Abstract

The residual shear strength is a fundamental parameter controlling landslide processes. In pure clays, this parameter is temperature-dependent according to the clay’s mineral composition and hydro-mechanical boundary conditions. However, landslide soils are typically heterogeneous mixtures with a variable content of clay minerals. Particularly for low-plasticity soils, the impact of changes in temperature on the mechanical response remains to be determined, and little can be said about possible macroscopic alterations of slope stability or landslide dynamics.
We conducted ring-shear tests using natural soils from the Melamchi catchment in central Nepal, which suffered widespread instabilities and rainfall-induced debris flows. We performed experiments under normal stresses of 50-150 kPa in water-saturated condition and under a constant rate of shearing. We controlled the temperature during testing and performed heating-cooling cycles (20-50-20 °C) only after attaining the residual shear condition. We explored the role of the clay fraction by preparing multiple samples from the same soil, retaining its finest portion under different cutoff grain sizes (0.125-0.020 mm).
We observed a decrease in shear strength with the clay fraction increasing. Samples with a coarser cutoff (and hence a lower clay fraction) did not exhibit any change in shear strength during the heating-cooling cycle. However, as the clay fraction increased, a heating-induced weakening emerged. In the specific case study, this weakening was minor (1° of friction angle) and would not affect evaluations of slope stability in simple assessments. Nevertheless, incorporating this effect into advanced physically-based models may provide useful insights into the complexity of thermo-hydro-mechanical responses and their effects on landslides at the local and regional scales.