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a. In vertical shaft sinking, blasting of drill holes so as to keep one
end of a rectangular opening deep (leading), thus facilitating drainage
and removal of blasted rock. Pryor, 3
b. Benches in tunnel driving are often drilled from the top with
jackhammers. The vertical shotholes are generally spaced 4 ft (1.2 m)
apart in both directions, fired by electric delay detonators, one row at a
time. When bench shotholes are drilled horizontally with the drifter
drills mounted on a bar, the charges are fired in rotation, starting from
the upper center. In some cases, a bench may be drilled both vertically
and horizontally, particularly where the benches are exceptionally high or
when the headroom above the bench is inadequate for handling drill steels
long enough to bottom the shotholes to grade. The lifters are drilled by
machines mounted on a bar across the bottom of the tunnel, in which case
the upper vertical holes will all be fired before the horizontal charges.
Hammond
Source:
Dictionary of Mining, Mineral, and Related Terms
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