Analysis of lateral rock spreading process initiation with a numerical modelling approach

The first erosional front of the south-eastern Alps has been affected by several deep-seated landslides. The movements affected a Jurassic limestone sequence underlain by an upper Trias unit (Keuper) mainly composed of marl and clays. Horst- and graben-like structures, newly formed gravitational faults, outflow and other field evidence suggest that such movements are consistent with lateral rock spreading processes. An example of such processes is the La Marbriere' slope, where field investigations have revealed that three distinct zones corresponding to three evolutionary stages of movement exist. The question is: What are the structural and mechanical modifications undergone by the slope between those evolution stages? To tackle this question, numerical models based on a 2D cross section of the La Marbriere' slope have been performed. Results show that, to fit the structural evidence of the precollapse situation, elasto-plastic behaviour of the Trias Keuper unit is needed. The transition between the precollapse and fresh post-collapse situations requires a modification of the mechanical behaviour of the Trias Keuper unit to Burger type, resulting in the formation of antithetic and synthetic gravitational faults (producing a graben-like structure).