Production blasting is designed to fragment rock for loading.
At the slope wall, this fragmentation results in backbreak, which
reduces the bench face angle and results in flatter slope angles
or narrower catch benches. To reduce this backbreak the frag-
mentation of the final wall must be reduced by controlled
blasting.
V = k(D/W )^b
where V is peak partial velocity, D is distance, and W is
instantaneous charge weight. The constants k and b are a function of the rock and the type of blasting, so are site specific.
Typical values for open pit blasting are b= – 1.6 and k = 26 to 260.
This relationship can be used to determine the maximum charge weight per delay required to keep the peak particle velocity below 25 in./sec. (625 mm/s).
Reducing the number of holes per delay
particle velocity, but for the perimeter row of holes and the
buffer row, a production hole charge is usually too large, and
must be reduced. To maintain the same powder factor, the hole
spacing must be reduced concurrently with the reduction in hole
charge. In practice, this method of controlled blasting has been
shown to increase the measured bench face angle by 5°
Presplitting is usually not
necessary and is not effective in closely jointed rock. where a closely spaced line of holes with a light Powder charge is shot before the mine blast, can produce a smooth face with minimum damage.
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