Multiscale observations of diurnal thermal effects on rock failure and crack dynamics in soft marl layers (La Cornalle molasse rock wall, Switzerland)

Landslides and rockfalls frequently result from the weakening of soft rocks, with marl deterioration being particularly noteworthy. Most research on marl weathering has predominantly relied on laboratory simulations, leaving its natural behavior inadequately explored. This study investigates the impact of thermal weathering on marls at the La Cornalle Molasse rock wall in Epesse, Switzerland, which consists of interbedded thin sandstones and marls. We conducted a 26-h monitoring campaign (March 1–2, 2022), utilizing bi-hourly LiDAR scans, a K-type thermometer to measure rock surface temperature at 10-min intervals, a pyranometer for solar radiation monitoring at 10-min intervals, and thermal imaging of the rock wall face at 20-min intervals. Additionally, microscope images were taken using a portable digital microscope every hour to monitor marl microcracks at three different spots on different dates. During these survey periods, we detected 66 small rock detachments (6 in sandstones, 60 in marls), with a total marl volume of 0.0677 ± 0.0088 m 3 and an average failure scar depth of 0.018 m. Notably, 63.3 % of marl detachments occurred during daytime hours. Three detachment peaks were observed: the first during peak solar radiation on the initial day, the second during rapid cooling in the late afternoon and early evening of the same day, and the third during rapid heating in the late morning of the subsequent day. Crack movement was most pronounced in the afternoon and diminished at night, resuming with sunrise and increased solar radiation. The absence of rainfall and strong winds during the study period suggests that thermal tensile stress induced by solar radiation was the primary driver of rock detachment and crack activity. Our findings indicate that even minor diurnal temperature fluctuations can promote crack development and detachments in marl. While thermal weathering primarily affects the shallow rock surface, its long-term and repeated action can undermine slope stability. • A multiscale observational framework was developed to study the daily degradation of marl under natural environmental conditions. • For the first time, diurnal micro-scale displacement patterns were observed in a real field setting. Microscope imaging detected opening-closing cycles with sub-micrometer amplitudes. • Solar radiation and temperature variations were found to significantly influence the detachment of small marl fragments, the gradual pre-failure deformation of marl blocks, and microcrack activity. • The typically overlooked and challenging-to-detect small-scale degradation of soft rocks may play a crucial role in understanding the macro-scale evolution of rock wall retreat.