Yielding and plastic behaviour of an unsaturated compacted silt

Within the framework of an extended elastoplastic constitutive model for unsaturated soils (loading-collapse (LC) model), an experimental programme was performed in an osmotically controlled suction triaxial apparatus. The laboratory behaviour of a statically compacted silt was studied, and particular attention was given to the volume changes monitored during shear. Isotropic loading tests confirmed the main features of the LC model related to the effect of suction on volume changes, and allowed a direct determination of the LC curve. Constant sigma(3) and a few constant eta shear tests were performed in order to study yielding and plastic flow at various increasing suctions, starting from the as-compacted condition. Several yield criteria were considered, depending on the type of test performed. Some similarities between compacted unsaturated soils and natural soft soils were shown, such as the inclined elliptical form of the yield curve, which results from the anisotropic state of stress prevailing during K-0 compaction. Some preconsolidation effects due to increasing suction were identified, and an approximately isotropic suction hardening phenomenon was evidenced, together with a nonassociated flow rule. The direction of the plastic strain increment seemed almost independent of the suction, and a hyperbolic plastic potential, similar to that of sand, was found satisfactory. Inclined ellipses were chosen for modelling the yield curves. As for any simple elasto-plastic Cam day type model applied to overconsolidated soils, the predicted stress-strain curves showed a sudden transition at yield, whereas a much more gradual transition was observed in practice. Volume change prediction appeared satisfactory, showing the validity of the hyperbolic plastic potential.