Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 273

fl ooding of the site. Only drainage channels were then put in place through
the coating on the walls. It was only in 1945, before the electrifi cation work
on the line, that the natural cavity that the tunnel cut across was recognized,
which enabled the catchment of the accessible water circulations and the
waterproofi ng of the other humid areas in the tunnel roof. The persistence
of water infl ows and of temporary loading due to hydraulic pressure behind
the coating led the SNCF (Railway company) to undertake a new survey
of the site from 1957 to 1959, based on intense speleologic exploration,
obstruction removal, and detailed topographic surveying. This investigation
enabled a solid understanding of the geometry of the karst networks and the
aquifer system's behavior. Complex catchment systems, through drainage
galleries oriented so as to intercept known outfl ows, were then created in
1963 and 1964, enabled the effective long-term drainage of the tunnel and
the elimination of dangerous loading for the construction (Remondet &
Marchand, 1965).
It is sometimes necessary to proceed with injection ahead of the tunnel
as it progresses, in terrain that does not hold up well, in order to reconstitute
a coherent and waterproof mass enabling the safe excavation of the tunnel.
These injections are done through borehole, either in circles around the
tunnel, or as a frontal crown (umbrella method). They can equally be put in
place preventively with the help of vertical boreholes from the surface (when
the depth of the tunnel allows it). This method enabled passage through a
very delicate area a hundred or so meters long during the construction of
the hydroelectric Pralognan (Savoy) gallery, built in 1943 to 1949 over a total
length of 12.9 km. The passage through crushed, water-bearing quartzites
occurred with no damage thanks to 1000 metric tons of cement and 100
metric tons of silicate, injected under pressure through 1 900 m of boreholes
(Rivière & Roger, 1950; Giron, 1955; Letourneur & Michel, 1971).
Freezing can also be used to cross delicate aquifer zones, when the terrain
is diffi cult to inject, and therefore very poorly permeable, and in urban areas,
when the environment is highly sensitive. This procedure consists of turning
groundwater to ice in the area affected by the construction and is therefore
considered a temporary consolidation method, which nevertheless remains
costly and diffi cult to put in place. This method was used successfully in
order to cross a 50 m wide zone of crushed cellular dolomite fi lled with
water, during the digging of a water conveyance gallery for the Hongrin-
Leman, in the Swiss Valais (1967-1970), as well as for the construction of a
25 m long sewage collection passage at shallow depth in the Bourget-du-Lac
agglomeration in Savoy (1975) (Boutitie & Meyer, 1981).
The complete drainage of groundwater and the creation of waterproof
screens may also be necessary in order to guarantee protection from rapidly
evolving formations. Such is the case in clayey terrain, susceptible to
signifi cant alteration in the presence of water, and particularly, in swelling