Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 279

on the Vasta plateau, belonging to the town of Sospel, and its drying was a
true tragedy for the fi fty or so agricultural operations affected. Numerous
litigations followed, and subsequent studies enabled the establishment,
as early as 1919, of a causal relation between the two events and of an
estimation of the spring's original discharge, which would have varied
between 40 and 60 L·s -1 depending on the season.
But it must be acknowledged that this catastrophe for some turned out
to be a blessing for the surrounding communities, which were able to benefi t
from the water collected in the tunnel, to provide their drinking water
supply. The use of this water was established by agreements between the
P.L.M. Railroad Company and the municipalities of Sospel (northeastern
entrance to the tunnel) in 1926 and l'Escarène (southwestern entrance to
the tunnel) in 1927. Indeed, the railway tunnel, given its passage over a
distance of 1 900 meters through an aquifer with high hydrostatic head, is
essentially a very productive catchment system, surpassing in production
any system that one could hope to create with classic methods in such a
compartmentalized aquifer with such a mediocre global permeability.
4.3.2 Passage through the Jurassic aquifer overriding the
Triassic
The Triassic formations encountered at the eastern mouth of the tunnel are
principally composed of anhydrite, which, once exposed to the water freed
during the passage through the overthrust Jurassic unit and fl owing along
the sloping tunnel fl oor, gradually turned into gypsum through hydration.
This transformation was aggravated by the water coming from the Turonian
aquifer and by the interruption of construction during WWI.
The alteration of anhydrite into gypsum, as it is accompanied by an
increase in volume and signifi cant swelling of the rock caused serious
damage to the stonework in the tunnel. It created the need for the
construction of sizeable drainage systems below the tunnel fl oor, which
did not manage to prevent the subsequent dissolution of gypsic masses,
which resulted in the creation of localized cavities and cave-ins. This new
situation led to the construction of reinforcements, repairs to the tunnel
fl oor and supports, and ongoing surveillance and maintenance.
4.4 Lessons to be drawn from this experience
The geologic structure of the area that the Braus tunnel crosses in particularly
simple, and the cross-section along its length differs only slightly from what
can be inferred from surface mapping.