A posteriori studies of rock slope instabilities generally show that
rockfalls do not occur at random locations: the failure zone can
be classified as sensitive from geomorphological evidence. Zones
susceptible to failure can therefore be detected. Effects resulting
from degrading and triggering factors, such as groundwater circulation
and freeze and thaw cycles, must then be assessed in order to evaluate
the probability of failure.
A simple method to detect rock slope instabilities was tested in a
study involving a 2000m3 rockfall that obstructed a mountainous road
near Sion (Switzerland) on 9 January 2001. In order to locate areas
from which a rockfall might originate, areas were assessed with respect
to the presence or absence of five criteria: (1) a fault, (2) a scree
slope within a short distance, (3) a rocky cliff, (4) a steep slope,
and (5) a road. These criteria were integrated into a Geographic
Information System (GIS) using existing topographic, geomorphological,
and geological vector and raster digital data. The proposed model
yields a rating from 0 to 5, and gives a relative hazard map. Areas
yielding a high relative hazard have to meet two additional criteria
if they are to be considered as locations from which a rockfall might
originate: (1) the local structural pattern has to be unfavourable,
and (2) the morphology has to be susceptible to the effects of degrading
and triggering factors. The rockfall of 9 January 2001, has a score
of 5. Applied to the entire length of the road (4 km), the present
method reveals two additional areas with a high relative hazard,
and allows the detection of the main instabilities of the site.