ACCESS RAMPS: DESIGN AND IMPLEMENTATION - The Westerschelde Tunnel: Approaching Limits - page 89

Fig. 8.3b
Construction of
starting shaft
boring machines could no longer be taken up in the proper manner by the
concrete structure.The floor was carried out in a single casting session. Due
to technical implementation reasons, the walls and the roof were cast in 4
sections. The expungency of the expansion joints also led to an increase of
the reinforcement percentage in the longitudinal direction.
The relatively slim concrete structure of the closed section of the access
ramp (floor and wall thicknesses of 1,100 mm) also lead, in combination
with the abovementioned design changes, to a substantial quantity of rein-
forcement, increasing to approximately 300 kg/m 3 in the floor. The adapta-
tion of the mixture was necessary in order to make a justified processing of
the concrete possible.
Due to the short amount of implementation time being available for the con-
struction of the southern access ramp - normative for the starting of the bor-
ing process - the choice was made for a single casting of the walls over the
full height per section. In order to avoid cracking in the walls as a result of
hydration shrinkage, the bottom sections of the walls were actively cooled.
Concrete work at the starting shaft; second phase
Dimensioning problems due to greater settlements than
expected
After both tunnel boring machines had started and the necessary structures
were removed from the starting shaft, the concrete internal work in the
starting shaft could be applied. Due to the boring process being underway
during the realisation of this concrete work, and the supply and removal of
equipment went via the access ramp, the design had a longitudinal phasing
per tunnel tube; by means of a 'detour' the logistical process of the boring
could take place undisturbed.
The 'second phase' concrete work encompassed the construction of a clean
water and dirty water cellar, the floors, walls and roofs of both driving tubes,
plant rooms and a stairwell/lift shaft. In order to link up this concrete internal
work to the concrete work carried out in the first phase, coupling anchors
were incorporated in the external work. During the implementation - when