Resilient modulus of unsaturated subgrade soil: experimental and theoretical investigations

TLDR

The resilient modulus of subgrade soil is a key stiffness parameter for pavement fatigue analysis, yet the influence of unsaturated soil suction on this modulus is largely ignored in current testing methods. This study aimed to determine how stress and suction affect the resilient modulus of subgrade soil using a suction‑controlled cyclic triaxial apparatus. Measurements were obtained with Hall‑effect transducers monitoring local axial and radial deformation, and a new semi‑empirical equation for stress‑dependent resilient modulus was proposed and validated on four soils. Results show that resilient modulus increases with repeated loading in contractive soils, decreases slightly in dilative soils, rises with suction due to water tension, varies strongly with stress state, is higher along wetting than drying paths, and the proposed equation accurately predicts these behaviors.

Abstract

The resilient modulus, M R , of subgrade soil is an important stiffness parameter for analysing fatigue cracking in either the asphalt or concrete layer of a pavement. Although subgrade soil is often unsaturated and subject to seasonal variations of moisture content and hence suction in the field, effects of soil suction on the resilient modulus are generally not accounted for in existing testing methods. In this study, M R values of a subgrade soil under various stress and suction conditions were investigated using a suction-controlled cyclic triaxial apparatus. To enhance the accuracy of measurements, Hall-effect transducers were employed to monitor the local axial and radial deformation of each specimen. It was found that M R increases with number of load applications when a soil contracts, but decreases slightly when a soil dilates. When suction increases, the soil response tends to change from contractive to dilative due to suction-induced dilatancy. Moreover, the measured M R is highly dependent on the stress state. It decreases with cyclic stress due to the nonlinearity of the soil stress–strain behaviour, but increases significantly with suction due to the presence of water tension. At the same stress and suction conditions, M R measured along the wetting path is generally larger than that measured along the drying path. A new semi-empirical equation representing the stress-dependency of M R is proposed and was verified using experimental results of four different soils.