In 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 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.
Cyclic behavior of natural fine grained soils under a broad range of strains were investigated considering the effects of plasticity index and changes in confining pressures based on cyclic triaxial tests.
The second part involves the investigation of the undrained stress–strain behavior of fine grained soils under irregular cyclic loadings.
The results showed that the elastic threshold is approximately equal to 90% of .
Another transition point was defined as the flow threshold where the value of tangent of shear modulus ratio changes for the second time.
Simple empirical relationships to estimate the dynamic shear modulus and damping ratio was formulated and compared with the similar empirical relationships proposed in the literature.
The results provide useful guidelines for preliminary estimation of dynamic shear modulus and damping ratio values for fine grained soils based on laboratory tests.