Downhill progressive landslides in long natural slopes: triggering agents and landslide phases modeled with a finite difference method

Abstract

A large landslide in Tuve (Gothenburg, Sweden, 1977) initiated the development of a model for slope stability analysis taking the deformation-softening of soft sensitive clays into consideration. The model studies triggering agents and five phases in progressive slope failure are identified: (1) in situ, (2) disturbance, (3) unstable “dynamic”, (4) transitory (or permanent) equilibrium, and (5) “global” failure. The clay resistance in these phases may differ widely; mostly due to different rates of loading. Two time-dependent failure criteria are defined: (i) the triggering load condition in the disturbance phase 2 and (ii) the transitory equilibrium in phase 4, indicating whether minor downhill displacements or a veritable landslide catastrophe will occur. The analysis explains why downhill landslides tend to spread over vast areas of almost horizontal ground further downslope. The model has been applied to landslides in Scandinavia and Canada. Three case studies are briefly discussed. The model is a finite difference approach, where local downhill deformations caused by normal forces is maintained compatible with deviatory shear deformations above — and, if relevant, below — the potential (or the established) failure surface. Software and an easy-to-use spreadsheet are introduced as well as recent developments.