The material, including soil and/or rock, as measured along a perpendicular from the tunnel crown to the ground surface. See also overburden.

In reinforced concrete or shotcrete, the least distance between the surface of the reinforement and the outer surface of the concrete or shotcrete

Factors by reference to which a decision is taken. Multi-criteria analysis may also be carried out, possibly incorporating cost-benefit analysis as one of the criteria.

Tunnel to give access between tubes for the operators and the emergency services

The highest point of an arched tunnel cross section; the roof or back of a tunnel.

The configuration of the upper portion of a tunnel section above the springline.

Timbers or steel members cantilevered from previously installed sets nearest the heading to temporarily support a rock tunnel roof while the next set is being installed.

Small pour made on the bottom and at each side of a tunnel and generally located outside the minimum concrete line; used as a reference point and support for invert and arch pour in the tunnel.

A drilling pattern in the tunnel face which provides relief for an explosive charge.

Underground construction involving open-cut excavation followed by eventual covering with a slab or arch structure to support a replaced ground surface. A temporary covering at ground level may be provided for the passage of traffic or environmental reasons.

A tunnel constructed by excavating a trench from the surface and then decking it over, usually with timber, so that traffic can be maintained while the structure is built within the trench.

The rotating front end of a mechanical excavator which actually cuts through rock or soft ground.