The Journal of Explosives Engineering - November/December 2024 - 28

Figure 3.
tiating system for underground (not factoring in downstream
costs). Therefore, most conventional underground perimeter
control methods relay on firing the perimeter blastholes on the
final delays of the blast, despite the impacts of cap scatter of
long period nonelectric detonators.
Pre-blast control methods are less common in underground
mining, but can produce the best results if an operation has
the right design and products. These are applied on the first
delays of a blast or in a separate blast before the stoping or
production holes are loaded and fired. Pre-blast control methods
include presplit blasting or air decking (more commonly
a cost-effective surface blasting method, but proven to work
underground). Line drilling, which incorporates unloaded perimeter
holes, can be included in this method. Finally, another
pre-blast control method that never gained much traction, is
the fracture control technique. All these methods will be discussed
in detail in subsequent articles.
How do the methods compare? Figure 4 shows the relative
impact on backbreak between bulk blasting (no perimeter
control), smoothwalling, and presplitting. The image also illustrates
that contacts, foliation, and bedding planes work well at
stopping backbreak...similar results to presplitting.
Figure 4. Data taken from a seismic refractive survey on backbreak at the
University of Missouri-Rolla by Dr. Paul Worsey.
Perimeter Control Methods
This article is meant to provide a high-level overview of
perimeter control methods. Follow-up articles will cover the
design details for each method.
Perimeter control uses a one or a combination of smaller
diameter drillholes or charges, reduced charge weight, tighter
spacing and burden, decoupling, accurate delay timing, and
adjusted timing and sequence to reduce the chance of backbreak,
overbreak, and dilution.
Geology always plays a factor, sometimes in such a way
that perimeter control might not lead to better results of an
individual blast. However, when used globally over time, perimeter
control design methods can decrease the overall cost
of an operation significantly.
In general, there are two approaches to perimeter control:
post-blast control and pre-blast control. What this means, is
that perimeter control blastholes can be designed to detonate
before or after the stoping or production holes in a blast. Both
approaches can work well, depending on your operation.
Post-blast control methods are common in underground
mining. These methods include smoothwall blasting and buffer
/ cushion blasting, where the perimeter holes are fired last.
Typically, long period nonelectric delays are used underground
for tunneling. Long period nonelectric delays are designed so
the shortest delay periods are used in the middle, or cut, of the
blast and the longest delay periods are used around the perimeter.
Nonelectric delays are also the most cost-effective iniNovember/December
2024
The Journal of Explosives Engineering
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The Journal of Explosives Engineering - November/December 2024

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