Thermally vs. seismically induced block displacements in Masada rock slopes

• Thermally induced wedging mechanism for rock blocks with filled tension cracks is proposed.• Seasonal temperature amplitude is sufficient to induce permanent plastic displacement.• For Masada slopes the wedging mechanism may lead to greater displacements than seismic loading over time.• Wedging mechanism should be considered when quantitatively assessing surface erosion of rock slopes.

We compare thermally and seismically induced sliding mechanisms of blocks that are separated from the rock mass by a tension crack and slide along a frictional interface. The rock slopes of Masada Mountain, Israel, are used to demonstrate our approach. Crack displacement coupled with thermal fluctuations is measured in the West slope of the mountain during two years (2009–11). Physical and mechanical lab tests provide the assumed depth of penetration of the heating front during seasonal cycles of exposure as well as the thermal expansion coefficient of the rock mass. These, along with the shear stiffness of the sliding interface, allow us to quantify the expected thermally induced displacement rate of blocks in Masada, through a proposed wedging–ratcheting failure mechanism. A distinct block in the East slope of the mountain exhibiting a tension crack opening of 200 mm was monitored for displacement and temperature during a single seasonal cycle in 1998. 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, discrete element, discontinuous deformation analysis (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. This 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.