Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 242

2 LANDSLIDES
2.1 Characteristics and particularities of landslides
These are mass movements mobilizing variable volumes, going from a few
m 3 (small-scale slips) to several million m 3 (hillside slides). They generally
affect plastic and granular terrain, but large-scale slides can also affect
unconsolidated or ground up rocky slopes (example of the Clapière slide,
chap. D2-4).
There are two principal types of slides, characterized by the geometry
of their rupture surface (or shear surface):
• planar slides, where the rupture occurs through simple translation
along a planar surface. This surface generally coincides with a natural
discontinuity: stratifi cation joint, tectonic break (diaclase, fault), or
tilted contact between two distinct formations (superfi cial cover over
rocky substratum, for example);
• circular slides, which develop due to rotation along a curved surface
and can affect relatively deep horizons.
The evolution velocity of these processes is generally fairly slow, from
a few millimeters to a few centimeters per day, and rarely exceed a few
meters per day, but their progression can occur over long periods of time,
marked by alternating periods of relative stability and more or less abrupt
remobilization, as well as regressions upslope.
When the liquid phase becomes predominant, some slides can
evolve into mudfl ows, characterized by rapid dynamics and signifi cant
propagation. These are highly destructive events, as can be seen from
the following two examples, which deeply marked the Alpes-Maritimes
(Perriaux, 1927; Menéroud, 1976):
• on November 24th, 1926, the village of Roquebilière, built on the left
bank of the Vésubie, was hit in the middle of the night by a fl ow with
a volume estimated at 2 million m 3 , after an accumulation of 440 mm
of rainfall over the 3 preceding days. The event mobilized morainic
clay deposits in large blocks thanks to the emptying of a water pocket
in the underlying gypsum, causing 19 deaths and destroying some 20
houses;
• on April 24th, 1952, the town of Menton was hit by several landslides,
following an accumulation of 306 mm of rainfall in 3 days. The clayey
eluvial cover of the fl ysch slopes, gorged with water, generated several
fl ows, the propagation of which caused the death of 11 people and the
destruction of 17 buildings.