Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 283

This sensitive passage recently showed worrisome new signs of
instability, likely tied to water pressure behind the tunnel walls and the
ageing of the initial treatment (fracturation of the coating, fallen blocks,
deformations of the rails, new infl uxes of water). The line was shut down
for 9 months (June 2003 to March 2004), in order to allow the construction of
appropriate reinforcements over the 50 meters concerned. This consolidation
consisted of metal arches every meter covered by a concrete shell over the
entirety of the section. The setup was completed with a catchment system
for the incoming water fl ow, with evacuation in a central collector (Doublot
et al. , 2004; Mario, 2004).
5.3.2 Drying up of the Ingram spring
On May 21st, 1962, the progress of the tunnel was halted by a large infl ux
of water which appeared at the top of the arched roof towards 2.6 km from
the southwestern entrance. The collected discharge reached 280 L·s -1 at the
most diffi cult point, and led to the drying of the Ingram spring, located 150
meters downhill and used for the Principality of Monaco's drinking water
supply (Maury & Carpentier, 1966; Varlan, 1981).
The construction had opened a breach in the base of the scree cover,
apparently within a paleo-valley fi lled by slope deposits. This opening
diverted the groundwater circulations coming from the Mont Gros mass,
initially relayed by the scree cover to the Ingram spring.
The continuation of the project required the emplacement of a catchment
system for the incoming water, and reparations for the arch, as well as
the conveyance of the collected discharge back into the Monégasque pipe
network.
The catchment is set up above the tunnel and includes a drainage
network made up of three transversal galleries and one longitudinal gallery.
The collected discharge is directed into an evacuation aqueduct leading
to a stonework well, and then brought back to the Ingram spring by a
150 m long gallery. The distribution of water was put back into service in
September, 1964.
5.4 Lessons to be drawn from this experience
The impact of hydrogeology on the construction conditions for the Monte-
Carlo tunnel confi rms the diffi culty of revealing local heterogeneities, even
in the context of a well-done study complemented by heavy mechanical
reconnaissance.