Why permafrost rocks become unstable: a rock-ice-mechanical model in time and space

In this paper, we develop a mechanical model that relates the destabilisation of thawing permafrost rock slopes to temperature-related effects on both, rock- and ice mechanics;and laboratory testing of key assumptions is performed. Degrading permafrost is considered to be an important factor for rock-slope failures in alpine and arctic environments, but the mechanics are poorly understood. The destabilisation is commonly attributed to changes in ice-mechanical properties while bedrock friction and fracture propagation have not been considered yet. However, fracture toughness, compressive and tensile strength decrease by up to 50 % and more when intact water-saturated rock thaws. Based on literature and experiments, we develop a modified Mohr-Coulomb failure criterion for ice-filled rock fractures thatincorporates fracturing of rock bridges, friction of rough fracture surfaces, ductile creep of ice and detachment mechanisms along rock-ice interfaces. Novel laboratory setups were developed to assess the temperature dependency of the friction of ice-free rock-rock interfaces and the shear detachment of rock-ice interfaces. Indegrading permafrost, rock-mechanical properties may control early stages of destabilisation and become more important for higher normal stress, i.e. higher magnitudes of rock slope failure. Ice-mechanical properties outbalance the importance of rock-mechanical components after the deformation accelerates and are more relevant for smaller magnitudes. The model explains why all magnitudes of rock slope failures can be prepared and triggered by permafrost degradation and is capable of conditioning long paraglacial response times. Here, we present a synoptic rock- and ice-mechanical model that explains the mechanical destabilisation processes operating in warming permafrost rocks