A real-time seismic and displacement monitoring system for rock instability assessment — case studies in the French Alps

Abstract

Cliff instabilities are a common problem for the local authorities in the French Alps, significantly affecting the local economy. When the implementation of protection against rockfalls is not a comprehensive solution, an early warning system remains an important alternative to monitor unstable rocks and to evaluate the safety of hazardous areas. Several instrumentation systems have been implement in the French Alps where the danger is resultant from the construction or improvement of roads, railway lines and buildings near steep slopes or cliffs. Some cases, when monitoring can be performed in real time, instrumentation becomes a key element for the authorities to take important safety decision. Indeed, the instant prediction of rockfall incidents can significantly help the local authorities to take all the necessary safety measures to manage the traffic congestion. The classic and most common method of monitoring cliffs is to follow the surface movements using displacement measurement sensors such as crackmeters, EDM or radar. The monitoring of internal deformation in a rock mass is a difficult variable to apprehend. The seismic noise methodology can meet that need and provide structure information to develop a full three-dimensional model of rock mass deformation. This paper presents a multi-sensor system that has been developed to provide a continuous acquisition of relevant parameters for monitoring the stability of rock masses. This self-contained device is capable of continuously measuring and storing high and low frequency data generated by different types of sensor over long periods (several months). This monitoring system has been installed on several sites in France. Two sites in the South of France, where this device has allowed monitoring of unstable cliffs for different purposes, are presented in this paper. For these two projects, it was seen that seismic and displacement measurements of the rock complement one another. Also shown are the first results of the seismic noise monitoring of the unstable cliff of Mont Bataille. In addition to the microseismic monitoring, the resonant frequencies of the unstable rock mass provide very useful information to evaluate the rock collapse.