Effects of inclusion contents on resilient modulus and damping ratio of unsaturated track-bed materials

For the French conventional railway lines, a layer, namely interlayer, was created in the substructure mainly by the interpenetration of ballast grains and subgrade soils. The in situ investigation indicated that the content of coarse grains decreases over depth. In this study, the resilient modulus and damping ratio of the unsaturated lower part of the interlayer soil were investigated at six different volumetric inclusion contents f_v (volumetric ratio of dry coarse grains to the whole sample) by carrying out cyclic triaxial tests following a multi-step loading procedure. The results show that the two dynamic parameters (resilient modulus and damping ratio) change significantly at the beginning of loading and the variation rate decreases as the number of cycles increases. Besides, a mean characteristic volumetric inclusion content f_v-cha ranging from 27.8% to 28.9% was identified, separating two zones with different inclusion effects. To verify this observation, X-ray microcomputed tomography (_μCT) scans were conducted on as-compacted samples. The results obtained strongly support the existence of f_v-cha: when f_v ≤ f_v-cha, the matrix of fines constitutes the skeleton of the sample with inclusions floating in it, leading to slight changes of the two dynamic parameters with f_v. By contrast, when f_v > f_v-cha, the inclusions dominate the skeleton of the sample, giving rise to significant changes of the two dynamic parameters with f_v.