Underground Construction - Hydrogeology: Objectives, Methods, Applications - page 280

It is the perfect illustration of a textbook case, where the geometry
of the aquifers can be easily determined based on fi eld observation and
where the hydrogeologic constraints can be defi ned through a rigorous
and quantitative inventory of the surrounding outlets.
The water encountered during the excavation of the tunnel through
the Turonian formation was, in particular, inevitable, given the trap formed
by the synclinal gutter and the hydrostatic head revealed by the position
of the outlets.
It is likely that the unknowns at the time of the tunnel's construction
were a result of a too-cursory hydrogeologic and geologic approach, which
would be totally unacceptable today.
5 EXAMPLE OF THE MONTE-CARLO RAILWAY TUNNEL
(PRINCIPALITY OF MONACO)
5.1 General characteristics
The Nice-Vintimille rail line originally passed through the entire length of
the Principality of Monaco, following the Mediterranean coastline at the
surface. The Principality, wishing to open land in use by the rail line to
urban development, concluded an agreement with the French Railways
Company (SNCF) in order to study the displacement of the tracks and their
relocation underground. Construction took place from 1958 to 1963, and
the line went into service in 1964.
The principal project for this relocation was the Monte-Carlo tunnel,
excavated over a length of 3056 meters at a maximum depth of 110 meters.
Its fl oor is at the average elevation of 24 m, and in cross-section, it has a
semi-circular roof, with a width of 8.60 meters and a height of 6.60 meters.
It was excavated simultaneously from both ends, as two rising galleries
meeting in the center, from within Monaco territory for the southwestern
entrance, and from within French territory for the northeastern opening
(city of Roquebrune-Cap-Martin).
The preliminary geologic studies apparently enabled a good
understanding of the general construction constraints, but periodic
diffi culties tied to sudden infl uxes of water punctuated the progression of
the project, causing construction delays and increased costs.
5.2 Geologic and hydrogeologic conditions
• The area through which the Monte-Carlo tunnel passes is located along
the southern front of the Arc of Nice subalpine ranges, immediately