Temperature and shear-rate effects in two pure clays: Possible implications for clay landslides

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Temperature fluctuations in landslide shear zones can originate from heat exchanges with deeper layers as well as with the atmosphere through the landslide body. Shallow landslides (depth <10 m) are especially subject to seasonal temperature oscillations and rapid climatic changes. Various hydro-mechanical properties of clayey soils are sensitive to changes in temperature. Few studies suggested that the residual shear strength may vary significantly even in temperature ranges typical of shallow layers in temperate and warm regions. Here, we verified the response of two pure clays (Ca-bentonite, kaolin) to shearing at temperatures up to ∼55 °C under various normal stresses (50–150 kPa) and shear rates (0.018–44.5 mm/min) by equipping a ring-shear device with a temperature-control system. Then, we performed experiments on an ideal slope to quantify the extent to which ground temperature can condition the stability of clay slopes, across the seasons and under prolonged warming. Considering the largest effects evaluated experimentally (change in residual shear strength by ±1.5 %/°C), we determined changes in global factor of safety by ∼20 % for rotational slides ∼6 m deep, solely attributable to seasonal heating-cooling. Warming of 5 °C over decades would change the stability condition by an additional ±7 %. Although these results were obtained under simplified geometry and boundary conditions, without considering changes in triggers, preconditions, and effects of other thermo-hydro-mechanical couplings, they provide an upper bound to the role of the temperature-dependence of the residual shear strength on the factor of safety. We argue that this role should not be neglected in slope stability and landslide hazard assessments in clay-rich soils, thus warranting in-depth experimental analyses and advanced modelling.