Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 264

• December 28, 1998: appearance of a collapse in a housing development,
160 m to the southwest, taking out the service road and fi ssuring a
building;
• October 24, 1999: development of a small collapse and of two
nearby depressions in the garden of a property, 210 m to the west-
southwest.
This was in fact a cluster of 5 collapses and 3 depressions that occurred
over the course of a year in the town of Mougins (Alpes-Maritimes), over
an area barely 200 m in diameter.
In-depth investigations on-site were rapidly undertaken by the Nice
CETE (Environmental Technical Studies Center) Mediterranean lab, in order
to enable treatment of the affected area and reopening of the speedway. The
boreholes drilled exposed large, highly karstifi ed gypsic masses under a
clayey marl cover around 10 meters thick, and enabled the location of the
cavities responsible for the problems, around 20 to 30 m below the surface.
A much larger study was then undertaken, from 2000 to 2002, in order
to specifi cally understand the deep geologic structure along the path of the
road, to defi ne and hierarchize the organization and role of groundwater
circulation in the development of karst cavities and in the origin of the
collapse mechanisms, and fi nally, in order to research the factors enabling
the possibility of predicting such events.
3.4.2 Geologic and hydrogeologic structure
The regional basement is provided by the sedimentary cover of the Tanneron
mass gneissic platform, evenly tilted to the northeast. The different horizons
of the Triassic series outcrop in successive rings and are capped by a Jurassic
age carbonate plateau (Figure 127). The series begins with the siliceous
Werfenian sandstone, followed by a hundred or so meters of Muschelkalk
limestone sandwiched between two marly dolomitic horizons, and ends
with the multi-colored Keuper marl, which encloses scattered dolomite,
gypsum, and cellular dolomite lenses, and is capped by thirty or so meters
of Rhetian marly limestone.
The overall structure is relatively simple, and can be described as a
monocline tilted to the northeast. It is, in fact, complex in the detail due to
superimposed tectonic deformations:
• folding with a N 20° to N 45° axis, induced by the Provençal phase;
• subvertical N 0° to N 20° faulting, messily breaking up and offsetting
ductile structures, with one set of Oligocene normal faults (fi rst Alpine
phase), then another set of thrust faults during the second Alpine
phase.