Civil Engineering Reference
In-Depth Information
Spacer block
Clamping bar
Bolt
Gina gasket
Counterplate
welded to end
frame
Figure 10.4
Gina gasket mounted on counterplate prior to compression.
• Change in sealing pressure due to opening of the joint due to
• Settlement
• Shrinkage
• Temperature when the joint is sealed and subsequent variation caus-
ing movement
• Backill friction restraining movement
• Accuracy of placing the tunnel element
• Realignment of the tunnel
• Seismic movements, if applicable
The information that can be obtained from the gasket supplier should
give the first two of these. The remainder will depend on the tunnel geom-
etry and the methods of construction. It is therefore very important to
engage the contractor in these discussions, or make reasonable allowance
for these effects, based on practical experience.
The sealing design criterion to be used for the Gina gasket is often
expressed as a factor of safety on the water pressure that exists on the
outside of the seal. This factor of safety is often set at 2.5 and is eas-
ily achievable for shallow tunnels. The sealing pressure caused by the
gasket compression needs to exceed the applied external water pressure
by this factor. This approach is quite conservative, but can be accom-
modated by the gaskets in most circumstances without leading to any
additional cost or complexity in the design or construction. However,
