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The 4 most costly blasting mistakes you can make

By |  April 9, 2021

4. Drill deviation

As a drill progresses, it naturally will wander in the borehole and cause holes to not be perfectly straight. This can be monitored using borehole-tracking systems, and it can be visually seen in half-casts left on the rock mass.

This drill deviation causes a number of problems to sites, including the production of boulders, problems achieving the proper grade, the generation of flyrock, and large increases to ground vibration. These problems are all a result of the location of the borehole at the bottom of the bench (at grade).

Drill deviation presents itself in three distinct ways: excessive burden, insufficient burden and improper spacing.

Excessive burden on a blast occurs when the hole wanders away from the burden. If this is in the middle of a blast pattern, then it leads to excessive burden in the specific hole and insufficient burden in the row behind that hole. Excessive burden can increase ground vibration levels, create flyrock from the top of the bench, leave a toe in the large burden, increase air overpressure, and lead to boulders forming.

Insufficient burden on a blast occurs when the drill deviates toward the free face (or previous row of holes). If this is in the middle of a pattern, it will also create excessive burden on the rows behind it. This insufficient burden leads to flyrock and increased air overpressure. The excessive burden on the next row often leads to additional problems.

The final situation is improper spacing, which features too much spacing between one set of holes and insufficient spacing between another set. This typically leads to vertical blowout of holes (clumping), a mixture of fines and boulders in the muckpile, and typically in a toe being left at grade.

The best-designed blasts can still perform poorly if the drill deviation is not kept in check. While geology does cause this deviation, another major cause is too much down pressure from the drill operator, who is often trying to drill a hole fast. The time it takes to drill each hole is a great KPI to keep, comparing it to drill deviation KPIs that are set up.

Another factor of having a stiffness ratio above 5.0 is that the slenderness ratio of the drill steel increases, and more deviation can occur. While some drills are equipped to deal with stiffness ratios above 5.0 – and several mines use stiffness ratios above this level – many sites put a limit in place to help alleviate problems of drill deviation.


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