Rate-dependency of residual shear strength of soils: implications for landslide evolution

Shear-rate weakening or strengthening behaviours can effectively control landslide runouts, defining sudden runaway sliding or years-long slow-creep phases. These behaviours are partly controlled by the properties of the basal material. Understanding its stress-strain-time response is crucial in physically-based assessments of landslide dynamics and the associated risk. We investigate the frictional resistance of a calcium bentonite, a kaolin and a quartz sand by means of a conventional ring-shear apparatus under normal stresses representative of landslide shear zones. Results for the fine-grained soils, in line with literature on pure clays, indicate important velocity strengthening, whereas small shear-rate effects were observed in sand. As long as effective stresses remain constant, a velocity strengthening response can exert a feedback that, under certain conditions, counteracts perturbations in boundary conditions and prevents fast runouts on pre-existing shear zones. Accordingly, we argue that specifically testing for shear-rate-dependency and incorporating observed behaviours in model formulations can be beneficial for better predicting landslide fates.

Automated summary

The shear-rate weakening or strengthening behaviors of landslides are influenced by the properties of the basal material and can control the speed and duration of the landslide. Understanding the stress-strain-time response of the basal material is crucial for assessing landslide dynamics and associated risks. In this study, the frictional resistance of different types of soil was tested under normal stresses representative of landslide shear zones. The results showed velocity strengthening behavior in fine-grained soils, while small shear-rate effects were observed in sand. Incorporating these observed behaviors into model formulations can improve the prediction of landslide outcomes.