Wedge failure hazard assessment by means of a probabilistic approach for an unstable sea-cliff

This paper deals with a probabilistic approach for the slope safety factor calculation that uses kinematic and kinetic instability assessment of wedge failures affecting an unstable sea-cliff. The cliff, located at Agropoli (province of Salerno, Campania) along a coastal stretch of Southern Italy, is about 240 m long with height ranging between 10 and 55 m. Detailed geological, geo-structural and geo-mechanical surveys were performed in situ, whereas joint and rock strength parameters were measured in the laboratory. Wedge failures are the most frequent episodic phenomena favoured by the geo-structural and geomechanical layout of the rock mass, and subordinately the assailing force of waves. Based on the orientation of the discontinuities, with respect to the cliff face, the main joint set intersections which may cause potential unstable wedges have been identified. As during heavy sea storms, wave energy may contribute to cliff instability, and also an in-depth study regarding wave characteristics and direction affecting the coastal stretch where the sea-cliff is located was performed. The probabilistic stability analysis, performed using the SWEDGE software, allowed the identification of joint set intersections with high wedge failure probabilities. The failure probability is strongly affected by the geo-structural and geo-mechanical layout of the rock mass. In this respect, the random properties of orientation of joints which define the wedges and their shear strength have an important effect in the safety factor calculation. As the deterministic approach does not take into account the variability of these parameters and can provide simplified and misleading results, the simultaneous analysis of kinematic and kinetic conditions can provide a better evaluation of the overall stability. Finally, a wedge probability failure zonation of the whole sea-cliff was proposed.