Page 408 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.8 THE WORK

Untamped Cartridges. If cartridges are not slit and tamped, capacity is figured on the basis of
the length and weight of the cartridges, and the number that can be placed side by side. Three of
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the standard 1 ⁄4 8 inch cartridges would fit in the hole if tied together. It would take 13 of these
bundles to fill the hole within 4 inches of the stemming line. There would be 39 cartridges, each
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weighing ⁄164 of 50 pounds, or about .3 pound per stick. The total load by this method would be
11.7 pounds, just a little more than half of the possible load.
Burden. The explosive in each hole is supposed to break out a section of the rock between the
line of holes and the face. Only experience with the particular rock and explosive will indicate
exactly the amount and type required.
In a general way, however, it may be said that 1 pound of 40 percent dynamite should break up
and move 2 yards of soft rock, or 1 yard of medium hard rock, on an open face. In soft, layered,
or rubbery rocks, 20 percent dynamite might move more per pound; while in very hard rocks, even
higher-strength dynamites might have smaller production. In tight holes, at edges, and in corners
heavier loading is required.
Figure 9.6(B) is a table showing the yardage of rock to be moved per foot of hole for various
burdens and spacings. As an example of its use, consider that the powder that packed 2.3 pounds
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to the foot of 3-inch borehole would move 1 ⁄ 2 yards of rock per pound, or 3.45 cubic yards to the
foot of borehole.
According to the table, 3.33 yards of rock would be moved if spacing were 9 feet and burden
10, or the other way around. An 8-foot spacing and 12-foot burden would yield 3.55 yards.

Pull. If a hole fails to move its burden, it is said that it did not “pull.” This usually occurs at the
bottom of the hole, and most often in edge or corner holes where the rock is held back on two or
three sides. Such failures may be due to too heavy a burden or too wide a spacing, to improper
stemming of a shallow hole, use of the wrong explosive, explosive not reaching the bottom of the
hole, or a partial misfire. It is generally necessary to remove the blasted rock, check for any unex-
ploded charge, then drill and shoot the bottom again.

Measurement. In order to drill and load holes accurately, it is necessary to know the height of
the face and the amount of the toe. With low faces, or in casual operations, or in working upper
lifts to temporary grades, depths may be estimated, although this is always risky.
Faces between 10 and 70 feet may be measured by the device and method shown in Fig. 9.7.
A 45° right triangle is carefully made of two-by-twos or two-by-fours, with the sides of the right
angle equal and from 2 to 4 feet long.

FIGURE 9.7 Measuring face height.