Monitoring Rain-Induced Stress Changes in a Limestone Cliff Using Ultrasonic Testing and Resonance Frequency

Rockfalls pose significant risks, with the potential to cause severe infrastructural damage and fatalities. Among the primary weathering agents - freezing, rainfall, and thermal variations - rainfall's impact on rock weathering remains poorly understood. The mechanical properties - damage and rigidity - are crucial determinants of long-term rock stability (2). This study investigates the effects of rainfall on the sonic velocities and apparent rigidity of a natural rock column.

Ultrasonic testing, a widely used method in structural health monitoring, was employed in situ on a 50-meter-high south-facing limestone cliff overlying the Chauvet Cave in the Ardèche Plateau, SE France. This cliff experiences a range of climatic solicitations, including solar illumination, temperature fluctuations, and rainfall events. Sonic velocity changes, obtained during repeated ultrasonic testing, are indicative of internal stress variations within the rock, driven by environmental factors (thermal-acousto-elasticity, (1)).

We combined ultrasonic testing with resonance frequency measurements to evaluate stress changes at both centimeter and decameter scales of a limestone cliff. While sonic velocities provide insights into local rigidity, resonance frequency measurements reflect changes in the apparent rigidity and fracture dynamics of the rock mass as a whole. Summer rain events caused a drop in resonance frequency, likely due to rock mass contraction and fracture adjustments, while sonic velocity responses varied depending on rainfall intensity. These results suggest an interplay between rainfall and rock properties, potentially involving pore space filling and increased local rigidity from micro-crack closure. This study underscores the value of sonic velocity measurements as a proxy for assessing rock damage and rigidity, emphasizing the need for further quantification to better understand damage evolution and rock stability.
 

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.