How Rainfall and Temperature Modulate Rock-slope Stiffness: Insights from Ultrasonic and Resonance Monitoring

Rockfalls threaten infrastructure and lives and are driven by progressive, climate-induced rock damage that weakens slopes until failure. Resonance frequency analysis can be to used to track stress evolution at the cliff (decameter) scale (1), but it lacks sensitivity to the near-surface zone where weathering initiates. We therefore combine resonance monitoring with high-frequency ultrasonic testing to resolve stress changes in this critical surface layer.

We deployed six ultrasonic transducers (two emitters and four receivers) over a few square meters on a 50-m-high south-facing limestone cliff above the Chauvet cave (SE France), while resonance frequencies were continuously recorded with a seismometer. On the one hand, repeated ultrasonic measurements provide relative sonic velocity changes as a proxy for near-surface stress changes and damage. On the other hand, resonance frequencies reflect the apparent rigidity and fracture dynamics of the entire rock column, which have been shown to track progressive damage at this site (2).

The data reveal pronounced diurnal velocity cycles driven by temperature-controlled opening and closure of micro-fractures. A major summer rainfall event caused an abrupt ~10% drop in sonic velocity, indicating a transient loss of near-surface rigidity. By constraining the surface contribution to resonance-frequency changes with the ultrasonic data and finite-element modelling, we could also show that rainfall promotes opening of the rear fracture of the cliff.

These coupled observations indicate that rainfall induces pore-pressure changes and fracture-deformation effects that temporarily reduce stiffness and accelerate sub-critical crack growth, promoting long-term slope weakening. The combined ultrasonic-seismic approach thus provides a powerful framework for quantifying climate-driven damage and improving rock-slope hazard assessment.
 

1 ) Guillemot, A., Baillet, L., Larose, E., & Bottelin, P. (2022). Changes in resonance frequency of rock columns due to thermoelastic effects on a daily scale: observations, modelling and insights to improve monitoring systems. Geophysical Journal International, 231(2), 894-906.

2 ) Guillemot, A., Audin, L., Larose, É., Baillet, L., Guéguen, P., Jaillet, S., & Delannoy, J. J. (2024). A comprehensive seismic monitoring of the pillar threatening the world cultural heritage site Chauvet‐Pont d'Arc cave, toward rock damage assessment. Earth and Space Science, 11(4), e2023EA003329.