Valley shape influence on deformation mechanisms of rock slopes

Stress distribution in mountainous areas is influenced by local morphology. Valley morphology and the relationship between main and tributary valleys strongly depend on geological characteristics and evolution. They may control the evolution of slope instabilities, especially when interacting with pervasive structural features. We performed parametric three-dimensional (3D) numerical modelling of simplified slope geometries with variable slope angle (from 21deg to 35deg;), length, combining different orientations for different slope sectors and changing attitude of pervasive planes of anisotropy (foliation, schistosity, bedding). Data used in the 3D models are the initial slope geometry, rock mass properties and internal anisotropy. We assumed Mohr-Coulomb behaviour, with the presence of ubiquitous joints and different piezometric levels. The model results show that plastic deformation initiates near the highest ridge just after deglaciation commences.A shear zone develops and propagates toward the toe of the slope, and its shape is strongly controlled by slope geometry, anisotropy and in situ stresses. The thickness of the failing mass, for model slope reliefs up to 3200 m, increases from 50 mto some hundreds of metres during glacier retreat, and it depends on geometry of slopes, anisotropy and in situ stresses. Results are compared to examples of deep-seated slope deformations from the Alps, which helps in the interpretation of such phenomena and in the understanding of their influence on valley evolution. © The Geological Society of London 2011.