Time-lapse ultrasonic testing for monitoring stress and structural changes in rock cliffs

Rock fracturing plays a key role in both the formation of mountain landscapes and natural hazards. Weathering agents, such as daily thermal variations and precipitation, are some of the main triggers of the weathering and fracturing process. However, the mechanisms involved are not well understood, and questions remain about the thermo-mechanical stress field in the near surface of cliffs. 

To better understand the effect of thermal variation on mechanical stress variation at centimeter to meter scales, long-term recordings were made using repeatable ultrasonic acoustic sensors to measure both esound velocity and waveform changes. The acoustic sensors were placed on a few square meters of a 50 m limestone cliff above Chauvet Cave in the Ardeche region of southeastern France.

The results show that daily cycles of velocity changes are evident and appear to correlate with thermal fluctuations and variations in solar radiation. We propose that the velocity variations are due to thermal stress variations in the rock. During the hottest part of the day, the velocity variation causes an increase in compressive hydrostatic stress. At the same time, the spectral analysis of the impulse response shows daily variations with the appearance of high frequency content during the hottest part of the day. Thus, we propose that the hottest times of the day have the effect of expanding the rock surface and thus closing the fractures, which increases the high frequencies content. Conversely, during cooling periods, we can detect tensile stresses that are likely to open fractures and contribute to progressive subcritical cracking within the rock mass.

This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project