Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 248

appears to be very steep over the majority of the area, with dips of 55 to 85°
to the northeast, is much less tilted and is sometimes even subhorizontal
in the area immediately surrounding the slides. It is, in addition, obliquely
cut across by Alpine schistosity. Finally, the rock mass is cut through by a
fracture network organized around several principal axes, inscribed within
the context of the transverse fault-overthrust system oriented N 120-140,
which dictates the course of the Tinée and which locally pinches narrow
Triassic basement synclines.
Figure 122 La Clapière landslide (Saint-Etienne-de-Tinée, Alpes Maritimes). Map view and
cross-section.
1-Anelle gneiss, 2-Iglière bar, 3-Permo-Werfenian, 4-Triassic cellular dolomite, 5-Triassic
limestone, 6-Alluvium, 7-Landslide, 8-Fault, 9-Overthrust.
Activity on the slope probably goes back to the beginning of the 20th
century, when it must have been only very gradually visible, but picked up
noticeably starting in the 1970s, then again starting in 1983 (with seasonal
alternations of acceleration and deceleration). During phases of activity, the
amplitude of the movement could reach 3 to 4 meters per year, with values
of 8 cm per day during the summer of 1987.
The causes of the landslide appear to be primarily mechanical, then
hydraulic (Follacci, 1987; Follacci et al. , 1988):
• initially, the melting of the Würmian glaciers occupying the Tinée
and Rabuon valleys would have, by eliminating buttressing support,
prepared the slope for its future evolution by allowing a large-scale
downturning of dip near the slope surface and a gradual destructuration
of the rock under the effects of decompression;
• subsequently, water acting in this highly fractured environment would
have engendered more or less stacked initial movements, gradually
evolving towards a deep rotational landslide.
Water is without a doubt the primary factor in the historical evolution of
the landslide, with an annual rainfall of around 1,000 mm in Saint-Etienne-