Geology Reference
In-Depth Information
The distribution of groundwater fl ow is perfectly aligned along the
extent of the karst units, and is represented by stepped outlets enabling
the underground drainage of the surrounding units:
• in the heights, the Justicier and Sainte-Marie plateaus are drained at
200 m and 220 m by the Bestagne and Fons Divina springs, which have
a combined average annual discharge of 10 L·s -1 ;
• the Mont des Mules feeds the Tour springs at an elevation of 65 m with
an average discharge of approximately 10 L·s -1 ;
• the Monte-Carlo overthrust is drained by the Larvoto springs against
the fault of the same name. These emergences are located at less than
one meter above sea level and provide an average discharge of 60
L·s -1 ;
• the Mont Gros, due to its intense tectonic partitioning, likely allows for
a more diffuse frontal outfl ow, which is then relayed by scree slopes
feeding the Ingram springs at an elevation of 18 m (average discharge
of 25 L·s -1 ) and the Marie springs and an elevation of 2 m (average
discharge of 10 L·s -1 ).
5.3 Hydrogeologic problems linked with the excavation of the
tunnel
The diffi culties caused by the hydrogeologic context during the excavation
work were a result of very localized complications, of a structural nature,
under the Mont des Mules overthrust, and of a paleo-geographic nature
immediately uphill of the Ingram spring.
5.3.1 Passage through the Mont des Mules overthrust
The base of the Mont des Mules overthrusting Jurassic limestone on the
Cenomanian marl led to water infl uxes over a distance of 50 m (1.015 km
to 1.065 km from the southwestern entrance), with a discharge around 14
L·s -1 . The affected zone was treated by injection (with the help of 8,000 m
of boreholes) and by complete coating of the tunnel walls, waterproofed
with extrados (Maury & Carpentier, 1966).
Examination of the map and cross-sections in Figure 132 reveals the
diffi culty of predicting such a particular structure. It is, indeed, a true basal
pivot for the limestone unit, sunk into the underlying marl. This corner
geometry materializes the contact between the weakly sloping frontal thrust
surface and the lateral subvertical thrust.
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