YIELDING AND PLASTIC BEHAVIOR OF AN UNSATURATED COMPACTED SILT

Within the framework of an extended elastoplastic constitutive model f or unsaturated soils (loading-collapse (LC) model), an experimental pr ogramme was performed in an osmotically controlled suction triaxial ap paratus. The laboratory behaviour of a statically compacted silt was s tudied, and particular attention was given to the volume changes monit ored during shear. Isotropic loading tests confirmed the main features of the LC model related to the effect of suction on volume changes, a nd allowed a direct determination of the LC curve. Constant sigma(3) a nd a few constant eta shear tests were performed in order to study yie lding and plastic flow at various increasing suctions, starting from t he as-compacted condition. Several yield criteria were considered, dep ending on the type of test performed. Some similarities between compac ted unsaturated soils and natural soft soils were shown, such as the i nclined elliptical form of the yield curve, which results from the ani sotropic state of stress prevailing during K-0 compaction. Some precon solidation effects due to increasing suction were identified, and an a pproximately isotropic suction hardening phenomenon was evidenced, tog ether with a nonassociated flow rule. The direction of the plastic str ain increment seemed almost independent of the suction, and a hyperbol ic 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 overconsolidat ed soils, the predicted stress-strain curves showed a sudden transitio n at yield, whereas a much more gradual transition was observed in pra ctice. Volume change prediction appeared satisfactory, showing the val idity of the hyperbolic plastic potential.