Lesson 2/Learning Event 2
QUARRY DIMENSIONS
Bench Height. Equipment limitations and geologic conditions most influence the decision as to what
bench heights are best suited to your quarry operation.
The characteristics of common military rock drills are given in Table 4. Most quarrying operations
employ track-mounted drills equipped with sectional drill steels. These are 10 or 12 feet (3.0 or 3.7
meters) long and can be coupled together as required. Drill holes are usually subdrilled 2 to 3 feet (0.6
to 0.9 meter) below the quarry floor. This assures complete rock fragmentation between holes during
blasting. For convenience, benches are often planned to accommodate some unit number of drill steels
(usually 2 to 4). Since drill efficiency decreases about 20 percent for each additional length of steel
used, lower benches are usually preferred when drilling is difficult. Relatively lower benches are also
preferred where rockfall and rockslide hazards exist along the face and where front loaders are used in
loading out blastrock.
TABLE 4. MILITARY ROCK DRILL CHARACTERISTICS
Where layers of undesirable material occur within a deposit, benches should be planned so that the
undesirable and desirable materials may be excavated separately. This prevents contamination of the
quarry product. Such surfaces reduce subdrilling requirements and aid in maintaining an even quarry
floor.
Face Width. As a minimum, quarry faces should be wide enough to meet daily blast rock requirements
and not narrower than the turning radius of the rock loading equipment. This radius normally ranges
upward from 40 feet (12 meters). Faces wide enough to provide blast rock for at least several days of
operation are preferred since weather conditions or other considerations may interfere with blasting
schedules.
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