ACCESS RAMPS: DESIGN AND IMPLEMENTATION - The Westerschelde Tunnel: Approaching Limits - page 93

The caisson - a closed concrete box structure which is provided with a cen-
tral wall - has a width of 34.60 metres, a length of 25 metres and a height of
24.10 metres. An open basin structure with a width of 29 metres and a length
of 260.50 metres connects onto the receiving shaft. This basin is carried out
in 15 sections with expansion joints with a length varying from 10 metres to
20 metres. The basin structure is built in a construction pit within anchored
sheet pile walls. At the 10 deepest sections, the bottom sealing consists of a
floor made of underwater concrete. At the remaining sections within the sheet
piling, drainage was applied in which use was made of a poorly permeable,
sand-containing clay stratum at approximately 7 to 9 metres

NAP.
The entire access ramp is surrounded by a surrounding dyke with a construc-
tion height of 6.50 metres NAP.

Fig. 8.7
Design access ramp
north

25 m 43 m 160 m

Open basin

Closed exit
open entrance

Infill

Impermeable block
Caisson

6.50 m

2.50 m

Tensile piles

Sheet piling

20.70 m Diaphragm wall

Adapting the building of the caisson - first phase

Initially the idea was that the caisson could be built at the surface level. To
this end, the soft covering layer (clay and peat) - present down to approxi-
mately 4.50 metres below the surface - had to be replaced by a ground
improvement consisting of sand. As a result of the higher rising height
in the aquiferous layers under this pack (in the construction phase up to
1 metre NAP, and thus approximately 1.50 metres above the existing sur-

face), the building of the caisson could not have been realised without
drainage or an enclosing dyke. That is why the choice was eventually made
for the construction of a construction pit in which the soft deposits could be
excavated. By making use of a water-braking layer situated approximately
8 metres NAP the drainage remained extremely limited. Due to this the

construction level of the caisson was also reduced by approximately 4
metres. After casting the floor of the caisson, the construction pit could be
removed.
As opposed to the receiving shaft of the access ramp south, the walls of the
caisson with a height of 16.60 metres were cast in 4 layers. An active cooling
of each casting was necessary in order to achieve a crack-free and thus
waterproof structure.The following facilities were applied for the immersion
of the caisson:
- the usual immersion facilities such as:
• a cutting edge and excavation chamber for the excavation under
increased air pressure;
• air locks for personnel and equipment in order to gain access to the
excavation chamber;
• bentonite piping located at the external walls for the reduction of the
soil friction;