Overcoming Oversize: Oversize and spacing (Part 3)

How the right spacing overcomes oversize

Improper blasthole spacing leads to blasts producing an increase in both oversize and fines in the final muckpile.

The blasthole spacing most mines use today is improper, causing an increase in oversize. Simply correcting the blasthole spacing can lead to large decreases in oversize produced by a blast. And in the majority of cases, the spacing is too close. This means expanding the pattern can reduce oversize and costs.

The first and simplest way to determine if an operation’s blasthole spacing is improper is to take the spacing (in feet or meters) and divide it by the burden (in feet or meters). If this ratio equals 1.0, then the spacing is too close and needs to be expanded in almost all cases. If this ratio is less than 1.1, it should be expanded in the majority of cases.

Additionally, simple field observations following a blast are important to determine if proper spacing is being used. The first requirement for this is that there not be a significant amount of backbreak.

The backbreak at a site should be less than 0.5 times the burden at the crest and less than 0.1 times the burden at the toe. If the backbreak is more significant than this, other avenues should be explored to optimize the blast design and achieve a better performance.

A next step is to analyze the face of the next bench after the blast has been fired. The key is to look for a sawtooth effect on the face. If there is a sawtooth effect with the boreholes being further out and cutting back into the rock, then the spacing is too small. If the boreholes are farther back and the rock in-between is closer out, then the spacing is too far and needs to be reduced.

This analysis sounds simple – and it is – but the difficulty is in determining where the blastholes are located before the blast. The original hole’s half casts are often not found throughout the entire shot.

Where tech comes into play

Fortunately, this is where modern software and drone imagery can greatly help in determining and measuring the effects from changes in the spacing.

The first part of the process is to log the actual, as-built location of the boreholes. This can be completed using different tools. The simplest tool is drone photography, along with an artificial intelligence system to log borehole locations. This can be completed with software that’s available on the open market today.

Drone photography can be used before a blast and then again afterward, generating a 3D model of the new face. These can then be lined up to determine the breakage pattern and quickly assess how the spacing is performing.

Sawtooth faces will only be present when the spacing has major problems. But the absence of this sawtooth effect does not mean the spacing is perfectly designed and that it is minimizing oversize (and fines) produced in a blast.

The next approach is to utilize the original Konya spacing equation for delayed boreholes. The equation is only for applications where the bench height is less than four times the burden. The equation to determine the spacing to burden ratio is: S/B = L + 7B/8B

This equation will take you to a good estimate for the spacing-to-burden ratio for a blast.

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