Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 274

clays (montmorillonite family), which can degrade tunnel walls due to the
pressure they cause when they become hydrated. The same phenomenon
can affect certain anhydrites, which, through hydration, turn into gypsum,
with a noticeable increase in volume. The 9.2 km long Courbaisse-Massoins
(Alpes-Maritimes) hydroelectric gallery, dug in 1947 to 1950, for example,
cut across a Triassic anhydrite mass over a distance of 600 meters, at the
base of an overthrusting Jurassic limestone unit above Cretaceous marly
limestone. This horizon's rapid transformation into gypsum led EDF
(National Electricity Company) to locally widen the gallery to a cross-
section of 17 m 2 , from the planned 10 m 2 , to isolate the passage of water
into a steel middle duct 3 meters in diameter, and to protect the walls
with coaltar distemper and with concrete in the crushed zones. The same
problem occurs in the NE end of the Braus railway tunnel, where the later
dissolution of gypsum masses has today led to delayed risks of cave-ins
under the tunnel fl oor (see chap. D3-4).
3 HYDROGEOLOGIC IMPACT OF TUNNELS ON THEIR
ENVIRONMENT
The crossing of an aquifer by a tunnel intercepts part of the aquifer's water
and totally modifi es the piezometry and the fl ow conditions. Such an event
can, depending on the real hydrogeologic conditions, cause a decreased
discharge in some of the aquifer's natural outlets, or can even dry them
completely. It can also stop or decrease the production of water wells and
lead to the emptying of lakes or the drying of small valleys at a higher
elevation than the construction.
Such a situation can be irreversible (see chap. D3-4) or can require
costly and diffi cult construction in order to restore the norm (see chap.
D3-5). The effective waterproofi ng of the tunnel walls or the creation of
injected curtains can, in certain cases, prevent such problems or at least
limit them temporally.
Another environmental effect of the drainage provided by tunnels in
permeable terrain is the inducement of soil movements capable of having
surface repercussions in the entire area of direct infl uence, whether the
movements are more or less pronounced slumps, or full-on cave-ins
characterized by the sudden opening of sinkholes. These problems can be
generated by the settling of suddenly dry unconsolidated formations, by
the collapse of unstable terrain due to the clearing of the gallery, by the
washing out of old karst fi ll, or by the reactivation of induced dissolution
of soluble formations (gypsum, rock salt).