Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 261

3.3.2 Reactivation of karst circulations
Prolonged pumping in a karst aquifer can signifi cantly perturb its hydraulic
regime when it creates a sizeable drawdown. The resulting disequilibrium,
partly tied to an increase in circulation velocity in the drainage axes and to
an increased renewal of the resources in the aquifer by aggressive water, can
be accompanied by the resumption of dissolution, the washing out of fi ne-
grained deposits, and the unblocking of fossilized conduits. These processes
can lead to various problems in the periphery of large-scale construction
sites where a sizeable drawdown is maintained over a long period of time,
or around sites of intensive exploitation of an aquifer.
Prolonged drawdown due to large construction projects can be
illustrated by the example of the recent Toulon (Var) highway tunnel,
where the diffi culties encountered in crossing through karstifi ed formations
could surely have been partly attributed to the continuous pumping
necessary to keep the construction site above water. The tunnel is, indeed,
20 meters below sea level, and only 750 meters away from the coast. The
drawdown of the water table was maintained over several years and
signifi cantly altered the fl ow systems and subterranean drainage circuits
of the aquifer, enabling, in particular, the reactivation of certain fossilized
ducts and the establishment of new connections with the sea (Mangan &
Gilli, unpublished).
The overexploitation of karst aquifers is also accompanied by more and
more frequent surface degradation. Such is the case in Tournaisis (Belgium),
where numerous collapses dot the banks of the Escaut, as a result of the
pumps exploiting the deep Carboniferous limestone aquifer. The onset of
the collapses clearly progressed over the course of the last few decades
(97 from 1955 to 1984) and their locations are correlated with the zones
of large drawdown due to pumping and with zones of recharge from the
alluvial aquifer (Kain hole) and from the losses of the Escaut, facilitated
by collapses, particularly during the events of January, 1977 and May, 1984
(Delattre N., 1985; Laurent E., 1985).
The same problem occurs in the region around Paris, where karst
reactivations affect the Lutetian gypsum under the effect of high volumes
being pumped from the underlying aquifer. Surface problems show a
marked increase in the last few decades, and can be grouped into clouds
geographically concentrated around the main cones of piezometric
depression (Toulemont M., 1984 and 1987) (Figure 126).
This type of evolution unfortunately tends to become more and more
common in the absence of suffi ciently strict regulation and currently
affects the most vulnerable aquifers in several regions, adding a resource
management problem to a serious safety problem for people and property.
In the Triassic formations of the Provence region, the recurrence of collapses