Investigating the effects of thermal weathering on bedrock cliffs using in-situ borehole monitoring in Yosemite Valley, California, USA

Thermal weathering has a long-term (decadal to millenia) effect on the physical and mechanical properties of rocks and can directly influence rockfall hazard. However, despite substantial research, the processes of rock degredation from thermal influences are not completely understood. This study analyzed data from two, 3-meter-deep boreholes spaced approximately 1.8 km apart in western Yosemite Valley, California (USA) subject to different thermal regimes. The boreholes were drilled horizontally into lithologically and macroscopically identical rock (El Capitan Granite) with no visible fractures. However, we located the two boreholes with different aspects, one on a sun-facing cliff with southern aspect, and one on a sun-shaded cliff with northern aspect. Sensors in the boreholes monitor temperature changes at 10-minute intervals and were installed from the surface to 3-m-depth at increasing increment intervals (i.e., at the surface, 5, 10, 20, 30, 50, 75, 100, 150, 200 and 300 cm). A comparison of the temperature data showed significant differences in rock surface temperatures and temperature gradients between the south- and north-facing cliffs. Between April and November 2024, the south-facing site showed a greater surface temperature range (51.2°C) and average (28.3°C) than the north-facing site (28.3°C range and 17.3°C average). At 3-m-depth, the south-facing site had a temperature range of 10.6°C (21.0°C average) compared to a 8.1°C range (13.1°C average) for the north-facing site. We also analyzed the borehole cores using applied ultrasonic P- and S-waves to calculate their dynamic elastic properties. Despite similar lithology and structure, significant differences were found between the sites. Samples from the south-facing slope, which receives more thermal energy given its location in the Northern Hemisphere, proved to be more weathered, coincident with rock of higher porosity and lower dynamic moduli. In contrast, rock samples from the north-facing and consequently wetter slope showed lower porosity, higher elastic moduli, and a more pronounced gradient of weathering towards the surface. We hypothesize that diurnal and annual thermal stress changes play a larger role in rock weathering than previously assumed, possibly exceeding the long-term influence of other factors, such as groundwater. However, further measurements of rock properties at multiple locations are required to confirm this hypothesis.