The role of discontinuities in the susceptibility, development, and runout of rock avalanches: a review

Rock avalanche is one of the most spectacular and catastrophic type of natural hazard phenomena. Those events typically start with a giant rock block or multiple blocks becoming detached from the rock slope, progressively fragmenting and transforming into rapidly moving cohesionless rock debris. Discontinuities are widely distributed in rock masses. Although research on rock avalanche phenomena is extensive, the role of discontinuities in different phases of rock avalanches, including the susceptibility, development, and runout phases, has not been systematically and comprehensively addressed, which has aroused a long-standing controversial issue. In this paper, the effects of discontinuities on the three phases of rock avalanches are systematically reviewed and discussed. The preexisting discontinuities influence not only the detachment of rock masses in the failure process but also their disintegration and propagation during runout. As a precursory factor, discontinuities control the kinematic feasibility of rock slope failure and the rock mass strength and thus control the susceptibility of the rock slope to failure as well as the size and spatial distribution of potential rock slope failure areas. During the development phase, the existing discontinuities will propagate and coalesce, increasing the slope fragmentation and decreasing the resistance to failure, and the kinematics of detachment evolve. It is worth noting that the evolution and failure phase would not happen, or just in moments in an earthquake-triggered events(s) or similar events. During runout, the control of discontinuities on rock avalanches is primarily reflected by shear and progressive fragmentation accompanied by heterogeneous distributions of stress and grain size, efficient energy transfer, and characteristic deposits. Nevertheless, dynamics of rock avalanches is complex, and controversial disputes remain; there is no straightforward conclusion. The inherent geology might play a dominant role in determining their strengthening or weakening effect in the various stages of rock avalanches. Several perspectives on future research are discussed, and approaches for focusing on the challenging research required to better our understanding of the role of discontinuities are suggested.