Civil Engineering Reference
In-Depth Information
construction. The first operations required in the joint space are cleaning
and erecting lighting and scaffolding to access the full perimeter of the
tunnel. Cleaning can be a significant operation, particularly if the tunnel
element has been afloat for a long period of time. Marine fouling will occur,
and high-pressure washing and other cleaning methods will be required to
get the immersion joint steelwork back to its original condition such that
the Omega seal can be clamped to a smooth, clean surface and therefore
achieve the necessary degree of watertightness. It is likely there will need to
be some minor repairs to paintwork and cleaning of threads to receive the
seal clamping bolts. Protection of threads in advance of flooding the cast-
ing basin with grease, tape, or caps is advisable to minimize the buildup of
corrosion products in the short term while the tunnel elements are afloat.
The immersion joint should be temporarily boarded over on the base slab
to allow easier access and to protect the components of the immersion joint.
When the joint has been cleaned, the Omega seal can be clamped into posi-
tion. The seal will need to be brought into the joint as one continuous ring
and is therefore heavy and cumbersome to maneuver. Similarly, the clamping
bars are heavy components, and it is advisable to install some winches and
simple lifting equipment on the scaffolding. The Omega should be lifted to
the roof of the tunnel in the first instance and clamped at the corners ini-
tially. Although these corners are preformed, and the clamping components
are specially designed to ensure that clamping pressure is exerted right into
the corners, this is the most difficult part of the operation. If there is any
difference in level between the two adjoining tunnel elements, there will be
a height difference between the adjacent clamping surfaces and the seal will
need careful maneuvering to get a good fit into the corners. When the corners
are clamped, the straight lengths between them can be clamped into position.
Testing of the Omega seal to ensure that the clamping has made the seal
truly watertight is required. This is usually done as an air or water pressure
test in which the space between the Omega seal and the Gina seal is pres-
surized to a pressure in excess of the water pressure likely to be experienced.
However, this may not necessarily be to the full factor of safety used in the
design. The test should be carried out over a 24-hour period, and initial pres-
sure drop-off needs to be corrected due to the short-term creep behavior in
the rubber materials of the seals. The test should be done while the bulkheads
are still in place as any failure of the test would require reclamping of the seal.
The testing facilities comprise simple pipework and valves from the joint
space through to the interior of the tunnel. If the test used is a water test,
the water will need to be fully evacuated after the test, and the pipework
should be positioned to enable this. The testing pipes can be capped off
when the test is complete. It is a good idea to do this rather than sealing
them permanently, so that the pipework can be accessed later in the life of
the tunnel. If a leak occurs at the tunnel joint in the future, this pipework
can be used to determine if the leak is coming past the Gina seal. They can
