Glossary - Immersed Tunnels - page 477

permanent seal or become the primary seal. It is bolted to the inter-
nal faces on each side of the joint. It may be replaced in a similar
manner on an as-needed basis. Because of its shape, it can sustain
fairly large longitudinal and transverse movements at the joint.
Soft-nosed gasket See “Gina gasket.”
Temporary immersion gasket This is usually an extruded rubber sec-
tion that acts as a seal when it is compressed. After completion
of the permanent joint, the seal is no longer needed. This type of
gasket is commonly used in the United States.
Gate Usually either hinged to a wall or floating, this structure is used to
close off the fabrication facility from the adjacent water to allow dewa-
tering of the facility.
Gravel (bed) foundation See “Screeded foundation.”
Graving dock An area that can be dewatered to form a casting basin.
Greenfield site An area above water level converted to enable the con-
struction of tunnel elements, usually steel shell tunnels. The elements
may be side or end launched into the water when capable of floating, or
may be incrementally launched.
Grouted foundation bed A foundation formed by filling the space between
the underside of an element and the preexcavated trench bottom with
grout. Until this operation is complete, the elements require temporary
support.
Haunch A thickening of a wall or slab to increase locally the bending
strength and shear capacity of the section.
Immersion The phase of construction covering the period between the
element floating on the surface and installed on its foundation or tem-
porary supports at bed level.
Incremental construction A method of construction whereby a short sec-
tion of an element is constructed, and then jacked along to enable the
adjacent section to be cast against the previous section.
Installation This phase of construction covers preparation for immersion,
immersion, foundation preparation, backfilling, and completion of the
interior works.
Jet fan A ducted propeller, usually mounted adjacent to or above the traf-
fic, that helps to maintain air velocity within that cell.
Joint
Closure or final joint A joint where the last element has to be inserted
rather than appended to the end of the previous element; a mar-
ginal gap will exist at the secondary end. This short length of tun-
nel will need to be cast in place and is known as the closure or final
joint.
Construction joint A horizontal or vertical connection between mono-
lithic parts of a structure, which is used to facilitate construction.
A waterstop is commonly placed in such a joint.