Effect of water content on resilient modulus and damping ratio of fine/coarse soil mixtures with varying coarse grain contents

Abstract

For the French ancient rail track, owing to the effect of traffic loadings, an interlayer was naturally created due to the interpenetration of ballast grains and subgrade fine soil. Under the effect of rainfall/evaporation, the water content of interlayer soil can vary, affecting its dynamic properties such as resilient modulus and damping ratio. In order to investigate the effect of water content on the resilient modulus and the damping ratio of interlayer soil, a series of cyclic triaxial tests were carried out, following a multi-step loading procedure with various deviator stress amplitudes of 10, 30, 50, 100 and 200 kPa. Two target fines water contents (17.6% and 10.6%) and five coarse grain contents (0%, 10%, 20%, 35% and 45%) were considered. Results showed that under a given coarse grain content, an increase of water content led to a decrease of resilient modulus owing to the decrease of suction while an increase of damping ratio owing to the increase of soil viscosity. Through the variations of resilient modulus and damping ratio with coarse grain content, a characteristic coarse grain content was identified, defining two distinct soil fabrics: a fine soil dominated fabric when the coarse grain content was smaller than the characteristic value, and a coarse grain dominated fabric when the coarse grain content was larger than the characteristic value. Moreover, a constant characteristic coarse grain content was obtained at a given water content: 25.5% at 17.6% water content and 32.0% at 10.6% water content. The increase of characteristic coarse grain content with the decrease of water content could be explained by the swelling-shrinkage of fine soil: with a decrease of water content, the shrinkage of fine soil led to an increase of the volume of macro-pores. More coarse grains were thus needed to constitute the global skeleton, giving rise to an increase of the characteristic coarse grain content. In addition, comparison of the values of characteristic coarse grain content obtained from cyclic tests with those from monotonic tests showed that at a given water content, cyclic loading led to a slightly larger value than monotonic loading, evidencing the more significant effect of cyclic loading on soil fabric.