Cracks and Voids: What the Hole Doesn't Tell You

Cracks are linear breaks in the rock. Voids are cavities, empty space inside the geology.

Both change blast outcomes dramatically, and both do it by giving explosive gas somewhere to go other than the rock you meant to break.

• Gas vents before it can do useful work.
• Confinement drops. The explosive can't pressurize the rock around it.
• Flyrock and blowouts. Energy escaping upward or toward an open face takes rock with it.
• Powder factor gets distorted. Explosives that run into a void inflates the local PF for the surrounding rock without breaking anything useful.

Some of these voids are natural, sitting in the ground before anyone touched them. Plenty are man-made, left by previous blasting.

The right tool is a borehole camera. Lower it down and watch for cracks crossing the hole, open voids, and water entry points, which often indicate cracks, or seams.

A lot of operations still use sewer cameras because they're cheap. They work, but they wear out fast in the abrasive, gritty conditions inside a blasthole, which is why MTi's Borehole Camera is purpose-built for the job.

The strategy is to plug above it, so explosive doesn't spill in and inflate the local powder factor.

• Underground: BLASTBALL or BLASTBAG ACE II.
• Open pit: BLASTBAG SOLO, EVO or ACE II.
• Small voids found during loading: shot bags will do.
• Large voids: a shot bag isn't enough. You need a sealed plug above it.
• Small-diameter holes: ROCKRIVETS block off a void in a small hole.

Sometimes the explosive column needs to pass through the crack or void zone rather than stop above it. Then the hole has to be sleeved.

BLASTSHIELD Plus is the liner for deep holes; Lay Flat Liner covers shallower ones. Either way the liner physically prevents explosive from escaping into the void.

A lot of cracks and voids are self-inflicted, created by energy directed at what will become a future bench. If you know where the next bench is going, design this blast to protect it:

• Air deck or dirt deck the row closest to the future bench, so less energy goes that way.
• Slow the timing on the holes nearest it, so energy has moved away before the back row fires.
• Time the shot to pull energy toward the open face rather than into the next bench.

You can't plug what you haven't seen. A camera down the hole is a great way to reduce how often you get caught by the one failure mode that gives the blaster no warning at all.

Drilling into ground you don't trust? Call us at 606-663-4069.