Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 263

karsts. Its effects can then be more noticeable and more rapid, through
the sudden emptying of preexisting fi lls, through the opening of old
passageways and cavities in the heart of the unit, or through the removal
of deposits fossilizing the ancient topographic surface (closed depression,
lapiaz, dry valleys).
Prolonged drawdown due to extraction pumping in an excavation or in
the overexploitation of an aquifer are not the only human actions causing
such effects. The appearance of surface damage can also be a result of a
large volume of water put into circulation in the upper part of the aquifer
(supply) or in the lower part of the aquifer (drainage).
In terms of supply, such evolution is frequently linked to point-source
leakage from sewer systems, water pipes, drainage ditches, or irrigation
canals. Problems can also be attributed to the dumping of storm water
runoff into karst bedrock. This practice, highly commendable in principle as
it uses the natural aptitude of the terrain for infi ltration, can result in well-
adapted and well-integrated setups when it is undertaken parsimoniously.
Its application on a large scale, based on the collection of runoff from a
large area of impermeable surfaces and on the occasional dumping of
large peak discharges into fractures, ponors, or chasms, inevitably causes
various problems, either due to the fl ooding of subterranean passageways
(temporary flooding), or following the gradual washing out of the
material fi lling ancient fracture networks (surface settling, problems with
equipment).
In terms of drainage, deep excavations and tunnels are the principal
causes of sizeable withdrawals and clearings that can be catastrophic (see
chap. D2-3).
3.4 Example of collapses in Mougins (Alpes-Maritimes) in
1998
3.4.1 Events observed and actions taken
Towards the end of 1998, two nearby collapses opened suddenly on the
northern lane of the R.N. 1085 (a broad speedway, connecting the cities of
Cannes and Grasse) and other accidents affected the bordering properties
during the same period and into the following year:
• October 24, 1998: opening of two funnels 20 m apart on the speedway
and collapse in a private property, 40 m to the north;
• November 30, 1998: creation of a collapse in a garden, 50 m to the
northwest;