A condition without physical danger. An area of analysis for alternate designs that can have significant public or private cost implications.

A foundation formed by filling the space between the underside of an element and the pre-excavated trench bottom with sand. The sand is placed hydraulically with the sand-flow or sand-jetting method. Until this operation is complete, elements require temporary support. A small gap may exist at the underside of the element after this operation, so that the temporary supports must be released or deactivated to lower the element onto the foundation.

A method of sand bedding whereby the sand-water mix is transported through a pipe system with fixed outlets in the soffit of the element. The mix is usually discharged through one outlet at a time. As the velocity of the mix decreases after leaving the outlet, sand is deposited by gravity to form a firm pancake-shaped mound almost touching the underside of the tunnel, with a small depression beneath the outlet. While pancake dimensions vary, an area of 100 square metres would not be unusual. The sand-water mix may be supplied either externally though inlets in the roof or walls, or from inside through non-return valves.

Miner who works in a tunnel driven under air pressure.

A method of sand bedding whereby the sand-water mix is transported through a jet pipe which can be moved anywhere in the void between the underside of the tunnel and the trench bottom. As the velocity of the mix decreases after leaving the jet, sand is deposited by gravity. The resulting density of the sand is less than by the sand flow method. The work can only be done from the outside.

The removal of loose pieces of rock adhering to the solid tunnel surface after blasting.

A slip form on an invert pour; anything used to strike off a concrete pour.

Following trench excavation and before immersing an element, a gravel foundation is prepared by screeding to close tolerances and onto which elements are placed directly without further adjustment. Temporary supports at bed level are not required.

The equipment required to be installed on diesel engines, whose purpose is to dissolve or neutralize exhaust gases.

A method of constructing a concrete wall in poor ground by means of a continuous row of concrete-filled boreholes. Alternate holes are bored in two successive series such that adjacent piles overlap.

A permanent tunnel lining of concrete that is usually placed after mining operations have been completed.

A condition safe from unwanted intrusion

A monolithic section of a tunnel element only separated from other segments by vertical joints. For concrete tunnels, a segment is typically the length of a single concrete placing operation. Some tunnel elements, particularly in the Netherlands, consist of a number of discrete segments held rigidly together during installation by temporary prestress and joined by expansion joints.

Sections of iron, steel, or precast concrete which can be bolted or keyed together to make up a ring of support or lining. Iron or steel segments are generally referred to as "liner plates"; concrete segments may be referred to as "panels".

See division of communities

One steel rib or unit of timber framing to support the sides and roof of a tunnel.

A pipe or conduit for transporting liquid waste for treatment or disposal

The network of sewers for a community

A vertical or near-vertical excavation or opening to provide access to an underground facility or construction operation

A device to transfer shear across a joint. Shear dowels are sometimes used in concrete tunnel elements across immersion, closure or expansion joints to provide continuity of alignment. Such shear dowels must permit relative longitudinal movement. For immersion and closure joints, the dowels would be embedded in cast-in-place concrete at the joint face after immersion.