Effects of temperature and shearing rate on the residual shear strength of two pure clays

The residual shear strength is the sole available strength in regular shear zones of landslides after large displacements. While it does not depend on the stress history, it has been shown to depend on the rate of shearing. Various mechanisms have been proposed to explain the shear-rate strengthening and weakening observed, in particular, in soils containing clay minerals. Frictional heating has been shown to be involved in shear weakening under very large shearing rates. However, changes in temperature (imposed as boundary conditions and propagating into the shear zone) also can affect the residual shear strength, even in drained condition, but evidence in the literature is scarce.

Here, we show results of temperature-controlled ring-shear tests on two pure clays (a commercial bentonite, very active, and a commercial kaolin, inactive), conducted under a wide range of shear displacement rates (0.02–45 mm/min) and normal stresses (50–150 kPa) typical of slow to rapid landslides. After attaining the residual shear strength under the chosen stress and displacement-rate conditions at room temperature (20 °C), we increased the temperature of the cell up to 55 °C and kept it constant over a sufficient shearing distance before gradually decreasing it back to the initial value.

We observed a clear effect of temperature on the residual shear strength of the active clay. We evaluated, in particular, a shear strengthening under slow shearing (up to +1.5 %/°C) which turned into a shear weakening under fast shearing (-0.5 %/°C) under any normal stress. We evaluated that the transition between the two behaviours occurred at a shear displacement rate of 0.1–1 mm/min, which is consistent with the range for the onset of shear rate-dependent behaviours. The effect produced by the increase in temperature was shown to be reversible, although in some cases we noticed a net decrease in strength that could be attributed to an improved alignment of the clay platelets resulting from the thermal cycle. Notably, little thermal effects were seen for the inactive clay, suggesting that such effects should originate from changes in physico-chemical forces of interaction at the microstructural level, which are indeed especially relevant in active clays.

Changes in residual shear strength with temperature could be related to changes in landslide activity (particularly for creeping landslides in clay soils) in terms of seasonal/progressive acceleration or deceleration driven by external hydro-meteorological forcing. Furthermore, these changes could also control the potential for runaway motion if a transition from a strengthening to a weakening behaviour occurs.