Figure 5-13.-Single-bench quarry.
A single-bench quarry (fig. 5-13) is one having the
entire floor on one level. The height of the bench will
depend on the reach of the equipment available. In the
NCF, the recommended bench height is 10 feet;
however, depending on the drilling equipment available,
type of rock, magnitude of operation, and experience of
the operating personnel, bench heights can range from
8 to 40 feet.
Military quarries are usually of the single-bench
type. This type offers greater safety and efficient
operation. All operations are on one level, a greater
amount of rock is shot at one blast, and less equipment
is needed in the overall process. In addition, this type
of quarry requires less training for the operating
Blast trial shots are made in both existing and new
quarries before installing equipment for two reasons.
First, to avoid possible damage to installed equipment
and the second, in the case of new quarries, is that the
trial shots will provide necessary ballast to construct
and for foundations to place equipment
Blasting must be supervised and controlled
directly by a qualified blaster. Also, all
personnel working around blasting should wear
hard hats, safety goggles, dust respirators,
earplugs, and hard toe safety shoes.
A multiple-bench quarry (fig. 5-14) is one having a
series of ledges or terraces resembling steps. The highest
bench is blasted and worked first. Then successive lower
levels are simultaneously developed as the work
progresses and as each bench is required.
Quarries are developed by the multiple-bench
method when the face is too high for single shots,
horizontal seams or separations are present, or deep and
narrow deposits exist. This method of development
permits equipment to be used simultaneously at more