Overcoming Oversize: Oversize and confinement (Part 4)

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 4 of the four-part series. You can also check out Part 1, Part 2 and Part 3.

Drilling & blasting Photo: SlavkoSereda/iStock / Getty Images Plus/Getty Images

If the row-to-row timing is too fast, or the sequencing of boreholes is incorrect, then the holes firing will have too little energy for the rock mass in front, and oversize will result. Photo: SlavkoSereda/iStock / Getty Images Plus/Getty Images

The blasting process is pieced apart in this final chapter of the “Overcoming Oversize” series, with a methodology for study relying on a 4D energy application.

This is a fairly simple process to understand. It all starts from a general understanding that the way an explosive breaks rock is through the application of a large amount of force derived from chemical energy. In order to break rock, the explosive energy must be greater than the strength of the rock mass in front of it at the time the explosive is detonated.

To understand this concept, let’s go back to the 1950s, when Gunnar Johnsson and Wolfgang Hofmeister studied the effects of stemming in underground rock salt blasting.

Trip back in time

Johnsson and Hofmeister selected rock salt because it was homogenous, meaning it did not have varying effects and that geology could not be claimed to affect the result of the study. Johnsson and Hofmeister designed a blast with a set burden and stemming, firing the blast and observing the breakage.

It was difficult at the time to know the actual fragmentation of the muckpile. Instead, the burden was increased on each successive blast while maintaining the same explosive and stemming – until the rock no longer broke from the explosive load.

The burden just below this, which was broken but beyond which no breakage occurred, was then considered the “maximum effective burden.” The results showed that based on the length of stemming, up to a certain length, the maximum effective burden increased as the stemming length increased.

When enough stemming was used that the stemming did not eject from the borehole, further stemming increases resulted in no change to the maximum effective burden.

This study revealed many important concepts, but one of the major ones is that the explosive produces a set amount of energy. For a proper blast, energy must be greater than the strength of the rock mass. Too little energy, or too much rock, leads to overconfinement and no rock breakage. In my purview, the problem will always be addressed as “too little energy,” which means the exact same thing as “too much rock.”

This analysis provides a distinct endpoint: too little energy, and there’s no rock breakage. Another way to look at this is that the maximum rock size after the blast is infinite – or at least very large. As more energy is applied, the maximum rock size will be reduced to a smaller value as an exponential function, which will have a set minimum size that can be achieved based on all other practical blast applications.

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