Geology Reference
In-Depth Information
of excess rainfall, or, inversely, lead to noticeable drops following periods
of defi cit.
Development projects must imperatively take into account these natural
fl uctuations, if they do not wish to suffer various problems or damages,
ranging from simple fl ooding due to overfl ow from the aquifer to structural
damage caused by hydraulic pressures or, inversely, by excessive draining
and settling of the land.
The basements and foundations of construction projects are frequently
fl ooded when the water table rises. This is, in fact, a very common case in
plains, where a river and its accompanying aquifer coexist, and the latter
can react very quickly to fl ooding in the former.
In the old city portion of Nice (Alpes-Maritimes, France), built on the
alluvial fi ll of the Paillon valley, most old buildings have overfl ow wells in
their basements to relieve pressure. These wells, otherwise used to draw
water from the aquifer, are built with rims high enough to contain the
known high-water level, in order to guarantee the safety and soundness
of the buildings.
Today, with the multiplication of basements buried several levels below
ground in urban zones, specifi c protection equipment is deployed, either
by waterproofi ng the entirety of the vulnerable portion of the building
with an impermeable casing, or by incorporating collection and evacuation
systems for groundwater (draining and collecting foundations equipped
with pumps).
Another example, very representative of the effects of a rise in the water
table, can be provided by a construction site where a buried pool had just
been installed in the pit opened for this purpose. The excavation, very near
a river, cut into silty and sandy alluvium containing an accompanying
aquifer. During a sudden fl ood, the pool was lifted up by pressure from
below due to the sudden rise in the water table, and was then dislocated
when the level dropped again.
In order to resist such piezometric pressures, buried constructions are
often weighted at their base, or welded to the bedrock by anchorages.
Such events are usually short-lived, and cause only scattered accidents
of limited severity. The disaster caused by the Somme's fl ooding in the
winter of 2001 is, however, of an entirely different scale. This phenomenon
was of an unusual intensity, both in its length (7 months) and in the extent
of the zones its affected (Somme, Aisne, and Oise departments, in North-
western France). The combination of, on the one hand, topography and
geology favoring infi ltration, and on the other hand, unusual and sustained
rainfall, led to the rapid saturation of the superfi cial silty cover and of its
underlying chalky bedrock. In addition to the overfl owing stream network,
a general rise in all of the aquifers submerged low-lying areas, fl ooded
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