The concept of strength degradation and progressive failure represents an important advancement in our understanding of deep-seated brittle rock slope failure. In conventional stability analyses, the strength properties of the rock mass or along a rupture surface on which sliding can occur are assumed to be constant over time. Failure is then explained through the influence of a triggering event, for example a heavy rainfall, which causes disequilibrium between the resisting and driving forces relative to the slope's constant strength. However, this does not properly explain the temporal nature of deep-seated rock slope failures. When a failure does occur, the triggering event rarely stands out as being exceptional, especially when compared to those that had occurred in the past. This suggests that the rock slope must be experiencing some form of strength degradation over time, driving the slope towards an unstable state (Eberhardt et al. 2004).
Strength degradation in these cases has been linked with the progressive failure of intact rock bridges and/or degradation of asperities along a sliding surface. Key studies by Terzaghi (1962), Robertson (1970), Einstein et al. (1983), Eberhardt et al. (2004) and Stead & Wolter (2015) demonstrate that the persistence of natural joints contributing to rock slope failure is generally limited, requiring intact rock bridges to fail to allow mobilization of the rockslide body. This may be associated with the development of a rupture surface and "rock slope collapse", which Hungr et al. (2014) define as sliding occurring on an irregular rupture surface consisting of randomly oriented joints and failed intact rock bridges (e.g., Fig. 1, left). It may also be associated with the development of internal shearing within the slide mass (Martin & Kaiser, 1984), for example in the case of "compound slides". Hungr et al. (2014) define compound slides, as cases involving a well-developed basal sliding surface comprised of multiple planes of uneven curvature such that sliding is kinematically possible only if accompanied by significant internal distortion and shearing of the moving mass (e.g., Fig. 1, right).