Civil Engineering Reference
In-Depth Information
of bored tunnels for a number of large infrastructural projects in the
Netherlands. For financial reasons earlier bored tunnel designs were only
considered as feasible with a single tube. However, from a safety point of
view such tunnels were not acceptable.
The boring of a tunnel became more attractive because there were also a
number of major objections for the tunnel-dam-bridge combination:
- the harbour at Antwerp could claim a substantial amount for the com-
pensation of the direct and indirect financial damages as a result of the
blocking of shipping during the immersion operations and its prepar-
ations (dredging activities);
- the impact on the environment and the landscape, particularly the dam-
bridge section, was more and more a subject of discussions. The bottom
of the bridge across the branch-fairway would be positioned at approxi-
mately 40 metres
NAP;
- the technical implementation of the immersed tunnel would take place
under extreme circumstances (the enormous depth and the soil conditions
at the spot of the channel to be dredged, the tide and the current) and was
therefore not without risk.
Based on the above, it was also decided to draw up a reference-design for a
twin-tubed bored tunnel across the full width of the Westerschelde.The mat-
ter was urgent, because the tendering procedure would soon be started.
Due to the short period in which it had to be concluded, the reference-
design therefore had a general outline character.
Reference design: bored tunnel
In respect of the tunnel-dam-bridge crossing, the horizontal alignment of
the bored tunnel could be relocated into a direction which was more north-
south; there was no need to cross the Pas vanTerneuzen at right angles any-
more. Although the local circumstances had obviously not changed, it was
necessary to carry out extra soil investigations in order to gain more insight
into the deeply positioned soil deposits in which the boring had to take
place. That is why extra drillings down to a depth of 70 metres below sea
level were carried out and the future boring route was subjected to seismic
investigation.
In the reference-design by the Civil Engineering Division of the Directorate-
General for Public Works and Water Management, the cross section of the
tunnel consisted of twin tubes with an outer diameter of 12.98 metres, in
which the tunnel wall had a thickness of 0.65 metres.The interior would con-
tain, amongst other things:
- a road surface on a concrete sheet with central support;
- electrical equipment cellars situated below the road surface spaced 1,000
metres apart and a pump cellar at the deepest point of the tunnel;
- facilities such as heat-resistant cladding so as to realise the same level of
safety as in an immersed tunnel;
- in the reference design, the two tunnel tubes were connected to each
other by cross connections every 500 metres. The electrical equipment
cellars were accessible from the cross connections. In order to gain access
to the pump cellars, separate annexes were provided.
- A separate longitudinal escape corridor with emergency doors 100 metres
centre-to-centre, and a cable gallery above it.
To a major extent the vertical alignment of the bored tunnel was deter-
mined by the necessity of adequate soil covering above the tunnel which
had to measure about the same as the size of the tunnel diameter. The bed
Search WWH ::
Custom Search