Before going into detailed discussions about tunnel engineering, brief knowledge of few of the basic terms is necessary.

Adit. A short, transverse tunnel between parallel tunnels or to the face of the slope in a sidehill tunnel.
Air Lock. A compartment in which air pressure can be varied between that of the compressed air used in shield tunneling and that of the outside air, to permit passage of workers or material.
Bench. Top of part of a tunnel section, with horizontal or nearly horizontal upper surface, temporarily left unexcavated.
Blowout. A sudden loss of a large amount of compressed air at the top of a tunnel shield.
Breast Boards. Timber planks to hold the face of tunnel excavation in loose soil.

Dry Packing. Filling a void with a stiff mortar, placed in small increments, each rammed into place.
Evase´ Stack. An air-exhaust stack with a cross section increasing in the direction of air flow at a rate to regain pressure.
Face. The surface at the head of a tunnel excavation. A mixed face is a condition with more than one type of material, such as clay, sand, gravel, cobbles or rock.
Grommet. A ring of compressible material inserted under the head and nut of a bolt connecting tunnel liners to seal the bolt hole.
Heading. A small tunnel, or tunnels, excavated within a large tunnel cross section which will be enlarged to the full section.
Jumbo. A frame that rolls on tracks or rubber wheels and carries drills for excavation of rock tunnels.
Lagging. Timber planks or steel plates inserted above tunnel-supporting ribs to hold back rocks or soil.
Liner Plate. A steel segment to support the interior of a tunnel excavation.
Lining. A temporary or permanent structure made of concrete or other materials to secure and
finish the tunnel interior or to support an excavation.
Mucking. Removal of excavated or blasted material from face of tunnel.
Pilot Tunnel. A small tunnel excavated over part or the entire length to explore geological conditions
and assist in final excavation.

Poling Boards. Timber planks driven into soft soil, over timber supports, to hold back material during excavation.

Scaling. Removal of loose rocks from tunnel surface after blasting.
Shield. A steel cylinder of diameter equal to that of the tunnel, for excavation of tunnels in soft material to provide support at the face of the tunnel, to provide space for erecting supports, and to protect workers excavating and erecting supports.


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