Civil Engineering Reference
In-Depth Information
Introduction
The southern and northern access ramps have both been designed and
constructed in a different manner. In Zeeuwsch-Vlaanderen, near the
southern bank of the Westerschelde, cement-bentonite walls have been
installed which reach from the surface level into the Boom clay stratum.
This clay stratum which lies at approximately 25 metres
NAP, forms the
(almost) watertight bottom sealing of the construction pit in which the
activities were carried out.
For the construction of the northern access ramp at Zuid-Beveland, use
could not be made of the watertight horizontal sealing of the Boom clay,
because this stratum was situated at a level which was too deep. That is
why the deepest closed section of the access ramp was constructed by
means of pneumatically immersing a caisson. The connecting open sec-
tion of the access ramp was constructed within a construction pit with
anchored sheet piling and - across the largest section - a bottom sealing of
underwater concrete.
During the immersion of the caisson, fossilized remnants of a woolly mam-
moth were discovered at a depth of approximately 19 metres
NAP. It con-
cerned a lower jaw and an ulna of the 'Mammuthus Primigenius', which was
estimated at between 20,000 and 30,000 years old. Parts of the lower jaw
have been built-in in the 'first stone' of the service building of the tunnel at the
toll square.
Design and implementation of the southern
access ramp; construction pit within a slope
The southern access ramp consists partly of a closed concrete box structure
with a central wall. This box is approximately 31.80 metres wide, 50 metres
long and 19.40 metres high at most.The structure served as the starting shaft
for the two tunnel boring machines, and at a later stage it was provided with
concrete internal work. Connecting onto the concrete box structure, an open
basin structure was constructed at a width of almost 30 metres and a length
of 88 metres. This (open) section was carried out in 6 sections provided with
expansion joints, of which 4 sections have a length of 12 metres and 2 sec-
tions measure 20 metres. The concrete structures have a natural foundation
and were constructed in a construction pit surrounded by cement-bentonite
walls which reach down to the sealing clay stratum (Boom clay) which is situ-
ated at approximately 25 metres below the surface. In the final situation, the
Fig. 8.1
Design access ramp
south
40 m
48 m
50 m
Closed exit
open entrance
Open basin
Starting shaft
6.50 m
Impermeable block
1.70 m
Infill
Boom clay
18.00 m
Cement-
bentonite
wall
Cement-bentonite wall with steel sheet piling
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