An historical, geomechanical and probabilistic approach to rock-fall hazard assessment

International audienceA new method (HGP for Historical, Geomechanical and Probabilistic) is proposed to estimate the failure probability of potentially unstable rock masses in a homogenous area, as a function of time. Analysis of a rock falls inventory yields the mean number of rock falls which may be expected in the area for the given time period and a given volume range. According to their geomechanical features, the potentially unstable rock masses are distributed in classes corresponding to different failure probabilities. The expected number of rock falls can be expressed as a function of these unknown probabilities. Assuming that only the ratio between these probabilities can be estimated, combining the historical and geomechanical analysis allows estimating the order of magnitude of the different failure probabilities. The method gives a quantitative significance to the evaluations which are usually attributed to potentially unstable rock masses. Rock-fall hazard can then be compared with other natural hazards, such as floods or earthquakes. The method is applied to a case study of calcareous cliffs in the area of Grenoble, France

An historical, geomechanical and probabilistic approach to rock-fall hazard assessment

International audienceA new method (HGP for Historical, Geomechanical and Probabilistic) is proposed to estimate the failure probability of potentially unstable rock masses in a homogenous area, as a function of time. Analysis of a rock falls inventory yields the mean number of rock falls which may be expected in the area for the given time period and a given volume range. According to their geomechanical features, the potentially unstable rock masses are distributed in classes corresponding to different failure probabilities. The expected number of rock falls can be expressed as a function of these unknown probabilities. Assuming that only the ratio between these probabilities can be estimated, combining the historical and geomechanical analysis allows estimating the order of magnitude of the different failure probabilities. The method gives a quantitative significance to the evaluations which are usually attributed to potentially unstable rock masses. Rock-fall hazard can then be compared with other natural hazards, such as floods or earthquakes. The method is applied to a case study of calcareous cliffs in the area of Grenoble, France

Seismic Tomography and Ground Penetrating Radar Applied On Rock Slope Instability Analysis. Application On A Limestone Cliff In The Chartreuse Massif, France

Abstract: Evaluating the stability state of a rock slope is a complex problem, mainly due to the number of possible failure mechanisms, and to the lack of knowledge of the real present state of the rock mass. Geophysical methods appear as a useful tool for inves-tigating the deep discontinuity pattern, which is poorly known from surface observa-tions. However they have seldom been applied on steep rock slopes. The aim of this study is to test some of these methods on subvertical cliffs. We investigate the quality of information that they can ..

Probabilistic approach to rock fall hazard assessment: potential of historical data analysis

International audienceWe study the rock fall volume distribution for three rock fall inventories and we fit the observed data by a power-law distribution, which has recently been proposed to describe landslide and rock fall volume distributions, and is also observed for many other natural phenomena, such as volcanic eruptions or earthquakes. We use these statistical distributions of past events to estimate rock fall occurrence rates on the studied areas. It is an alternative to deterministic approaches, which have not proved successful in predicting individual rock falls. The first one concerns calcareous cliffs around Grenoble, French Alps, from 1935 to 1995. The second data set is gathered during the 1912–1992 time window in Yosemite Valley, USA, in granite cliffs. The third one covers the 1954–1976 period in the Arly gorges, French Alps, with metamorphic and sedimentary rocks. For the three data sets, we find a good agreement between the observed volume distributions and a fit by a power-law distribution for volumes larger than 50 m3 , or 20 m3 for the Arly gorges. We obtain similar values of the b exponent close to 0.45 for the 3 data sets. In agreement with previous studies, this suggests, that the b value is not dependant on the geological settings. Regarding the rate of rock fall activity, determined as the number of rock fall events with volume larger than 1 m3 per year, we find a large variability from one site to the other. The rock fall activity, as part of a local erosion rate, is thus spatially dependent. We discuss the implications of these observations for the rock fall hazard evaluation. First, assuming that the volume distributions are temporally stable, a complete rock fall inventory allows for the prediction of recurrence rates for future events of a given volume in the range of the observed historical data. Second, assuming that the observed volume distribution follows a power-law distribution without cutoff at small or large scales, we can extrapolate these predictions to events smaller or larger than those reported in the data sets. Finally, we discuss the possible biases induced by the poor quality of the rock fall inventories, and the sensibility of the extrapolated predictions to variations in the parameters of the power law