Fully-coupled hydrologic/geomechanical simulations of slope failure in a prototypical steep mountain catchment

This work presents a physics-based framework for continuum modeling of hydrologically-driven slope failure. The analyses employ a mixed finite element formulation for variably-saturated geomaterials undergoing elastoplastic deformations. The deforming soil mass is treated as a multiphase continuum, and the governing mass and momentum balance equations are solved in a fully-coupled manner. This tight coupling is necessary to capture key features of slope behavior. To test the coupled formulation, we present a three-dimensional slope analysis motivated by a 1996 landslide that occurred at a steep experimental catchment (CB1) near Coos Bay, Oregon. Simulations are used to quantify the rainfall-induced slope deformation and assess the failure potential. Results of parametric studies suggest that for a steep hillslope underlain by bedrock, similar to the CB1 site, failure would occur by a multiple slide block mechanism, with progressive failure surfaces forming at the bedrock interface and propagating to the surface. Extensive field observations and experimental measurements made at the CB1 site provide a rich data set to calibrate and evaluate the proposed numerical model. We take the opportunity, however, to point out those features of the model that are not well-constrained by available field data, but which may play an important role in determing the timing and location of failure. These observations are used to assess the current state of predictive capability of the slope simulations, and to inform the design of future field experiments.