Thermally vs. seismically induced block displacements in rock slopes

Annual temperature fluctuations are responsible for generating irreversible displacements of removable rock blocks that are separated from the rock mass by the intersections of pre-existing discontinuities. A new mechanism, referred to here as "ratchet" model, is proposed to explain how cyclic thermal oscillations induce intermittent expansion and contraction of the tension crack, thus causing seasonal translations of rock blocks. Analytical model is suggested in terms of a thermally-induced wedging failure that essentially operates as a "ratchet" mechanism involving the rock mass, tension crack, sliding block, and sliding surface. We test the model on a distinct block in the East slope of Masada Mountain, Israel, exhibiting a tension crack opening of 200 mm. Based on the assumed seismicity of the region and the known topographic site effect, along with the laboratory measured frictional resistance and shear stiffness of the sliding interface, we subject the mapped geometry of the block in the East face to simulated cycles of earthquake vibrations utilizing the numerical DDA method. We find that for a time window of 5000 years, the observed 200 mm displacement of the East slope block is more likely to have been thermally, rather than seismically, controlled. The result implies that in climatic regions where the temperature amplitude over a seasonal cycle is sufficiently high, thermally induced displacements play an important role in rock slope erosion.