Stability and deformation mode of a rock slide along interbeds

The paper analyzes the stability and deformation mode of a large planar slide (140,000 m3) within a limestone formation, slipping along clay-filled bedding planes. Monitoring data extending over almost five years indicate that movements of the unstable wedge, firstly detected in 2003, are still proceeding and alternate phases of quiescence and reactivation associated with critical rainfall thresholds. The role of water thrust within the tension crack and of the pore pressures along the sliding surface was initially investigated through limit equilibrium analyses. Stress-strain analyses by means of a 3D model were successively carried out in order to evaluate distribution of stresses and deformations within the wedge. Results are consistent with the measured displacement pattern and support their dependency on strength conditions at the wedge boundary. Analyses provide elements for understanding the disarrangement observed within the sliding mass, which can largely influence the evolution towards failure and its relationship with hydraulic conditions and hence with the rainfall regime.