The Journal of Explosives Engineering - May/June 2022 - 31

flyrock from stemming ejection. Each hole depth in the main
body was measured and uploaded into the bulk emulsion
truck which allowed the truck to deliver an average density of
1.22g/cc depending on the depth. The overall powder factor
in the crusher pattern was 0.85 lb/lton (0.38 kg/tonne) which
is increased over the typical waste rock loading of 0.63 lb/lton
(0.28 kg/tonne) to compensate for the extended stemming
column.
Burden & Spacing
As previously mentioned, the blast hole depths varied
greatly. This alone made the determination of the burden and
spacing difficult, but when the 16 inch (406 mm) diameter of
the blast holes was considered, the burden and spacing determination
became even more difficult.
It was determined that no two blast holes should be within
a straight-line distance of 24 ft (7.3 m) due to concerns about
adverse interactions between blast holes. After careful consideration
and calculation, it was concluded that a 22 ft by 24 ft
(6.7 m by 7.3 m) staggered pattern would adequately meet
the requirements for rock breakage while at the same time
minimizing rock movement and preventing adverse interactions
between blast holes.
The west (crusher) facing crest was drilled on an average
of a 23 ft (7 m) spacing controlled by the burden profiles
completed. To maintain full control of horizontal movement
a minimum crest burden 37 ft (11.3 m) was established at the
top of the explosive column to allow a maximum horizontal
movement of up to 100 ft (30.5 m) in the direction of the
crusher. This was further optimized utilizing the variable density
gassed emulsion. Due to an extended stemming length in
the west face to control movement confidence was increased
that hole to hole interruption would be avoided.
Timing
Since minimal blast vibrations, particularly at the crushers,
was one of the stated goals of the project, considerable effort
was put into determining the optimum blast hole timing. In
the past, nearby production blasts have triggered the electric
motor vibration monitors resulting in SAG mill shutdowns in
the concentrator on site. From the beginning of the project it
was recognized that electronic detonators would be required
to achieve the necessary level of control of the blast. Dyno Nobel's
Digishot Plus 4G detonators were used as the initiation
system. A signature hole had been performed with a similar
1,200 lb (544 kg) load of Titan 1000G in a waste rock layer
with seismographs recording the vibration at all the structures
of concern. However, the signature hole traces were recorded
from a blast hole in the nearest portion of the mine and at
a distance greater than the distance between the proposed
blast and the crushers. Since it would not be possible to obtain
vibration traces from a signature hole at the proposed blast
pattern, the nearby signature hole information was used. Signature
hole analysis (SHA) was performed for each structure
location with the crushers receiving priority in selecting the
delay times for consideration. Based on the SHA and again
considering the rock breakage requirements and flyrock limitations,
it was decided an intra-row delay of 40 ms and an inter-row
delay of 128 ms would provide the best overall results.
With this timing pattern there would be a maximum of two
blast holes per delay period. However, the SHA determined
May/June 2022
The Journal of Explosives Engineering
31
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The Journal of Explosives Engineering - May/June 2022

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