The stage of construction of a tunnel element before it can float.

Vertical wall at the end of the excavation in a tunnel.

The advance end or wall of a tunnel at which work is progressing.

The ratio of the weight of a tunnel, or a portion thereof, to the buoyancy. Different required factors of safety may be specified depending upon whether backfill is included or removable items are excluded, and depending upon the stage of construction. Water density must be specified, since buoyances will vary with changes in water density.

A tunnel portal built in the open air, possibly by the cut and cover method. Free standing structure cast against the ground to ensure stability at the start of a tunnel drive.

Machine that allows to remove the polluted air of the tunnel.

A fracture zone in the rock mass within which differential movement of the two sides has occured relative to each other.

A small-diameter exploratory hole drilled ahead of the tunnel face in order to determine ground conditions.

Concrete or shotcrete (sprayed concrete) containing randomly dispersed fibers that are added while mixing.

Fibers for reinforcing concrete or shotcrete (sprayed concrete) are defined as short, discrete length of steel, glass, carbon, or synthetics, with any of several cross sections. They are sufficiently small to be randomly dispersed in an unhardened concret.

Filter elements to eliminate dust particles in engines and vehicles.

Long-term shaft or tunnel support installed for permanent stability or other user requirement, often incorporating the initial support elements; also referred to as "permanent lining", permanent support", final support".

The comparison is restricted to expenditures and financial benefits (receipts)

Traveling support for the concrete finishers when repairing concrete and applying curing compound to cure and repair arch concrete.

Methane (CH4) - Also called explosive gas or marsh gas. It may be encoutered in coal regions; often is associated with shales; and occurs in the neighborhood of oil fields or rock salt deposits.

A method of breaking rock practiced in ancient Egypt and medieval Europe. A hot fire is lit next to the rock and, after the rock is hot, it is rapidly cooled by pouring water over it.

Consolidated sediments or soft sedimentary rock in which the tunnel heading can be advanced without any (or with only minimal) roof support, and the permanent lining can be constructed before the ground begins to move or ravel.

Also known as outfitting, this term refers to work that is carried out while the element is afloat. It may consist, for example, of completing any remaining necessary construction of the element prior to immersion, the addition of ballast, the installation or removal of temporary equipment such as navigation lights, survey beacons, and access shafts, and adjusting the trim of the floating element. Some of the work may be necessary before transportation (towing), but the remainder must be completed after towing. Additional construction applies mainly to steel tunnels where much of the internal structural concrete may not be completed until the element is close to its final destination. Some of the work may not be carried out until the element is supported by the immersion equipment.

The rock underlying a stratified or nearly horizontal deposit, corresponding to the footwall of more steeply dipping deposits.

That part of any underground opening upon which one walks.