Water and Construction - Hydrogeology: Objectives, Methods, Applications - page 238

concentration of underground construction is such that the aquifer, initially
unconfi ned, can become confi ned by the built structures.
It is when structures are built transversal to the fl ow direction in the
aquifer that perturbations are the most noticeable. They can result in a
piezometric rise upslope and a drop downslope, as well as in a change in
the fl ow direction of the aquifer and an increase in its hydraulic gradient,
between structures and at their extremities. The most common consequence
for the surrounding buildings is the fl ooding of basements and of lower
levels in the area immediately upslope, due to piezometric rises, as well
as localized slumping induced by internal erosion caused by the washing
away of fi ne-grained particles due to the increase in fl ow velocities in sites
with steep hydraulic gradients.
In such a context, construction can proceed only when the site is
surrounded by an impermeable belt, which can be created through a
number of procedures (grout curtains, diaphragm walls, belt of sheet piling,
secant pile wall), and kept dry through pumping in collection wells or in
perforated boreholes.
The mechanisms put in place to maintain the fl ow of the aquifer
generally enable the limitation of induced piezometric variations to a few
tens of meters or even to a few meters, whether they are permanent or
temporary.
In certain cases, drainage of the aquifer is assured by the buried portion
of the structure itself, either by open drainage channels in the impermeable
barrier, by a draining foundation, or even by a mixed arrangement. The
outfl ow, collected in sumps and moved out by pumps, is then dumped into
the storm water runoff network or re-injected into the aquifer downslope,
if the surrounding context allows it.
In other cases, as much during construction as afterwards once the
impermeability of the structure is total, hydraulic continuity is ensured by
systems external to the building, whether these consist of fi ltering units
enveloping the underground structures, or of subhorizontal radial drains
created below buildings. Haffen (1977) mentions adaptations of this type for
the underground sections of the Munich and Duisburg subways in Germany,
and for the Vienna subway in Austria, buried in the sandy-gravelly alluvium
of the Isar, the Rhine, and the Danube respectively.
In Nice (Alpes-Maritimes), the underground Cours Saleya parking
lot, built in the alluvial aquifer of the Paillon, near the coast, is equipped
with an original system of four ducts ferrying the aquifer's water from the
upslope side of the lot to its downslope side (Pline, 1991). This setup did not,
however, stop the rise in the piezometric level, which was responsible, in
particular, for the destabilization of the Miséricorde Chapel, the preservation
of which required the recent re-anchoring of the foundations with micro-
piles.