Kinematics of the 1991 Randa rockslides (Valais, Switzerland)

Abstract

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...

Identification of active release planes using ground-based differential InSAR at the Randa rock slope instability, Switzerland

“…An orientation of 095/70 was used for the lateral release plane, while an orientation of 135/40 for the basal shear zone was found to best match the trace observed with GB-DInSAR. This orientation also matches the orientation of the basal failure surface of the second 1991 event (Sartori et al, 2003) and the azimuth of geodetic displacement vectors. For the back boundary to the north-west a nearly vertical plane as given by Willenberg et al (2008a) was assumed.…”

“…Possibly only a few discontinuities are active within lower parts of the instability, whereas most of the deformation is localized along the basal rupture plane. We interpret this displacement pattern as translational movement on a planar or stepped shear surface, as hypothesized by Willenberg et al (2008b), Jaboyedoff et al (2004), and Sartori et al (2003). The transition between toppling and translational failure occurs at a sharp change of the slope angle in the 1991 failure scarp from about 80 • at the top to about 60 • below.…”

“…The resulting talus cone dammed the river and formed a lake, which flooded the nearby village (Schindler et al, 1993). The kinematics and mechanisms of the 1991 events were studied by Sartori et al (2003) and Eberhardt et al (2004). One legacy of these failures was the formation of an 800 m high scarp, behind which an estimated 3-9 million m 3 (Ischi et al, 1991) of rock remains unstable.…”

Design of a geodetic database and associated tools for monitoring rock-slope movements: the example of the top of Randa rockfall scar

“…On 18 April 1991, 22 million m 3 of rock fell from the steepest cliff of the Matter valley near the village of Randa (10 km north of Zermatt, Switzerland) (Schindler et al, 1993;Sartori et al, 2003). On 9 May 1991 a retrogressive rockslide of about 7 million m 3 fell down from the upper part of the 18 April scar (Fig.…”

Mesoscale characterization of coupled hydromechanical behavior of a fractured-porous slope in response to free water-surface movement

“…Although poorly documented, water is often mentioned as a triggering mechanism for failure of rocks with well-developed pre-existing fractures networks, and thus the cause for rock slides [1]. Sartori et al [2] observed that "explosion-like failure of rock slabs and sprays of water under pressure" characterized the events preceding the 1991 Randa rockslide (22 × 10 6 m 3 ) in Switzerland. Similar observations have been made in other cases in which events have been triggered close to and above the main spring draining the aquifer contained in a slope.…”