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

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 Cu of coarse grains on the mechanical behaviors of interlayer soils was investigated by carrying out monotonic triaxial tests. Five volumetric contents of coarse grains fv (5, 10, 20, 35, and 45%) and three Cu values were considered. Also, X-ray μCT scans were performed on representative samples to visualize the grain distributions. Results show that: (a) at each Cu value, from the variation trend of qmax with fv, a characteristic volumetric content of coarse grains fv-cha was identified, defining two soil fabrics (namely fine-fine contact structure and grain-grain contact structure). When Cu decreased, fv-cha increased since fewer grain contacts were developed at smaller Cu; (b) in the case of grain-grain contact structure, the decrease of Cu increased the maximum deviator stress qmax, friction angle, Poisson’s ratio and dilatancy angle, since more large grains were involved at smaller Cu; (c) in the case of fine-fine contact structure, the Cu decreasing led to decreases in qmax, 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 Cu shared the same pattern: the smaller the Cu, the larger Young’s modulus and the cohesion.