Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 278

4.3 Hydrogeologic problems in the excavation of the tunnel
The geologic cross-section along the length of the tunnel shows that the
Lutetian aquifer was not affected by the construction, given its perched
location. Indeed, the discharge of its outlets remained unchanged.
However, abundant infl uxes of water marked the progress of the tunnel
as it passed through the other aquifers:
• on the one hand in the karstifi ed Jurassic unit, both in the western
part, in the southern fi nger of the Mont Saint-Sauveur, towards km
1.0, and in the eastern part, in the southern extremity of the Cime de
Pénas, towards km 4.7;
• on the other hand, in the fractured Turonian unit, mainly towards
km 3.3.
The outfl ow discharge reached 200 L·s -1 at its peak, and its residual
magnitude varies between 20 and 80 L·s -1 depending on the season, since
the tunnel was put into use.
The most notable hydrogeologic consequences concern the passage
through the Turonian aquifer and the water infl uxes in the eastern Jurassic
next to the Sospel Triassic units.
4.3.1 Passage through the Turonian aquifer
The largest infl uxes of water were generally located in the most fractured
part of the aquifer, between the km 3.32 and 3.34, where the discharge
reached 90 L·s -1 during excavation. Despite a noticeable decrease in
magnitude, these outfl ows are still today the major source of the water
collected by the tunnel.
Their evacuation required the creation of galleries parallel to the tunnel,
of periodic catchment systems against the tunnel supports, and of boreholes
through the walls in order to limit the pressure against the tunnel. The
collected discharge is evacuated towards each entrance by a central gutter
and various pipes.
Based on the local geologic structure and the position of the springs
fed by the aquifer, the original piezometric height of the water table above
the tunnel must have been around 200 to 400 m depending on the area, and
the water head is still fairly signifi cant today.
Drainage of the unit by the tunnel is likely to have dried multiple small
outlets and decreased the discharge of others, but in particular, it caused the
complete and permanent drying, in 1914, of a sizeable emergence located
1 500 m north of the tunnel at an elevation of 800 m, so 390 m above the
tunnel (see location on the map in Figure 131). This spring fed irrigation