Analysis of the flexural failure of an overhanging rock slab

Bending and toppling of rock slabs is an instability process that can occur in natural or excavated overhanging rock slopes. The paper discusses the flexural failure of a gneiss slab from a quarry face. The failure developed both due to the breaking of the intact rock and to the linking of preexisting joints. The mechanical parameters of the gneiss were evaluated through lab. testing, of above all the flexural strength, deformability and the toughness. A suitable, though arbitrary, reduction of these parameters was applied for the analysis of the slab failure. For stress analysis, the inclined slab was modelled by using a thin plate FEM model. The stress intensity factors were evaluated according to the plate results, by using a beam -of equivalent length- with a slit located at the beam clamped end. The LEFM approach was applied to the beam scheme using FEM models. The results show that the ordinary load condition (self weight) should satisfy slab stability, while small defects in the resisting section of the slab could be triggered by momentary, heavy load intensity (water or seismic events), and could eventually lead the slab to failure.