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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.

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The small-strain stiffness of compacted residual lateritic subgrade soil was determined. A bender element apparatus and the filter paper method were integrated to develop a systematic testing procedure to investigate the influence of matric suction on small-strain stiffness, including small-strain shear wave velocity and small-strain shear modulus, with different wetting paths. Soil specimens were compacted at three moisture contents (optimum moisture content, dry of optimum, and wet of optimum), then wetted to various moisture contents by using an environmental simulation apparatus developed to simulate in-service moisture conditions. Test findings indicate that specimen moisture contents increase gradually and finally attain equilibrium in the wetting process. Suction decreases with increasing moisture content and degree of saturation; consequently, small-strain shear wave velocity and small-strain shear modulus decrease. The specimens compacted at dry of optimum have a much lower small-strain shear wave velocity and small-strain shear modulus than specimens compacted at wet of optimum. Nevertheless, the small-strain shear modulus exhibits higher linearity with matric suction than with moisture content. Results indicate that matric suction is a key parameter for evaluating the dynamic properties of compacted soil.