Incorporating the Effect of Moisture Variation on Resilient Modulus for Unsaturated Fine-Grained Subgrade Soils

Moisture content and stress state, which includes the effects of matric suction for unsaturated soil, exert a significant effect on the resilient modulus (M-R) values of fine-grained subgrade soils. The moisture content and, consequently, matric suction vary periodically in subgrades in response to seasonal variation. The relationship between M-R and matric suction is investigated through repeated load triaxial testing on fine-grained soils to obtain M-R values and evaluation of the soil-water characteristic curves (SWCCs). The SWCCs, which represent the relationship between suction and degree of saturation, were evaluated with a combination of axis-translation and chilled-mirror hygrometer techniques to allow SWCC representation across the entire range of saturation. The plasticity index (PI) had a significant impact on the matric suction-water content relationship such that the SWCC shifted to the left as the soil PI decreased. The test results indicated a significant relationship between M-R and matric suction. The results also were used to analyze the relationship between M-R and moisture, which showed that the degree of saturation was superior to gravimetric water content in its ability to capture the effect of moisture variation on M-R values. A modified constitutive M-R-matric suction model was proposed to capture the effect of moisture variation on M-R while accounting for the stress state of unsaturated soils. The proposed model performed better than existing models in reducing the number of regression constants and providing a better fit to the measured M-R data.