Physical Model of the Mechanism for Thermal Wedging Failure in Rocks

Cyclic diurnal and annual temperature variations acting upon rocks are rarely considered among triggers of slope movements. The importance of temperature change is viewed mainly as a precursor of failures, where the triggers are rainfall or seismic activity. This paper aims to determine the limit conditions in which plastic deformation develops in a situation where one or more blocks fallen into an open crack create a wedge, causing non-elastic displacement of a block resting on an inclined plane. A physical model was prepared to study this phenomenon in a thermal dilatometer, in which the displacements were measured using linear variable differential transformer (LVDT) sensors for blocks with different block/wedge ratios, while temperature was varied in a controlled manner. Nine physical models of sandstone blocks were tested over a cyclic temperature change of ΔT = 35 °C while measuring the permanent displacements of a block in order to confirm the existence of this type of failure mechanism. Further, a series of cyclic tests were performed on all nine physical models to determine the threshold temperature change at which plastic deformation occurs for different block/wedge ratios. Results showed plastic deformation resulting from a cyclic wedging mechanism for a block/wedge ratio 0.5 and total model size of 50 mm, reaching a permanent displacement of 4.23 × 10⁻³ mm for a block resting on an inclined plane with a slope of 7°. For these conditions, a temperature change which caused permanent block displacement by thermal wedging was as low as 6 °C. The results of the physical model are in agreement with a proposed analytical solution by Pasten (2013) and measurements of Bakun-Mazor et al. (2013) at a site at Masada, Israel.