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| Titel |
Kinematics of the 1991 Randa rockslides (Valais, Switzerland) |
| VerfasserIn |
M. Sartori, F. Baillifard, M. Jaboyedoff, J.-D. Rouiller |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 3, no. 5 ; Nr. 3, no. 5, S.423-433 |
| Datensatznummer |
250001239
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| Publikation (Nr.) |
 copernicus.org/nhess-3-423-2003.pdf |
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| Zusammenfassung |
| About 22 mio m3 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 m3
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 preexisting 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
late-and 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 surface, 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 orthogneiss base cut by the 18 April event. |
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