Analysis of a rock slope instability mechanism combining wedge and circular failure in a soft rock mass

Abstract One of the most common failure mechanisms in rock slope stability analysis is wedge failure, where sliding of an element of a rock mass occurs along two pre-existing discontinuities, whose intersection daylights on the slope face. The kinematic analysis of this mechanism prescribes that failure can only take place when the intersection of both discontinuities at stake dips less than the slope face. However, when the intersection line of a wedge dips slightly more than the slope face, a modified version of this mechanism can occur. In this paper, the authors describe in detail an actual case study of this type of failure mechanism that occurred in a 25-m high, 38°-dipping bench of an abandoned open-pit mine in Spain. To illustrate how this phenomenon indeed takes place, a timeline description of events, intact rock and rock mass characterization and a detailed slope failure analysis are included. Moreover, indicative limit equilibrium and more accurate numerical modelling approaches are presented guiding how to estimate factors of safety against this complex type of instability mechanisms. Furthermore, some situations associated with soft rock where these and similar slope failure mechanisms could take place are discussed. These approaches can be of help for the purpose of back analyzing similar case studies and for designing slopes when potential failures of this type are prone to occur.