Rock slope dynamics in flysch formation under cold climate (part 1) : rock cracking and failure mechanism

Rockfalls are major natural hazards for road users and infrastructures in northern Gaspésie (Eastern Canada). In the last 30 years, more than 17 500 rockfalls have reached the two major road servicing the area. Rockfalls come from 10 to 100 m high flysch rockwall conducive to differential weathering. The retreat and settlement of weak rock strata (shale, siltstone) causes the gradual cantilevering of stronger rock strata (sandstone, greywacke), contributing to the development of tension cracks. The block, separated from the cliff, will eventually slide or topple on the eroding rock strata. These dynamics have been observed, but rarely studied with the objective of 1) determining the mechanical stresses and weathering conditions that promote rock cracking and 2) identifying the geometric conditions that control the final failure mode. We use the cantilever beam theory to model critical cantilever length (block size) and rock tensile strength. A frost cracking model (Rempel et al., 2016) was then used to explain the overestimation of the critical cantilever length and to verify whether the development of microfractures caused by frost damage can explain the decrease of the rock tensile strength over time. The results show that the areas of frost damage concentration correspond to those of maximum stress in the overhanging blocks. In order to identify the type of failure of these blocks, tests using a tilting table were carried out in laboratory. 405 tests were performed on 10 blocks characterized by different roughness coefficients and geometric ratios (height / length ratio, overhang length / total length of the block). The results, validated on natural blocks in the field, were used to identify the geometric conditions for stability, sliding, and toppling failure of overhanging block on an inclined plane. Such stability criteria could support the development of rock instability detection algorithm using high resolution 3D model.