Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 247

When injection at depth is not possible, the water collected by the
vertical drain can be extracted by pumping, with the inconvenience of
requiring permanent maintenance. Pumping is generally done with
submerged pumps installed at the bottom of the boreholes. Surface suction
pumps (sand point wells) or continuously primed siphons, which guarantee
gravitational extraction (siphon-drains), are sometimes used, but these
methods are limited to boreholes under 10 meters deep.
2.3.5 Drainage galleries
When the slip surface is very deep or the local topography is not conducive
to surface-based drainage operations, these operations can be undertaken
with the help of a gallery, the main inconvenience of which is its high
cost.
The gallery can enable a significant lowering of the water table
saturating or threatening the zone in movement. Its effectiveness can also
be increased through the creation of peripheral subhorizontal drains drilled
from the gallery (example of the Quiaus landslide, see chap. D2-6).
2.4 Example of the La Clapière landslide (Saint-Etienne-de-
Tinée, Alpes-Maritimes)
The La Clapière landslide, one of the largests in Europe, was a result of deep
ruptures and set in motion a volume of over 50 million m 3 of material over
an area covering around a hundred hectares and over an elevation change
of 700 m. It resulted on-site in an upper escarpment 800 m long and 100
m high, in a general stair-step morphology shaped by multiple fractures
and numerous slope reversals, and by a clear basal rise at the level of the
valley (Follacci, 1987).
This slide is located in the northwestern portion of the Argentera-
Mercantour external crystalline mass, immediately downslope of the
village of Saint-Etienne-de-Tinée. It affected the slope on the left bank of
the Tinée valley until its confl uence with the Rabuons valley, and disorders
developed between the elevations of 1100 and 1800 meters, while the mass
of mountains reaches 3,000 meters (Figure 122).
The slope is made up of Variscan metamorphic material, caught up
in Alpine tectonics. It consists of plagioclastic gneiss with two micas, said
to be “of Anelle”, within which is interbedded a stratiform metadiorite
body, making up the Iglière bar. This last unit, distinctly more massive
and compact, is marked by a noticeable projection mid-slope and provides
a useful cartographic reference point. The foliation of the rocks, which