Numerical analysis of a landslide in soils with strain-softening behaviour

The results of an analysis concerning a landslide of great dimensions, which occurred at Senise (Southern Italy) on 26 July 1986, are presented. The landslide was of a translational type, and the failure developed within a thin clayey silt layer interbedded by a slightly cemented sand formation. Both these soils experienced a pronounced strain-softening behaviour during laboratory tests. From the analyses performed using the traditional limit equilibrium approach, it has been found that the average operational resistance along the sliding surface was less than that of the peak and greater than the residual resistance deduced from the laboratory tests. As a consequence, it has been postulated that a progressive failure had occurred. Such a failure process can be appropriately studied using methods that are capable of simulating the formation and development of shear zones, where strain is localised. From a computational point of view this presents many difficulties, because the numerical procedures currently used are very often affected by a lack of convergence, and the solution may depend strongly on the mesh adopted. In order to overcome these numerical drawbacks, in the present study use is made of both an elasto-viscoplastic constitutive model and a nonlocal elasto-viscoplastic model implemented in a finite element code. The Mohr–Coulomb plastic law is adopted, and the strain-softening behaviour of the soil is simulated by reducing the strength parameters with the accumulated deviatoric plastic strains. The results of the analysis show that a progressive failure occurred owing to deep excavations carried out at the toe of the slope. In addition, the soil mass involved in the sliding and the location of the failure surface deduced from the numerical simulation are found to be very similar to those observed.