Impulsive shear Strength of de-structurated Italian clays

The knowledge of the residual shear strength of clayey soils during fast shears is particularly significant to verify the possibility of co-seismic re-activation of landslide bodies. In seismically active areas, such as the Italian Apennines, the landslide reactivation can be one of the most important ground effects. On this basis, residual impulsive shear strengths were obtained by means of the ring shear apparatus, opportunely modified. The kinematic parameters, which characterized shear motion, were related to co-seismic displacements of rigid landslide bodies, as defined by Newmark’s approach. Test results performed on samples of typical Italian clays, characterized by different index properties, highlight the influence of the composition of the material and of the spatial disposition of particles on shear strength. Increase of the viscous strength in the residual condition determines viscoplastic strength greater than that in the fully softened condition. Consequently, an exceeding of viscous strength is obtained. Such an effect makes it possible to hypothesize new co-seismic failures both along the shear zone and in weak layers. The obtained values of shear strength, in the different conditions, were used in analysing co-seismic landslide displacement.