Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 252

A complete and broadened study was rapidly undertaken (topographic
survey, topometric monitoring, fi eld surveys, geophysical prospection, core
and destructive drilling, inclinometers, piezometers) and enabled the precise
understanding of the roles played by the structure and hydrogeology of
the area on the setting off of the landslide.
The ancient landslide's shape fossilized an undercut in the Cretaceous
substratum over a thickness of 10 to 15 meters along the axis, reaching up to
25 meters at the level of the basal toe. The material consists of scree mixed
with reworked Cenomanian marl and clay, making up a very hydraulically
heterogeneous mix of old slide material. The 1977 landslide affected the
site over an average thickness of 10 meters, in an area rich in clayey slide
material. It was stopped downslope on the draining slide material making
up the ancient toe, but its own toe progressed laterally and was blocked in
a little valley as a result of local topography.
The supply of water is principally secured by the overlooking Jurassic
aquifer unit, which temporarily collects signifi cant quantities of water until
it passes into the superfi cial slope aquifer through transverse thrust faults.
Giant rockfall plays an important role in the transfer of these circulations,
as they form a continuous hump in the southwestern part of the site. They
are made up of large-scale carbonate chunks, the consolidation of which
frequently occurs through concretion and the emplacement of which is
likely of catastrophic origin (ancient cliff collapses, linked to tectonics and
seismicity). Their thickness can reach 40 meters and they are more or less
deeply set into the Cenomanian marl, prolonged by diverging branches in
the heart of the slide material. Their high permeability enables the hydraulic
relaying of the water fl owing out of the Jurassic units, then its progression
thanks to draining lenses in the slide material, causing sizeable increases
in head in culs-de-sac without an outlet.
In addition to the emergency measures put in place immediately
after the emergence of the disorders, the shoring up of the slide depends
essentially on drainage projects and includes the following mechanisms
(Figure 124):
• a deep drainage ditch Y, located in the center of the landslide as a
function of the piezometry, so as to cut across the maximum number
of lenses and drainage appendages. It has a trapezoidal cross-section
of 100 m 2 (base of 5 meters and legs at 45°), a depth of 8 to 10 meters,
and a high longitudinal slope. With a total length of 370 meters, it is
made up of a common lower trunk and two divergent upper branches,
as well as a lateral branch cutting across the end of the giant rockfall.
This setup creates a mechanical armature in the heart of the landslide