Resilient modulus and hardening/softening behavior of subgrade soil under repeated loading

Standard test methods for resilient modulus of subgrade soils regulate the experiment with limited number of load repetitions to reduce testing time. However, a pavement is likely to incur millions of traffic loads in its service life. This study conducted triaxial tests on 2 cohesive soils for 10,000 load repetitions, with some specimens subjected to 100,000 or 500,000 load repetitions, to characterize the behavior of cohesive subgrades under repeated loading. Experimental results show that, at optimum moisture content (OMC), the resilient modulus of cohesive subgrades exhibits gradual hardening behavior under low stress level. At high stress levels, soil tends to soften after a specific number of load applications. At the wet side of OMC, the resilient modulus of soil descends initially and then rises as the number of load application increases. To describe the behavior of subgrade soil under repetitive loading, a prediction model incorporating the hardening/softening behavior for resilient modulus is established.