A spontaneous and violent detachment of a slab or slabs from over-stressed rock. See also popping rock.

A 5- to 40-ft long steel reinforcing bar inserted in drilled holes around the periphery of a tunnel and anchored or sealed with mortar or polyester resin to tie rock-blocks or strata together and prevent their loosening or falling out.

The provision of larger stone or rock to prevent erosion or dredging

A modified core recovery percentage in which only sound pieces of rock core 4 in. or more in length are counted as recovery. RQD is considered a more accurate gauge of a rock's engineering "quality" or competence than is the gross recovery percentage. It is stated as the cumulative percent of the core run occuring in pieces greater than 4 in. long.

The distance broken rock will be thrown down the tunnel when the round is exploded.

A tunnel driven in consolidated natural material (i.e., "rock") which requires use of rock excavation methods such as blasting, channeling, wedging, or barring, or a tunnelling machine making use of specially hardened cutters.

A cutter consisting of a circular metal disk with hardened rim or teeth, mounted on bearings set in the rotating face of a TBM and rolled in an arc across the rock face under force; the resulting concentration of force or of stress spalls the rock. See also disc cutter.

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

Bolts equipped with an expanded anchor at one end and a nut and washer at the other. Installed in drilled holes to tie rock together. Also known as rock bolts

Protection provided to the waterproofing membrane on the roof against accidental damage.

A cycle of rock excavation consisting of drilling blast holes, loading, firing, and then mucking.

A measurement of alignment that distinguishes between single tube and multiple tube configurations for the purpose of equating the utility of borehole surveys. For example, one borehole may be used to survey (theoretically) twice as many linear ft of tunnel for a double tube system as for a single tube system. Thus, one route ft in a double tube system comprises two linear ft of tunnel alignment; in a single tube system, route ft and linear are the same.

An operation of transporting and discharging unsuitable materials arising from the excavation, into a site previously selected for unsuitable materials.

Relatively sudden, uncontrolled flow of material into a tunnel from the face or the tunnel circumference.

Perfectly cohesionless materials (such as dry sand or clean, loose gravel) above the water table which flow into the excavated area while the tunnel is being excavated.

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.