Hydromechanical triggering of landslides: From progressive local failures to mass release

Water infiltrating during intense rainfall on steep slopes gradually weakens the wet soil mass, inducing localized failures that may initiate a cascade of load redistributions and successive failures propagating across a hillslope. The challenge of linking the progressive nature of local events culminating in an abrupt landslide is addressed by a new hydromechanical triggering model that links key hydrologic processes with threshold-based mechanical interactions. A hillslope is represented as assembly of soil columns interconnected by frictional and tensile mechanical bonds represented as virtual bundles of fibers. Increasing water load exerted on mechanical bonds causes gradual failure of fibers until restraining forces are exceeded. Following failure at the soil-bedrock interface, the load on a column is redistributed to its neighbors via intact mechanical (primarily tensile) bonds which, in turn, may also fail and transmit the load downslope as compressive stresses. When soil internal compressive strength is exceeded, a load-bearing column may liquefy and initiate a landslide release that could propagate downslope or retrogressively upslope. The model reproduces observed power law frequency magnitude relationships of landslides with exponents ranging between -1.0 and -2.2 in agreement with landslide inventory data. We applied a criticality measure defined by Ramos (2011) to evaluate the specific influences of slope angle, soil texture, and root reinforcement on attainment of hillslope criticality in which a small local failure may trigger release of a large soil mass. The model provides new insights on the conditions giving rise to an abrupt transition from a seemingly stable hillslope to a catastrophic landslide.