Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 258

punctured by sudden collapses termed “clapes” (1890, 1920, 1978, 1983).
More recent events can also be cited, particularly the collapse in Tourettes
that, in 1987, destroyed a residence, and the one in Bargemon that opened
in 1992 in the middle of a housing development (Nicod, 1993 and 1999).
The prediction of such accidents and the adaptation of construction
projects to this type of risk require a solid knowledge of the hydrogeology
and the karstifi cation mechanisms of the relevant aquifers. This is rarely
the case in practice, where the level of information most often remains
fragmentary and where the time limit on interventions is very limiting.
In carbonate formations, problems linked to karstifi cation can often
be resolved and treated with a good degree of reliability, given the high
mechanical resistance of the unweathered rock, the fi nite limits of the
responsible heterogeneities, and the compatible evolution speed of bedrock
conditions with respect to the mechanisms constructed (Mangan, 1985;
Amar et al ., 1988).
The diffi culty is much greater in the case of gypsic rock, where the
incidence of active karstifi cation constitutes one of the most delicate to
resolve problems in soil engineering. In addition to the weak mechanical
resistance of the rock and its high solubility, the possibility of rapid
evolutions in the subterranean network must be taken into account, as
well as, in some cases, horizontal and cross-sectional variations in its later
path under the effects of base level variations. A number of hypotheses can
frequently be considered which can alter the performance of a treatment
solution. The most commonly used methods consist of transferring the
load of the building to supports outside of the dangerous zone (piles),
fi lling the threatening cavity, or treating the entire unit through injection.
If the underground network is the seat of permanent hydraulic activity, it
must also be kept in mind that the process is irreversible and that certain
operations (concrete fi lling, injections) are likely to divert groundwater
circulation from its initial path and to move the problem to initially stable
peripheral areas.
An innovative and clever treatment enabled the support and protection
of the railway viaduct crossing the Suès valley on the Nice-Breil line (Sospel,
Alpes-Maritimes). Built in 1913 over Triassic gypsum, this construction
originally bridged a permanent valley, which was gradually deepened
through headward erosion, abandoning its superficial bed. Studies
undertaken by the SNCF in 1963 enabled the surveying, after numerous
obstruction removals and widening excavations, an underground river
fl owing through a 300 meter long gallery, and the observation that in
its lower regions, the viaduct's foundations were partially suspended
in mid-air. In addition, this stream received seven small tributaries, also
subterranean, and the peak discharge of the network was evaluated to be
approximately 3 m 3 ·s -1 . The solution to be implemented had as its goal