Rate, Water Pressure, and Temperature Effects in Landslide Shear Zones

Abstract The behavior of slow‐moving landslides is controlled by the residual strength of the shear zone. Despite their mobile state, such slides can often withstand extreme events such as heavy rainfalls and earthquakes. A rate‐hardening in the shear zone is suspected to be one of the main stabilizing factors and has been well investigated, in particular for clayey soils. Soils from steep alpine landslides, on the other hand, are often dominated by silts and sands with various clay contents, and while understanding of their rate dependency is less advanced, it remains critical for the reliable risk assessment. In this article, an improved ring shear apparatus is presented and applied to investigate the rate, pore water pressure, and temperature effects in landslide shear zones. The testing program on samples from two alpine landslides in Switzerland revealed a moderately positive rate effect explaining their mobile state. An achieved insight into the thermo‐hydro‐mechanical processes of fast shearing provides physical evidence to the widely debated hypothesis on the generation of excess pore water pressure due to frictional heating. However, for the tested materials, this effect only occurs after extended shearing and does not lead to a complete loss of shear strength. Based on that and the observed positive rate effects, catastrophic failure seems unlikely for these landslides. At the same time, the results suggest that for less permeable soils, frictional heating may indeed be a source of negative rate effect during very rapid shearing.