Resilient modulus and plastic strain of unsaturated cohesive subgrade soils

Construction specifications generally require that subgrade soils be compacted in the field at or near optimum moisture content (OMC). As such, subgrade soils should be treated as unsaturated soils. In addition, moisture content in the pavement components changes over time as a result of environmental and traffic factors. Numerous studies indicated that the moisture content of the subgrade, after construction, would attain an equilibrium with the environment and this is referred to as the equilibrium moisture content (EMC). However, the influence of water content or soil suction on the unsaturated cohesive subgrade soils has not been systematically considered in the conventional subgrade soil design. This paper aims to evaluate the variations of resilient modulus and plastic strain with the post-construction moisture content and soil suction for cohesive subgrade soils. This study carried out the resilient modulus and plastic strain tests via the MTS cyclic triaxial system. To simulate subgrade soils at in-service conditions, soil specimens were compacted at the optimum moisture content, and then placed in the damp room to increase the moisture content to equilibrium condition for resilient modulus and plastic strain test. Based on the above test results, the characteristic of resilient modulus and plastic strain of two unsaturated cohesive subgrade soils were investigated. The experimental results demonstrated that the stress state, moisture content, and soil suction influenced the resilient modulus and the plastic strain. Unsaturated subgrade resilient modulus reduces with increasing deviator stress and decreasing matric suction. The subgrade plastic strain increases as the deviator stress increases and the matric suction decreases. Furthermore, the high subgrade moisture content or low soil suction would result in sharp decrease in subgrade resilient modulus, and should be avoided for pavement design. Finally, based on the concept of effective stress of unsaturated soils, a prediction model incorporating cyclic deviator stress and matric suction for unsaturated cohesive subgrade soils was established. The matric suction of soil proved to be a good prediction variable for resilient modulus. Also, the effects of seasonal variation of moisture content on the resilient modulus of subgrade soils are reflected in the deviator stress-matric suction model for the prediction of resilient modulus. Copyright ASCE 2006.