The short answer
Fly rock is stone thrown beyond the area cleared for a blast. It comes from three places: the face, when the front-row burden is too thin; the collar, when stemming is too short or too fine; and weak zones such as mud seams and cavities that let the charge vent. The controls are a surveyed face, measured stemming of crushed stone, a checked charge in every hole, correct delays and, close to property, covering the shot.
Of everything that can go wrong with a blast, fly rock is the one that kills people. It is also almost never bad luck. In the investigation afterwards there is nearly always a hole that was shorter, closer or fuller than the design said, and somebody who did not measure it.
01Three ways rock leaves a blast
| Mechanism | What happens | Usual cause |
|---|---|---|
| Face burst | Rock is shot horizontally out of the free face | Thin burden on the front row: an uneven face, or a hole that drifted forward |
| Cratering | The collar zone lifts and scatters upward | Stemming too short for the hole diameter, or weak rock at the top of the bench |
| Rifling | Stemming and stone are ejected up the hole like shot from a barrel | Fine or wet stemming that will not lock, such as drill cuttings |
02Before drilling
Look at the face. A face left ragged by the last shot has hollows where the burden in front of a hole may be half the design figure. On a high bench or near anything that matters, profile the face with a survey instrument and move or angle the front-row holes to suit. Clean loose rock off the crest, because the first row is collared on it.
03During charging
- Tape every hole before loading. A short hole given the full charge has no room left for stemming.
- Check for water and for blockages.
- Watch the column rise. If a hole takes more explosive than its depth allows for, the product is running into a cavity or a seam. Stop, and bridge the zone with an inert deck.
- Measure the stemming length. Around 0.7 to 1.0 times the burden is normal, and more in the front row and near structures.
- Stem with crushed angular stone of about 10 to 15 mm. It wedges under pressure. Cuttings blow out.
- Keep a charging sheet with the explosive and stemming in each hole, signed by the person who loaded it.
04Timing
The delay between rows has to be long enough for the row in front to have started moving. If the second row fires into rock that is still in place, its energy cannot go forward, so it goes up. Too little delay between rows is a common cause of cratering in the back rows of deep shots.
05Covering the shot
Near houses, roads and plant, the blast is muffled. Heavy rubber mats, or wire mesh weighted with sand-filled bags, are laid over the holes so that anything lifted is caught. Covers do not replace correct design. They are there for the hole that went wrong in spite of it.
Charge weights come down at the same time, for vibration as well as for throw. The arithmetic of that is in controlled blasting near structures.
06Secondary blasting
Breaking boulders with small charges, whether in a short drilled hole or laid on the surface under mud, throws stone further and less predictably than any bench blast. The charge is barely confined and the rock has free faces all round. Where anything is nearby, break oversize with a hydraulic breaker instead. Better still, reduce the oversize at source, which is the subject of oversize at the hopper.
07The danger zone
Every shot has a danger zone that must be clear of people before firing. The Metalliferous Mines Regulations define it as 300 m around the shot, and DGMS has asked mines to treat 500 m as the zone for deep-hole blasting. A zone on paper protects nobody. It needs guards posted on every approach, a warning signal that everyone on the site knows, a shelter for the shotfirer, and one person with the authority to stop the shot if somebody walks in.