Civil Engineering Reference
In-Depth Information
'Tour de force', a pioneering project
In many respects the construction of the Westerschelde Tunnel was a pio-
neering project. Due to the length of 6,600 metres, the large diameter (the
internal diameter measures 10.10 metres) and the extremely deep positioning
(up to 60 metres
NAP) in combination with the complex geotechnical cir-
cumstances, the tunnel by European standards is quite unique. Exceptional
too, is that during the boring process the civil fitting out of the tunnel was
started directly behind the tunnel boring machines. This was also a reason
why the logistics of the project were a tremendous challenge.
Extremely stringent safety requirements resulted, amongst other things, in
the construction of 26 cross connections between both main tunnel tubes,
in which use was made of freezing technology.This technology had not pre-
viously been used in the Netherlands on such an enormous scale and
applied under such extreme conditions.
For the construction of the Westerschelde Tunnel, trendsetting research was
carried out into the life span of the tunnel wall and the fire-resistance by
means of heat-resistant cladding. In the Netherlands there were no stan-
dards as yet for bored tunnels. The necessary experience of boring into soft
soil (clay and sand) had already been gained elsewhere in Europe but, aside
from the Danish Störebaelt Tunnel, a bored tunnel of this magnitude had
not previously been realised in soft soil in Europe.
Fig. 1.3
The finished tunnel
Geology
The Westerschelde is 55 kilometres long and the width varies from
1.5 kilometres to 5.5 kilometres. It is a tidal river with an average tidal vari-
ance of 4.5 metres.The bed of the river continually changes due to the sand
transportation as a result of the tidal current which causes erosion and
sedimentation. For a large part the tunnel is bored through Boom clay. Its
layer thickness varies from approximately 8 metres under the main fairway
to approximately 38 metres under Middelplaat. Below the Boom clay there
is sand, the so-called Berg sands.
Above the clay deposit the sand and clay deposits alternate in various
compositions and thicknesses. From recent sea sand and clayish sand
to extremely tight glauconitic sand. At the main fairway, the Pas van
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