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 destabilization 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 destabilization 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 that incorporates 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. In degrading permafrost, rock-mechanical properties may control early stages of destabilization 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 para-glacial response times. Here, we present a synoptic rock- and ice-mechanical model that explains the mechanical destabilization processes operating in warming permafrost rocks.