Overcoming Oversize: Oversize and spacing (Part 3)

By |  September 17, 2021

Pit & Quarry’s “Overcoming Oversize” series discusses the generation of oversize in the blasting process and techniques to reduce or eliminate it. Below is Part 3 of the four-part series. In Part 4, author Anthony Konya will analyze the three major causes of over-confinement: burden, stemming and timing. You can also check out Part 1 and Part 2.

Simply correcting the blasthole spacing can lead to large decreases in oversize produced by a blast. Photo: P&Q Staff

Simply correcting the blasthole spacing can lead to large decreases in oversize produced by a blast. Photo: P&Q Staff

The first two articles of this series address some of the major causes of oversize and methods to reduce it in the blasting process.

While the steps covered previously represent the first measures that should be taken to overcome oversize, the improper spacing of blastholes is often completely overlooked. Many times, spacing is designed improperly when aiming to reduce oversize from blasts.

In addition, the traditional design approaches for blasthole spacing lead to more oversize and increases in the drilling and blasting process. Spacing, then, becomes one of the most important factors in not only overcoming oversize, but in reducing a site’s drilling and blasting costs. Spacing is one of the few things that can lead to better blast performance while spending less money.

In Part 2 of this series, Calvin Konya’s research on stiffness ratio was discussed in relationship to how a low bench leads to more oversize and a high bench leads to less oversize. This month’s discussion picks up there, because Konya’s most important findings have yet to be discussed in this series.

Still, before diving into those findings, it is critical to first understand why a discussion on blasthole spacing first began.


Determining blasthole spacing was a challenge for a long time. Significant field experimentation was required to get to a ballpark spacing that would work and produce decent fragmentation.

This was the case, at least, until powder factor blast design was reintroduced in the early 1900s. Blasters could then use a formula to determine the burden and design the spacing based on achieving a certain powder factor.

Problems, however, were that the powder factor approach never truly worked and site-specific optimization was still required. That is, until Richard Ash decided he was going to develop a new way to design blasts.

Ash surveyed quarry operators and developed simple relationships between different blast variables. For example, most quarries at Ash’s time used a burden of around 20 to 25 times the borehole diameter when they used ANFO as an explosive.

This method gave a fairly narrow range to design a blast’s burden based on empirical data, but Ash had one major problem. When analyzing the survey data, he found that blasthole spacing at quarries ranged from one to two times the burden. Based on this large difference in spacing, no simple equation could be developed to determine blasthole spacing.

Konya set out to discover why this large deviation existed in the spacing-to-burden ratio. What he found was that the difference in spacing was due to spacing being influenced by both blast timing and a blast’s stiffness ratio. This led to the spacing design equations that are used worldwide today, determining blasthole spacing based on stiffness ratio, bench height, burden and hole-to-hole timing.

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