Estimating the Contribution of Suction on the Resilient Modulus of Subgrade Soils Using Capillary Saturation

The matric suction is well known to significantly affect the resilient modulus of unsaturated subgrade soils. However, the contribution of matric suction on the resilient modulus varies with moisture conditions and needs to be properly estimated. In this paper, the capillary saturation was used for this purpose and investigated by laboratory tests. Three subgrade soils were prepared with different saturation conditions and dry densities. Mercury intrusion porosimetry (MIP) tests, the filter paper method, and repeated load triaxial (RLT) tests were conducted to obtain the pore structures, matric suction, and resilient modulus, respectively. The results show that three tested soils present typical bimodal pore size distributions. The soil with finer particles forms more intra-aggregate pores, which results in greater differences between capillary saturation and degree of saturation. The product of capillary saturation and matric suction reaches a close correlation with resilient modulus for all tested suction levels. Based on these findings, resilient modulus prediction models incorporating capillary saturation are proposed using both the effective stress method and the independent stress state variable method. Both models provide good agreement with the laboratory test data.