Civil Engineering Reference
In-Depth Information
The problems of horizontal planarity appear during the placement of
the tunnel. During the immersion process, when a tunnel element is being
pulled up to an already placed tunnel element, it will try to orientate itself
at 90° to the plane of the immersion joint, to achieve a uniform compres-
sion in the Gina seal. If either of the end frames is not set at 90° to the line
of the tunnel element, the element will naturally position itself off of its
intended alignment. This can normally be corrected by forcing the tunnel
element back to the intended alignment and locking it in place with backfill.
In practice, this can be achieved by simply lifting the free end and moving it
slightly to correct the alignment or by using jacks within the immersion joint
to move the element back to its intended position. However, the Gina gasket
will be over-compressed at one side of the element and under-compressed at
the opposite side, reducing its possible movement range thereafter.
When elements are being constructed, it is possible to account for some
out-of-tolerance by undertaking as-built surveys of the elements within the
casting basin or dry dock. If there is any out-of-tolerance detected, it can
be compensated for by offsetting the plane of the end frame counterplate in
the adjacent tunnel element.
All of these tolerance issues must be considered when designing the Gina
gasket, to ensure it retains sufficient sealing pressure under all circumstances.
An assumption on tolerances needs to be made in the design of the Gina seal,
but once designed and selected, this will dictate the accuracy of construction
required and should be reflected in the specifications for construction of the
end frame and in the immersion procedures. At the stage of choosing the
tolerances to allow, it is important for the designer to either consult with the
team which will be responsible for placing the tunnel, or to have good knowl-
edge of the likely construction methods and the tolerances that could result.
Local surface tolerances must be considered in terms of the possible
reduction in sealing pressure in the Gina gasket that could lead to leakage.
The Gina gasket will be able to cope with many variations, but sharp steps
in the surface or abrupt deviations in the line could lead to leakage.
Detailing
The careful detailing of the joint is an essential part of its success in ensuring
that watertightness is achieved. Particular features to be aware of include:
• In sizing the steelwork to accommodate compression of the Gina gas-
ket, ensure that opposite flanges of the end frame do not clash or that
the Gina clamping bars do not touch when the joint is fully compressed.
• Provide pipework from the void between the Gina and Omega seals to
the inside face of the tunnel to allow for pressure testing of the joint.
• Maintain the pipework used for joint pressure testing as access points
for maintenance inspections and for checking water ingress. Should
