Back-Analysis of a Failed Rock Wedge Using a 3D Numerical Model

In the present paper, a rock wedge failure that occurred in the Rosandra valley (Trieste, NE Italy) has been back-analysed to compare the results of a traditional rock wedge analysis with those obtained using a 3D finite difference model (FDM). The simulations performed considering a purely frictional model of the rock wedge show that the values of the factor of safety (FOS) are in good agreement with those of the strength reduction factor (SRF). For the failure condition, the calculated values of FOS and SRF are very similar and both <1 (FOS = 0.92 and SRF = 0.93), suggesting that another type of localised strength has to be considered to guarantee block stability, such as that provided by a rock bridge. A second numerical model considers this localised rock bridge, whose presence, location and size have been determined through a detailed field investigation of the detachment surface. The parametric approach indicates that mean values of the shear stress acting on the rock bridge range from 1 to 2 MPa. The shear strength contribution given by the rock bridge is about 10-15 % of the whole resisting system. Calculated block displacements before the collapse vary from 0.1-0.5 mm to 2-3 cm, depending on different characteristic stiffness assumed for the basal rock joints and the rock bridge. Finally, the rock bridge cohesion at failure ranges from 0.8 to 1.2 Mpa, depending on different friction angles assumed for the rock bridge.