About
30 million cubic meters of rocks fell from a rock face near the village
of Randa (10 km north of Zermatt) in two main stages: the first one
on April 18, 1991, and the second one on Mai 9, 1991. No fatalities
were reported except for a few horses, cows and sheep. 31 chalets
were buried. Both events caused the deposit of 10 to 40 cm of dust
in a radius of approximately 1 km from the rockslide area.
The April
18th event interrupted the railway line connecting Zermatt
to the Rhône Valley. The Mai 9th event was forecasted using
detailed field, seismic and geodetic surveys; the area was evacuated.
The railway line was buried for 800 m and the road for 200 m. The
fallen rock mass dammed the Vispa river. About 30 houses were flooded,
due to heavy rainfalls and snowmelt. The Swiss army succeeded in digging
a channel before a potential catastrophic failure of the dam, which
world have destroyed all the downstream villages.
The rockslide
is located in a busy alpine valley, especially frequented by tourists
visiting Zermatt and the Matterhorn (Cervin). The global economical
impact is difficult to estimate, but the cost of works and surveys
exceeded Swiss francs 110’000’000.
Fig. 2 : View of the cliff before the rockfall in 1970
and different important features of the site (modified from Schindler
et al., 1993 and after Sartori et al., 2003).
Fig.
3 : Cross section through the rockfall (modified from
www.crealp.ch).
Both
1991 rockslide events are the results of particular slope conditions
and structures (Figs. 1–4). The structural settings were particularly
unfavorable: a moderately steep continuous fracture cut the base of
the rock face and three persistent joint sets divided it into large
blocks, preventing a rock-avalanche-type deposit. Those joints promoted
the development of two ancient landslides above the rock face. These
pre-existing instabilities had as results on the one hand to provoke
an excess load on the top of the rock face, and on the other hand
to increase the permeability of the substratum and therefore the infiltration
rate. This promoted groundwater circulations that favored rock weathering
and water overpressure. Those processes led to rock fatigue and to
the creation of big blocks that suffered small movements, breaking
progressively the remaining rock bridges. The rockfalls led to fragmentation
of those larrge falling blocks, representing probably 5-22% of the
energy dissipated during the fall (Locat et al., 2003).
Observations
of precursory small rockfalls during the 10 years before the 1991
events illustrate the progressive mobilization of the rock blocks.
The 1991 rockslides finally occurred during a snow-melt period. Jets
of water near the basal slip plane were observed just before and during
the 1991 events, indicating that water overpressure acted as releasing
process.
At the
present time (Fig. 5), the upper part of the scar is still slowly
moving towards the Southeast at a maximum speed of 1.5 cm/year (Ornstein
et al., 2001). The movements are monitored by geodetic and extensometric
survey. A database was specially designed to facilitate the monitoring.
The present day instability forced the authorities to move the road
and the railway line, to prevent them to be hit by the potential new
rockslide.
(Link
for pdf abstract)
Fig. 4 : View of the upper part of the present scarp.
Fig.
5: View of the deposit and of the scarp to the north.
References:
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