Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 276

infl uxes of water, which led to long construction delays and multiple
adaptations to the construction (reinforcements and drainage).
4.2 Geologic and hydrogeologic conditions
This project is located in the heart of the sub-Alpine ranges of the Arc of
Nice, shaped by Alpine tectonics.
The sector under consideration is the Braus syncline, with a NNW-
SSE direction, presenting a nice overall symmetry with few tectonic
deformations (Figure 131).
Its heart consists of Nummulitic deposits (Priabonian marl over Lutetian
limestone) forming the backbone of the relief, and the underlying Cretaceous
formations outcrop in the surrounding slopes (Senonian marly limestone
over thinly bedded Turonian limestone, then Cenomanian marl).
The Jurassic carbonate basement support is, however, frequently
cut up by series of thrust duplexes, resulting in overthrusts spreading
concentrically outwards, one part following the principal direction of the
compressive constraints on the western border, and the other at the front
of the Triassic gypsic clays, which accumulated through diapirism in the
Sospel depression, on the eastern border.
The geologic series traversed by the tunnel contains three distinct
aquifers separated by impermeable or less permeable layers (Figure 131):
• in the upper part, the fractured to karst Lutetian aquifer, perfectly
bounded by the roof of the Senonian marly limestone acting as an
impermeable fl oor. It is drained by two side outlets, the Pissaour
spring to the SW at an elevation of 850 m and the Paraïs spring to the
NE, which is relayed to a lower elevation by a scree mantle (elevation
820 m). These outlets have an average annual discharge on the order
of 10 to 13 L·s -1 ;
• in the middle part, the fractured Turonian aquifer, limited at its base
by Cenomanian marl. This reservoir is highly compartmentalized as a
function of the distribution of the fracture network and of interbedded
marl. Groundwater fl ow is poorly hierarchized and drainage occurs
through multiple scattered outlets with a limited discharge. The known
springs are staggered across the space between the elevations of 640
to 800 m in the northern area, and 550 to 680 m in the southern area;
• in the lower part, the karst Jurassic aquifer resting on Triassic gypsic
clay (Keuper). The Cime de Pénas is drained at 450 m by the Piaon
springs in the gorges of the Bévéra and the Mont Méra is drained by
the springs in the Méras valley at 600 m and by the Sambora spring at
520 m. The discharge of the outlets is not well known, but is, for each,
greater than 10 to 15 L·s -1 as a yearly average.