The Journal of Explosives Engineering - January/February 2022 - 28

Figure 10. A cast blast being initiated from seven locations.
protect the coal without creating hard dig. Multipoint initiation
has also been successfully used to mitigate vibration and
overpressure issues (Goswami, et al. 2015) at several environmentally
sensitive operations. The presence of in-hole water
must be given due consideration. Dewatering holes, gas bagging
and selecting explosives that produce less gas are found
to be good practices.
Initiating a Cast Blast to Protect Coal
With the usual practices of initiating a cast blast using nonelectric
initiators there are small delay intervals between the
spacing and larger delays between rows. Once a practice is
deemed successful at a site, it is generally adopted as standard
for all cast blasts and is seldom changed.
With the widespread acceptance of more accurate electronic
initiators, the industry is evolving, and attempts are being
continuously made to not only avoid coal damage but also
to increase productivity. The latter is achieved, in part, through
muckpile profiling by dynamically producing ramp, cast and
stand-up within the one blast event. Cast blasts with longer
strike lengths, reduction in the number of blast events, reduction
in stoppage times and environmental events, 'compliance
to license to operate' are some other benefits that are claimed.
Electronic initiators do not require a burning front because
once the detonators are programmed, all forms of communication
(predominantly leadwires) with the initiators become
redundant. This provides greater flexibility and opportunities
for initiating a blast from multiple locations.
For example, a blast 0.8 mile (1.3 km) long, 200 ft (60 m)
wide and 180 ft (55 m) high is shown in figure 10. It was a
requirement to blast carefully to protect an upper thin 2 ft (0.6
m) layer of premium coal seam from any damage or loss, no
vibration exceedance on the nearby powerpoles. There were
approximately 1400 blastholes in this blast. Out of these, 60
gamma logged holes were drilled first, prior to commencing
drilling the pattern. Of these 60, ten randomly scattered holes
were drilled through the floor of the coal seam. Holes were
drilled 25 ft (8 m) from the seam floor to accommodate the
gamma tool.
All the gamma logs were analysed for the three previously
discussed important aspects which control the coal damage
and movement: the location of the roof of the coal, determined
with the help of natural gamma, density, and calliper
data; the rock density 16 ft (5 m) above the coal roof, and,
the quality of the coal floor, i.e., soft, hard etc. A stand-off
distance for each hole was ascertained from these measurements
to establish the proximity of the explosive charges.
These variable stand-off distances in effect form various
shearing planes (horizons) which are then used to generate
loadsheets and initiation details for blasting from multiple locations.
It is then vitally important that the blast is loaded by
following the loadsheets exactly with each hole being dipped
several times as backfilling holes accurately is key to averting
any coal damage/loss.
The blast in figure 10 which followed the above design
procedures is an exemplary example of a multiple-point cast
blast initiated from nine separate locations designed to control
vibration, to form a ramp, to maximise cast and to stand-up
the last rows to facilitate the dragline operation. And most importantly
protect premium coal seam from any blast induced
damage/loss. This blast involved:
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The Journal of Explosives Engineering
January/February 2022
The Journal of Explosives Engineering - January/February 2022

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