The bi-planar failure, sometimes referred to as "bi-linear failure", is a particular type of rupture of rock slopes that occurs when a steep rock joint intersects a discontinuity having a lower inclination and that daylights at the rock face. The bi-planar configuration requires, differently from other well-known failure types (such as planar, wedge and circular failures), a considerable inner deformation and/or rock fracturing to make the block movement and the subsequent collapse possible. In the present paper, a forward analysis has been performed on a high natural rock slope (height = 150 m) made up of stratified limestone and characterised by a bi-planar sliding surface. The slope stability has been investigated adopting a 2D finite difference analysis (FDA). Two specific failure mechanisms (1 and 2) have been identified, based on the different strength parameters assumed in the models. In failure mechanism 1, a combination of internal shear and tensile fracturing occurs so as to form a deep, curvilinear rupture surface that links the two pre-existing planar surfaces. The block kinematism is an en-block roto-translation that, in turn, causes additional internal fracturing to accommodate deformation. In failure mechanism 2, a large shear band with obsequent dip enucleates within the unstable block, thus subdividing it into two main sub-blocks with different kinematisms. Model results demonstrate that bi-planar rock slope failures are associated with internal block damage that can also determine possible inner block splitting and differential movements between the secondary blocks. Stress-strain modelling is a very effective study approach that can be used to understand the key role played by rock fracturing and inner deformation occurring during the long preparatory phase that precedes the final collapse.
