Influence of climate-dependent variables on deformation and differential erosion of stratified sedimentary rocks

Rockfalls are major natural hazards for road users and infrastructures in northern Gaspe & PRIME;sie (Eastern Canada) where nearly 25 km of road runs along imposing rockwalls. Since 1987, the Ministe`re des Transports du Que & PRIME;bec (MTQ) has reported >17,500 rockfalls that have reached the roadway. While rockfalls are primarily gravitational phenomena, other factors contribute to their development. The long-term interaction of a series of processes leads to rock instabilities. They are never caused solely by the most recent visible change. This study aims to better understand the influence of climate-dependent variables on the mechanical deformations that leads to the erosion of stratified sedimentary rock sequences. To achieve this, a flysch rockwall was instrumented with crack meters and a network of hydrometeorological sensors. The major deformations affecting stratum of siltstone and sandstone were identified and correlated with climate-dependent variables. Results highlighted that irreversible deformations in the first decimeters of rock wall surface are mainly induced by rainfall and snowmelt, by temperature fluctuations around the freezing point and to a lesser extent, by large thermal variations. The water content fluctuations at the rock surface are more effective at generating irreversible deformations in the clayey siltstone strata than freeze-thaw cycles and high amplitude thermal variations. In the sandstone strata, irreversible deformations were primarily caused by freeze-thaw cycles and water content fluctuations at the rock surface. The long-term trends in the sandstone and siltstone strata recorded by crack meters, along with field observations, highlight mechanisms that lead to the development of superficial rock instabilities in this type of geology. Our results show that the retreat and settlement of the weak rock strata (clayey siltstone) causes the gradual overhang of the resistant rock strata (sandstone). As a result, sandstone blocks will eventually slide or topple on the eroding weak rock strata below. The results of this study underscore the importance of studying rock wall dynamics as a whole to understand the mechanisms that lead to the development of hazardous largescale rock instabilities.