Water and Construction - Hydrogeology: Objectives, Methods, Applications - page 239

2.2 Induced drawdown
Drainage induced for large-scale construction projects can have distinctly
more sizeable repercussions on their environment than that of construction
sites limited by impermeable barriers and therefore proceeding in a confi ned
environment.
The infl uence of this drainage can, indeed, result in drawdown of
several tens of meters and can be felt in relatively distant locations, with all
of the consequences associated with such modifi cations of the groundwater
system:
• drying of springs, lowering of the water level in wells and boreholes,
drying of lakes or of hanging valleys, as a direct effect of the excessive
drawdown of the aquifer;
• slumping or induced collapses, in relationship with the drying of the
ground and with the increase in hydraulic gradient of the aquifer
(washing out of fi ne particles and of unconsolidated fi ll).
These effects are in large part tied to the long duration of certain
construction sites in which induced drawdown is necessary (several months
to several years), or to the great spatial extent of the aquifer affected by the
construction (a few hundreds or thousands of meters). The damages are,
additionally, irreversible when the characteristics of the structure do not
allow the reestablishment of initial hydraulic conditions.
The role of tunnels passing through aquifers perfectly illustrates this
problem (see chap. D 3-3 to D 3-5).
The origins of the drawdown can also be involuntary and result from
poorly-adapted actions or from the beginning of construction without
really taking into account the various components of the natural and
human environment. The case of the use and development of the lower
Var valley in the area around Nice (Alpes-Maritimes) is revealing of this
state of being. In its lower part, the Var River fl ows through a plain ending
at the Mediterranean Sea, and the alluvial fi ll of which contains a large
accompanying aquifer, which is intensely exploited for the drinking water
supply of coastal urban areas. At the end of its course, the river initially
spread out over a wide bed, varying in width from 600 to 1,000 m, that it
occupied fully in fl ood, and within which it meandered during the dry
season. The progressive channeling of its course between 1850 and 1970
reduced its width to an average of 250 m, enabling the use of part of the
surrounding previously-fl ooded land for cultivation and various other
purposes, but also favoring erosion and the lowering of the channel as its
fl ow velocity increases. In parallel, a large quantity of alluvial material was
extracted from deep gravel pits, enabling the mining of a volume of material
estimated at 30 million m 3 between 1946 and 1980. The intensive usage of