org/mepdg/guide.htm. 12. Drumm, E. C., and R. Meier. LTPP Data Analysis: Daily and Seasonal Variations in Insitu Material Properties. Final Report, NCHRP Project 20–50. TRB, National Research Council, Washington, D.C., 2003. http://trb.org/publications/nchrp/nchrp_w60.pdf. Accessed May, 1, 2005. 13. Johnson, T. C., R. L. Berg, E. J. Chamberlain, and D. M. Cole. Frost Action Predictive Techniques for Roads and Airfields. A Comprehensive Survey of Research Findings. CRREL Report 86–18. U.S. Army Corps of Engineers, Cold Regions Research & Engineering Laboratory, Hanover, NH, 1986. 14. Bishop, A. W. The Principle of Effective Stress. Teknisk Ukeblad, Vol. 106, No. 39, pp. 859–863. The Engineering Behavior of Unsaturated Soils Committee sponsored publication of this paper.