| Project Detail |
The general idea that a rock mass cracks when the external stress-loading exceeds the rock strength, leading to catastrophic failure (critical cracking), is challenged when considering outcrop erosion. Many observations in the field and the laboratory evidence slow, incremental crack developments well below the yield stress of the material, for which repetitive stress load and fatigue induce progressive failure, also called sub-critical cracking. It is largely accepted that the weather (temperature changes, rain, frost) is a major source of stress cycles and slow cumulative damage, but the literature shows too limited quantitative field observations to confirm the mechanics behind rock outcrop cracking. Today, we cannot predict rockfalls nor quantify the relation between daily or seasonal weather forcings to rock crack production. And yet, rock fracturing plays a leading role in most surface processes, from landscape building to vegetation, hydrology and natural hazard prediction. There is an urgent need to reveal the mechanics of sub-critical failure within rock outcrops. Fracturing processes initiate and develop at small scales (from the grain size to a few mm), and meteorological forcing evolve over daily to seasonal time scales, but existing field observation tools hardly meet these scales. In order to lift this issue, the project will deploy an original active multisensory acoustic imaging technique on natural rock outcrops, to map and monitor stress changes and crack production (in m2/m3/time) over timescales ranging from a few hours to a few months, with a centimetric spatial resolution and a stress sensitivity down to a few kPa. Stress will be deduced from acoustic waves, thanks to the acousto-elasticity principle, allowing us to determine the physics of sub-critical cracking of rock outcrops in their natural environment. This work will pave the way for predicting rockfalls and, further, the impact of climate change on rock erosion. |
