Influence of temperature on the residual shear strength of landslide soil: role of the clay fraction

Om Prasad Dhakal, Marco Loche , Ranjan Kumar Dahal, Gianvito Scaringi 

The shear strength is a fundamental parameter of soil that controls the occurrence and propagation of landslides. In pure clays, it depends on temperature according to the mineralogy, stress history, and hydro-mechanical boundary conditions. Landslide soils, however, are typically very heterogeneous and have a variable content of fines. The sensitivity of the residual shear strength of low-plasticity soil to temperature, in particular, is poorly understood, leaving significant uncertainties on the potential role of thermal forcing in landslides. We conducted ring-shear experiments on remoulded low-plasticity soil samples from the Melamchi catchment in central Nepal, where a large disaster occurred in 2021, with fifteen simultaneous landslides along the river corridor and a destructive flood. We performed the experiments in water-saturated conditions under representative normal stress values (50-100-150 kPa) and a constant rate of shearing (0.1 mm/min).
During each test, we controlled the temperature and performed a heating-cooling cycle (20-50-20◦33 C) with the shearing ongoing. To evaluate the role of the clay fraction (grain size <0.002 mm), we obtained specimens from the same soil samples by retaining the finest portion under three different cutoff grain sizes (0.125, 0.063, and 0.020 mm). The collected data were analysed statistically, using variance and skewness to evaluate the goodness of interpretation. A t-test was also implemented to exclude data close to the experimental uncertainty, delineating the experiment’s significance at a 68% confidence interval (1σ). Our results revealed a decreased residual shear strength upon heating (thermal weakening), with the magnitude of this weakening correlating with the specimen’s clay fraction and normal stress. Notably, a response to heating only emerged in specimens with a clay fraction of at least 10% and higher clay fraction and normal stress were conducive to greater weakening. Yet, the observed effect was relatively small, corresponding to a decrease in friction angle of just ∼1◦. This suggests a minor role of temperature in the response of sheared low-plasticity soil; however, more experiments are needed, covering the wide range of mineral compositions of clayey soils, to understand the role of temperature in the shearing process and formulate robust empirical laws to quantify thermal effects.