Civil Engineering Reference
In-Depth Information
then also be used to pump out any water that might collect between the
two seals and to permit the introduction of corrosion inhibitors. For these
reasons, the end of the test pipe should be as low as possible in the tunnel
wall and readily accessible for inspection.
Bulkhead breakout
Breaking out of a tunnel bulkhead can be started when there are a suffi-
cient number of bulkheads in place to offer the necessary level of safety to
operatives working in the tunnel. The minimum number of bulkheads that
should always be left in place until the tunnel is complete is usually con-
sidered to be three. This number comprises the single bulkhead at the free
end of the tunnel as far as it has been completed, and the pair of bulkheads
at the first immersion joint in from the free end of the tunnel. Controlled
entry through these bulkheads can be allowed to perform operations such
as ballasting. This access is through the bulkhead door, which is manned
to ensure that it can be closed quickly should any leakage occur at the joint
or into the last element placed. Provided the last element has been ballasted
securely, the risks of movement of that element and any consequential leak-
age should be minimal, but the retention of the bulkheads is a prudent
safety measure that has been adopted worldwide.
The method of breaking out a bulkhead depends on the type being used.
Concrete bulkheads are simply demolished with pneumatic tools and the
supporting steelwork is dismantled. The steel beams can be recovered and
the concrete debris removed. Needless to say, breaking out the concrete in
such an enclosed space requires the dust and debris that is created to be man-
aged carefully. Steel bulkheads are probably quicker to remove. If they are of
modular form, they can be unbolted and removed for reuse in later elements.
The perimeters are often welded to supporting steelwork for watertightness.
These welds will need to be burned through, so good ventilation of the area
and possibly fume extraction are important. It is also vital during these
bulkhead removal operations that the joint seals are adequately protected.
Shear keys and joint infill
There are a variety of shear key solutions available for tunnel immersion
joints, which are described in Chapter 9. Whichever form of shear key is
used, it can only be installed once the Omega seal is in place. There is no need
to wait until the bulkheads are removed before the shear keys are installed.
The timing for their installation may depend on the likely settlements that
are anticipated, and it is common for some of the settlement to be allowed
to occur before the shear keys are formed to reduce the magnitude of forces
they have to carry. Therefore, they are more typically constructed some time
after the bulkheads are removed, the tunnel has been backfilled, and much
