The Journal of Explosives Engineering - January/February 2015 - 31

Table 1. Pattern designs.

Methodology
The whole idea of the design was to put as much of the
explosive energy into breaking and casting the rock as possible
to reduce the amount of vibrations escaping the blast pattern. Explosive energy likes to take the path of least resistance.
The less contained a blast is the more energy goes into breaking and casting the rock in the direction of the free face than
goes into the material behind the blast. The bigger the bench
height to burden ratio, the more tensile stress is exerted onto
the rock. Rock tends to break the best under tensile stress. This
is like trying to break a tall skinny pencil in half and a short fat
pencil in half. The tall skinny pencil is a lot easier to break. The
plan was to increase the powder factor by decreasing burden
and spacing and increasing face height. This in theory would
increase movement of the material, increase fragmentation,
and decrease ground vibrations.

Design
The bench elevation that the dozer drove off was on the
5,740 ft (1,750 m) elevation. The front dozer blade caught
on the 5,680 ft (1,731 m) catch bench below. This meant the
blast would have to fragment 60 ft (18.3 m) of material to be
excavated to create a pad to work on the dozer. Two types of
blasting were designed for creating the pad, one being for the
initial drop and the other for removing the material closest to
the dozer.
Since we had to drop down 60 ft (18.3 m), the drop cut
was made by shooting two levels. The first level was drilled to
5,697 ft (1,736 m) and the second was drilled to the 5,677 ft
(1,730 m). This was because we used normal production design for the drop because it was far enough away to not be as
concerned with moving or hurting the dozer. This helped out
the speed of the mining cycle.
Signature hole analysis was done on a 40 ft (12.2 m) bench
using normal production practice of down hole cord and on a
60 ft bench using a down hole electronic detonator. An explosives supplier was used to analyze the signature hole data and
they came up with 33 ms hole to hole and 62 ms row to row
for the 40 ft (12.2 m) bench, and 25 ms hole to hole and 53
ms row to row for the 60 ft bench. These situations simulated
well at 100 ft (30.5 m) and 200 ft (61.0 m) locations from the
blasthole.
Normal production patterns used at the mine site are 16
ft x 18 ft x 23 ft (4.9 m x 5.5 m x 7.0 m) (burden x spacing x
depth) in ore and 18 ft x 18 ft x 44 ft (5.5 m x 5.5 m x 13.4
m) in overburden. The average powder factor on site is around
January/February 2015

0.4 lbs/ton (0.2 kg/tonne). The decision was made to double
the powder factor to 0.8 lbs/ton (0.4 kg/tonne) for the special
panel shots by decreasing the burden and spacing to 13 ft x
15 ft (4 m x 4.6 m) and increasing depth to 63 ft (19.2 m).
The pounds of explosives were limited in the 63 ft (19.2 m)
face by using a 6.75 inch (171 mm) hole instead of normal
7.875 inch (200 mm) hole. A buffered blend with a density
of 1.15 g/cc was used due to reactive ground potential. Unfortunately getting nice crushed stone was not an option for
stemming so drill cuttings were used for stemming the holes.
The quality of the drill cuttings for stemming was decent due
to the damp conditions of winter and stemming ejection was
minimal.
The panel shots were limited to three rows to minimize
constipation of the shot. After three rows, relief caused by
the row timing and material moving, starts to decrease. This
causes an increase in vibrations going back into the wall. The
pattern designs of the drop cuts and panel shots are shown
in table 1. Figures 2 and 3 show a plane view of the pattern
designs.

Results
Unfortunately, there are no regulations on the maximum
vibrations for a D10 dozer sitting on the edge of a high wall.
The engineers had no starting place besides trial and error.
Since the material with the dozer did not fail due to weather
conditions changing, it was assumed that the dozer could
take quite a bit more than the regulation for structures of
2 in/s (50.8 mm/s). Table 2 shows the distances away from
the blast of the seismographs and seismograph data. Notice
that the last three blasts had significantly more ground vibrations. This was due to the proximity of the blasts. From data
collected vs. what was estimated, vibrations near the dozer
were significantly reduced by using signature hole data and
increasing powder factor by decreasing burden and spacing
and increasing hole length. Now in a perfect world the hole
diameter would of have been drastically reduced. This would
have decreased pounds per hole to be less than production
and still doubled the powder factor. With this operation going
lower than 6.75 inch (171 mm) diameter was not an option.
The first blast went well. Laser profile scans were taken before and after the blast and showed minimal movement. Figure 4 shows the first blast. Notice the dozer in the lower right
hand corner. The dozer was 166 ft (50.6 m) away from the
blast. We did not decide to bring the next pattern back from
the crest edge because the scans did not show any movement
in the material between the dozer and the blast. The blast had

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The Journal of Explosives Engineering - January/February 2015

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