Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 275

Such damage often has a limited scope in totally natural environments
with no human development. It can, however, have very serious
consequences in urban and peri-urban zones. Taking it into account therefore
appears as important as considering diffi culties in the construction of the
project, during preliminary studies and monitoring of the construction.
The diversion gallery for the Tinée (Alpes-Maritimes), built in 1989 to
1991 near the village of Saint-Etienne-de-Tinée, based on the possibility
of the river valley being obstructed if the La Clapière landslide were to
suddenly be reactivated, encountered this type of problem (see chap.
D2-2.4). The gallery is 2.4 km long and has a cross-sectional area of 10 m 2 ,
and is in the right bank of the river. It follows the gneissic Mercantour
basement, principally dug into Permo-Werfenian quartzitic sandstone and
Triassic gypsum and cellular dolomite, under a variable cover of clayey,
rocky fl uvial and fl uvio-glacial deposits. During construction, large water
infl uxes (from 100 to 250 L·s -1 ) were drained by the tunnel. They were
relayed from the aquifer in the overlooking Muschelkalk carbonate mass,
which experienced a drawdown of 40 meters, and led to the drying of
the Laus spring, which fed a fi sh farm and a horticultural establishment.
Additionally, these permanent circulations facilitated multiple high-volume
evacuations of sand in the passage through quartzitic formations and the
racking of fl uvio-glacial deposits blocking old karst cavities in the passage
through gypsum and cellular dolomite. It is regrettable that there followed
the delayed appearance of 3 sinkholes having reached the surface, and the
evolution of several zones of active settling.
4 EXAMPLE OF THE BRAUS RAILWAY TUNNEL (ALPES-
MARITIMES)
4.1 General characteristics
This underground portion of the Nice-Breil line (Alpes-Maritimes) has a
length of 6 kilometers, at a maximum depth of 700 meters. It crosses, at the
average elevation of 410 m, a ridgeline with a maximum elevation of 1,330 m
separating the Paillon basin to the west from the Roya basin to the east.
It was built by the P.L.M. Railroad Company in 1912 to 1922, with an
interruption during WWI. It was excavated simultaneously from each end,
as rising galleries which met towards the middle of the tunnel, the highest
point of the fl oor. The rail line, which went into service in 1928, is now
used by the SNSEE.
The construction of the tunnel was slowed by signifi cant diffi culties,
tied to the geologic nature of the terrain being traversed and to abundant