Air Blast vs Ground Vibration: Why the Complaint Is Almost Never What They Say

A neighbor calls. Windows rattled, a picture moved, somebody's ears popped. The word they use is almost always “vibration.”

Your seismograph says you were under the limit. Both of those things can be true at the same time, because the thing they felt probably wasn't vibration at all.

Somewhere between 90 and 95% of blast complaints from neighbors are air blast, not ground vibration. People feel air overpressure more strongly than they feel ground shake.

Air blast is the pressurized wave of air travelling out from the shot. It rattles windows, pops ears, moves pictures on walls, and at high enough levels it cracks glass.

Ground vibration is the seismic shock moving through the earth. It's measured as peak particle velocity, and the usual regulatory limit at a protected structure is 1 inch per second.

These are two different phenomena with two different mitigation strategies. Confusing them is why a site can be comfortably inside its vibration limit and still be fielding complaints every shot.

Ground vibration has predictive tools. A scale distance chart tells you how many pounds of explosive can fire per delay for a given distance to the nearest structure, with a delay being 8 milliseconds by industry standard.

Unlike vibration, air blast formulas can't take into consideration the many variables you encounter on the bench. Mitigation is entirely about blast confinement.

Better confinement means less air overpressure. Worse confinement means a bigger air blast. When the energy stays in the rock and does its job breaking material, the pressure component is small. When energy escapes the hole, that escaping energy is the air blast.

1. Not enough stemming. Not enough material capping the explosive column, so gas vents straight up out of the collar.
2. Too much explosive for the stem column. The load exceeds what the stemming length can confine.
3. Blowouts. Face holes drilled closer to the open face than the design called for. Overloaded face holes fracture sideways and blow out laterally instead of breaking the bench properly.
4. Cracks and voids nobody identified. The driller went through one and it wasn't logged. The explosive vents into the void, or out through the crack to surface.

The most useful mitigation tool for air blast is top-hole air decking.

Research by MTi and Itasca puts the reduction in upward pressure on the stemming at up to 65%. That is the number that matters here, because stemming can only do its job if it isn't being driven up and out of the hole by the gas column underneath it.

Take the upward pressure off the stem column and the same stemming length suddenly confines the shot properly.

Which bag depends on the hole. BLASTBAG SOLO is the premium dual-speed aerosol and covers 4.5" to 12 1/4" (114 to 311 mm). BLASTBAG EVO covers 4" to 9 7/8" (102 to 251 mm) across just two variants, which is why drill and blast crews running mixed diameters tend to prefer it. BLASTBAG ACE II is the chemical-inflation option with no metal and no hazardous materials, for sites that need non-DG or run metal detectors.

Below about 4", or where a mechanical plug suits the job better than a bag, ROCKRIVETS grip the hole wall under pressure instead. On a 90 Mtpa iron ore operation in South America they returned a 38% reduction in P80 fragment size and a 15% reduction in top-size material, along with reduced flyrock range.

• Verify burden and spacing against the design. No holes drilled closer to the face than specified.
• Get the powder-to-stem ratio right. Don't overload the column; leave room for adequate stemming.
• Look at the face before you drill. 3D modeling and drone surveys are increasingly used pre-drill to profile the face before holes go in to be sure sufficient burden is there for each hole.

Air blast still happens underground, but it's much less of a concern. The pressurized air dissipates through the tunnel network, the way blowing through a straw connected to many others goes nowhere in particular. There's always somewhere for the pressure to go.

Worst case underground is usually ventilation damage in the immediate blast area, and that's normally an easy repair.

If the complaints keep coming and your seismograph keeps saying you're fine, stop trying to solve it with charge weight per delay. That's the vibration lever, and vibration probably isn't the problem.

Look at the drill log first. Then the stem column. Then the face holes.

If you want help picking a bag or a plug for the hole sizes you actually shoot, call us at 606-663-4069. We'd rather get it right than ship you the wrong box.