Investigation Into the Mechanical Behavior of Track-Bed Materials with Different Grain Size Distributions of Coarse Grains

Abstract

In French ancient railway substructure, the interpenetration of ballast and subgrade soils formed a new layer, namely interlayer. Along the depth, a decreasing trend of ballast content was identified in field investigation. In this study, the effect of C_u of coarse grains on the mechanical behaviors of interlayer soils was investigated by carrying out monotonic triaxial tests. Five volumetric contents of coarse grains f_v (5, 10, 20, 35, and 45%) and three C_u values were considered. Also, X-ray μCT scans were performed on representative samples to visualize the grain distributions. Results show that: (a) at each C_u value, from the variation trend of q_max with f_v, a characteristic volumetric content of coarse grains f_v-cha was identified, defining two soil fabrics (namely fine-fine contact structure and grain-grain contact structure). When C_u decreased, f_v-cha increased since fewer grain contacts were developed at smaller C_u; (b) in the case of grain-grain contact structure, the decrease of C_u increased the maximum deviator stress q_max, friction angle, Poisson’s ratio and dilatancy angle, since more large grains were involved at smaller C_u; (c) in the case of fine-fine contact structure, the C_u decreasing led to decreases in q_max, friction angle, Poisson’s ratio and dilatancy angle due to smaller quantity of coarse grains; (d) for two soil fabrics, the variations of Young’s modulus and cohesion with C_u shared the same pattern: the smaller the C_u, the larger Young’s modulus and the cohesion.