Permanent deformation of track-bed materials at various inclusion contents under large number of loading cycles

In the French conventional railway substructure, the interpenetration of ballast grains and subgrade fine soils that occurred over years of operation has created a new layer, namely, an interlayer. The in situ investigation on this interlayer showed that the content of ballast grains decreases over depth, with the lower part characterized by a matrix of fines with inclusions of coarse grains. In this study, the permanent deformation of a material simulating the lower part of interlayer soil was investigated at six different volumetric inclusion contents f_v (volumetric ratio of the inclusion grains to the total sample) by performing cyclic triaxial tests. The results indicate that at a given cycle, the permanent deformation decreases with the increase of f_v, and the evolution of permanent deformation is strongly influenced by the loading history. To further investigate the effect of inclusion content, a method that allows eliminating the influence of loading history and estimating the end-stage permanent deformations at different stress levels was applied. Quadratic polynomial and bilinear fitting methods were used to fit the variations of the estimated end-stage permanent deformations with f_v, showing a proper similarity between the two methods at high loading amplitudes (higher than 15 kPa). Furthermore, the testing results revealed the existence of a characteristic volumetric inclusion content f_v-cha by bilinear fitting method, separating two zones with different inclusion effects. This observation is strongly supported by the X-ray microcomputed tomography (μCT) scans performed on the as-compacted samples. When f_v ranges from 0 to 20% (smaller than f_v-cha), the fines constitute the skeleton of the sample, and the permanent deformation decreases rapidly with the increase of f_v. By contrast, when f_v increases up to 35-45% (larger than f_v-cha), the inclusions dominate the skeleton of the sample, leading to a slight decrease of permanent deformation with f_v.