In the sub-substructure of conventional tracks in France, an interlayer was created with time.The soil in this interlayer is characterised by the presence of both coarse elements from ballast mainly and fines from subgrade.The mechanical behaviour of this coarse soil was studied in the laboratory using a large-scale triaxial cell.The soil taken from an interlayer was re-compacted to a dry density of 2.01 Mg/m3, at three water contents (4, 6 and 12%) corresponding to three values of initial degree of saturation (32, 48 and 100%).Both monotonic and cyclic triaxial tests were performed under constant water content conditions.The experimental results evidenced the significant effect of 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 a 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 developed, that accounts for the stress level, number of cycles and soil water content.