Shear mechanisms and viscoplastic effects during impulsive shearing

In clay formations, the co-seismic reactivation of pre-existing landslides involves dynamic shear actions on sliding surfaces characterised by attritive states. Impulsive shear motions were applied, after the attainment of the fully softened and residual conditions, by means of an appropriately modified ring-shear apparatus. The kinematics parameters that characterise shear motions are related to co-seismic displacements of rigid landslide bodies, as defined by Newmark's approach. The results of the tests, performed on clay samples with different index properties, highlight the influence of the initial spatial disposition of particles on the shear mechanisms. In addition, it seems that these mechanisms control both the inviscid and viscous components of the shear stress. For high shear rates, an increase of the viscous strength in the residual condition determines a greater viscoplastic strength than that in the fully softened condition. On this basis, different behaviours in seismically induced reactivation of landslides can be explained.