Glossary - Immersed Tunnels - page 478

Earthquake joint An immersion joint of special design to accommo-
date large differential movements in any direction due to a seismic
event. It is also applied to a semirigid or flexible joint strengthened
to carry seismic loads and across which stressed or unstressed pre-
stressing components may be installed.
Expansion joint Also known as segment or dilatation joint, this term
refers to a special moveable watertight joint between the segments
of a tunnel element.
Immersion joint The watertight joint that is dewatered when an
element is installed at the seabed. It may remain flexible or can be
made rigid, as is usual with steel tunnels. An immersion gasket or
soft-nosed gasket is usually used, and an Omega seal is installed
later.
Keel clearance The least vertical distance between the deepest part of a
floating body and the bed beneath.
Keel concrete Concrete, often ballast, placed in the lowest portion of an element.
Lifting lugs Temporary lifting points from which an element is sus-
pended during immersion, usually removed after an element is set on
its foundation.
Locking fill Backfill, usually granular, placed carefully around the lower
part of a tunnel to hold it in position.
Outfitting See “Fitting out.”
Portal The structure or the end face of the structure at the two ends of the
tunnel at the interface of the covered and open sections.
Prestress, temporary Used mainly in concrete tunnels to temporarily lock
a flexible joint, to modify stresses until immersion, or to provide addi-
tional strength during transportation and installation.
Pulling jack Device attached to the primary end of a tunnel element being
immersed, which connects to the secondary end of the previous tunnel
element and enables the tunnel element to be pulled up to the previous
element and create the initial seal of the Gina gasket.
Pumped sand foundation See “Sand bedding.”
Rigging A system of lines, winches, and hoists used to control the posi-
tion of an element, both horizontally and vertically, especially during
immersion. Lines may be attached indirectly to the shore, anchors,
pontoons, derrick barges, or other lowering equipment.
Rock protection or armor The provision of larger stone or rock to prevent
erosion or dredging of required backfill or bed. The term is also applied
to systems for protecting a tunnel against potential collisions and drag-
ging anchors.
Roof protection Protection provided to the waterproofing membrane on
the roof against accidental damage. This term is also applied to combi-
nations of backfill and rock protection placed above the roof to protect
against sinking or grounding vessels.