Abstract. About 22 mio m 3 of rock fell from a cliff near the village of Randa (10 km north of Zermatt, Switzerland) on 18 April 1991. A second retrogressive rockslide of about 7 mio m 3 followed on 9 May 1991. At present, a rock mass situated above the scarp is still slowly moving toward the valley, involving several mio m 3 of rock.A kinematic approach to study of this well-documented rockslide was made "a posteriori" in order to identify the parameters relevant to the detection of such failures involving large volumes of rock. A 3-D model of the pre-rockslide geometry is presented, and is used to interpret the geostructural, hydrogeological, and chronological data.The steepness of the cliff, the massive lithology (mainly orthogneiss), the location on a topographic ridge outcropping at the confluence between a glacial cirque and the main valley, and the existence of previous events of instability were the pre-existing field conditions that affected the stability of the area. The structural cause of instability was a 30 • dipping, more than 500-m-long, persistent fault, which cut the base of the rock face. Together with a steeply dipping set of persistent joints, this basal discontinuity delimited a 20-mio-m 3 rock block, with a potential sliding direction approximately parallel to the axis of the valley. To the North, the fractures delimiting the unstable mass were less persistent and separated by rock bridges; this rock volume acted as key block.This topographic and structural configuration was freed from glacier support about 15 000 years BP. The various mechanisms of degradation that led to the final loss of equilibrium required various amounts of time. During the lateand post-glacial periods, seismic activity and weathering of the orthogneiss along the fissure network due to infiltration of meteoric water, joined to reduce the mechanical resistance of the sliding surfaces and the rocks bridges. In addition, crystallisation of clay minerals due to mineralogical alteration of the fault gouge accumulated along the sliding surCorrespondence to: F. Baillifard (francois.baillifard@crealp.ch) face, reducing its angle of internal friction, and sealing the surface against water circulation. Once this basal fracture began to act as an aquiclude, the seasonal increase of the hydraulic head in the fissures promoted hydraulic fracturing on the highly stressed edges of the key block. Acceleration of this mechanical degradation occurred during the 20-year period before the 1991 rockslides, giving rise to an increasing rockfall activity, that constituted a forewarning sign. The final triggering event corresponded to a snow-melt period with high water table, leading to fracturation around the key block.On 18 April 1991, the key block finally failed, allowing subsidiary orthogneiss blocks to slide. They fell in turn over a period of several hours. The 9 May 1991, rockslide was the first of a series of expected future retrogressive reequilibrium stages of the very fractured and decompressed paragneisses, which lie on the ort...
The dynamic response of four unstable rock compartments in the Alps has been studied using the ambient vibration technique, with the aim of identifying precursors to rockfalls. The test sites present various geological settings (limestone, argillite, and shale-sandstone series), failure mechanisms and volumes. The ambient vibration spectra measured on the unstable compartments systematically showed clear energy peaks at specific frequencies, in contrast with records made on the adjacent stable rock masses. These predominant frequencies were interpreted as resonant frequencies of the unstable compartments, in agreement with 2-D modal analysis. In the horizontal plane, ground motion at the fundamental frequency was found to be systematically parallel to the line of maximum slope gradient, and perpendicular to the main bounding fracture observed at most of the sites. The fundamental frequency of each prone-to-fall compartment shows reversible variations related to temperature fluctuations at different timescales, with a significant contrast in magnitude and phase shift between sites. At the more fractured site, resonance seems to result from a contrast in internal rigidity between the compartment and adjacent rock mass, rather than from decoupling along a rear fracture, which is the mechanism observed at the three other sites. No change in fundamental frequency resulting from damage was observed over the period of study.
Abstract. The need for monitoring slope movements increases with the increasing need for new areas to inhabit and new land management requirements. Rock-slope monitoring implies the use of a database, but also the use of other tools to facilitate the analysis of movements. The experience and the philosophy of monitoring the top of the Randa rockfall scar which is sliding down into the valley near Randa village in Switzerland are presented. The database includes data correction tools, display facilities and information about benchmarks. Tools for analysing the movement acceleration and spatial changes and forecasting movement are also presented. Using the database and its tools it was possible to discriminate errors from critical slope movement. This demonstrates the efficiency of these tools in monitoring the Randa scar.
This paper contains a seismic assessment at urban scale of the cities of Sion and Martigny in Switzerland. These two cities have been identified for the present research based on their importance regarding size and the characteristics of the building stock for which information was available. Moreover, microzonation investigations are available for both cities. This results in a more accurate characterization of local expected ground shaking, which is expressed through specific response spectra. Sion and Martigny represent, respectively, the capital and second largest city of the canton of Valais. This region is characterized by the highest seismicity within Switzerland. The paper focuses on the assessment using Risk-UE methodology, namely the empirical method LM1 and the mechanical method LM2. The obtained results are compared in order to assess the related accuracy. Firstly, buildings of the two cities were surveyed in order to collect main structural characteristics in a database. Building stock is typical of that region and can be found similar to many other medium-sized Swiss cities. Around half of the buildings are unreinforced masonry buildings, while several others are reinforced concrete buildings with shear walls. Results show the most vulnerable part of the cities regarding earthquake. There are significant differences in global results between LM1 and LM2 methods. The mechanical LM2 method is more pessimistic since it predicts damage grades of about one degree higher than LM1 method. However, the main drawback of the empirical LM1 method is that an a priori determination of an adequate value of the macroseismic intensity is required. Nevertheless, LM2 method may lead to a global overestimation of damage prediction.
Abstract. Based on the assumption that major class of rock instabilities are created by discontinuities, a method is proposed to estimate the fracture density by means of a digital elevation model (DEM). By using the mean orientation, the mean spacing and the mean trace length of discontinuity sets potentially involved in slope instabilities and a DEM, it is possible to calculate the mean number of discontinuities of a given set per cell of the DEM. This would allow for an estimation of the probability of the presence of at least one discontinuity in a given area or simply in a topographic cell of the DEM. This analysis highlights sites potentially affected by rockslides within a region. Depending on the available data, the mean number can be calculated either by area, or along a line parallel to the mean apparent spacing. The effective use of the probability of occurrence is dependent on the size of the discontinuities because short and closely spaced discontinuities will have a 100% probability of occurrence in each favorable location. The a posteriori prediction of a recent rockslide is discussed as an example.
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