Current understanding of rock slope shear zones

Large rock slope deformations (RSD) are controlled by basal shear zones that are rarely exposed and difficult to sample. This review synthesises current understanding of their genesis, geometry, materials, and time-dependent behaviour, with emphasis on the roles of lithology, pre-existing structures, shear-zone material and hydrology. Basal surfaces are commonly stepped, stacked, multi-planar and locally curved, rather than single planar failures, and shear zone architecture reflects litho-structural anisotropy and persistence. Rock bridges, typically a small fraction of the surface, are critical for temporary stability; failure mode shifts from tension upslope to shear at the toe as confinement increases. Shear zone materials consist of thin clayey gouge bounded by crushed bands with gradual transitions; grain size reduction, mineralogical effects, and gouge maturity govern residual friction. Hydro-mechanical coupling modulates slip rates: pore-pressure changes can trigger transient accelerations and, under undrained conditions and rate-weakening materials, rapid motion. We outline a practical framework that links geomorphology—extensional head, high-strain transition, and compressional toe—to likely basal architecture, enabling first-order subsurface inference without drilling. This review advances practice by providing a framework to link observable shear-zone material maturity indicators to indicative residual-strength ranges, formalising a geomorphology-to-subsurface workflow that integrates structural mapping and surface kinematics and providing a concise field checklist to classify shear-zone maturity. Case syntheses, including Åkneset and Downie Slide, illustrate how integrated geomorphology, structural mapping, and displacement data support hazard assessment and monitoring. The review provides transferable concepts for engineering geologists to interpret RSDs and improve risk mitigation where direct subsurface data are limited.