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International audienceIn the track substructure of ancient railways in France, a fouled ballast layer has often been created with time. The mechanical behaviour of this coarse soil was studied in the laboratory using a large-scale triaxial cell. The soil taken from the fouled ballast layer of an ancient railway was re-compacted to a dry density of 2.01 Mg/m(3) at three water contents (4, 6, and 12%) corresponding to three values of the initial degree of saturation (32, 48, and 100% respectively). Both monotonic and cyclic triaxial tests were performed under constant water content conditions. The experimental results gave the following evidence of the significant effect of the water content on the soil mechanical behaviour: (i) the lower the compaction water content, the higher the shear strength; (ii) a permanent axial strain of 0.4% was found after a large number of cycles at a water content of 4%, while it was 1.4% at the higher water content of 6%. For the saturated soil specimen, failure was even observed after a limited number of cycles. Based on the results obtained, a constitutive model for permanent deformation was elaborated, that accounts for the stress level, the number of cycles and the soil water content. (C) 2012 The Japanese Geotechnical Society

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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 fv (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 fv–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 fv–cha: when fv ≤ fv–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 fv. By contrast, when fv > fv–cha, the inclusions dominate the skeleton of the sample, giving rise to significant changes of the two dynamic parameters with fv.

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