In short
- Flyrock is any fragment thrown beyond the area cleared for the blast. It is the blasting hazard that reaches people who are outside the pit.
- The causes are known: too little stemming, too little burden in the front row, weak zones and voids in the rock, overcharging, a wrong delay sequence and holes that deviated.
- Prevention is mostly measurement before charging: the face surveyed, the burden measured, each hole checked.
- The danger zone is the back-stop, not the control. The 1961 regulations put it at 300 m; DGMS Circular 2 of 2003 advises 500 m and substantially built shelters.
Flyrock is rock thrown by a blast beyond the area that was cleared for it. Vibration cracks plaster and brings complaints. Flyrock is the hazard that injures and kills people who are nowhere near the bench: on a road, in a field, in a house beyond the boundary.
It happens when explosive energy finds a shorter way out than through the rock it was meant to break: up the collar of the hole, through a front row with too little burden, or along a weak seam. A blast is designed to prevent it by closing those three exits, and the danger zone exists for the shot where the design fails anyway.
Three ways rock leaves a blast
The published studies describe three mechanisms. Rifling is the hole behaving like a gun barrel: the stemming gives way and gas, stemming and collar rock go straight up. Cratering is the top of the bench breaking upwards because the charge sits too close to the surface. Face burst is the front of the bench blowing outwards where the burden is thinner or weaker than the design assumed.
Knowing which of the three happened tells you where to look. Rifling and cratering point at the collar. Face burst points at the front row and the face survey.
Causes and what prevents them
| Cause | What happens | What prevents it |
|---|---|---|
| Short or poor stemming | Gas escapes up the hole and takes the stemming and the collar rock with it | Stemming length and material held to the design in every hole, using angular crushed stone |
| Too little burden in the front row | The charge is nearer the free face than drawn, and the face bursts outwards | A face survey and a measured burden for each front-row hole before charging |
| Weak zones, mud seams, cavities and loose rock | Gas vents through the weakness at high speed and does not break the rock around it | A driller's log of voids and soft bands, and holes checked before charging so the shotfirer can allow for them |
| Overcharging | More energy than the burden can absorb, and the surplus throws rock | A charge worked out for each hole from its measured depth and burden, not repeated from the last shot |
| Wrong delay sequence, or too short a delay between rows | Back rows fire before the front rows have moved, so the rock can only go up | A sequence in which every hole has relief when it fires |
| Inaccurate drilling and hole deviation | The bottom of the hole ends up closer to the face, or to its neighbour, than the pattern shows | Holes set out and drilled to the design angle, with deviation measured and not assumed |
Causes and mechanisms as listed in published technical papers on flyrock. Whatever the cause, the control is specific to the bench and is the licensed shotfirer's decision.
Checks before the holes are charged
Most flyrock is decided before any explosive reaches the bench. Start with the front row. A face is never the flat plane on the drawing: it has hollows where the last shot overbroke, so a hole drilled at the design burden from the crest can be much closer to open air halfway down. A face survey finds those holes while there is still time to change what goes into them.
Then each hole is checked: its depth against the design, whether it holds water or is blocked, and whether the driller logged a void or a soft band on the way down. A hole nobody checked is a hole that will surprise the shotfirer. It is the practical argument for one firm doing the drilling and the blasting: the log and the person who needs it are in the same crew.
Deviation is the hidden version of the same problem. Hole deviation and what it does at the hopper explains how a small error at the collar moves the toe of a hole, and with it the burden.
Stemming and sequence
Stemming is the inert material in the top of the hole above the charge. Its job is to hold the gas in long enough to work on the rock. As a first estimate, the Dyno Nobel Blasting and Explosives Quick Reference Guide gives a stemming length of at least 20 hole diameters, or 0.7 to 1.2 times the burden, in angular material sized at a tenth to a twentieth of the hole diameter. Those are textbook starting points that the rock and a trial decide. Burden, spacing and hole depth sets out the rest of the pattern.
Sequence matters as much. Each hole needs somewhere for its rock to go, and the delay order is what provides it.
Muffling, the danger zone and shelter
Close to buildings and roads, the shot is also covered. Muffling means laying heavy material over the blast area, old conveyor belting under sandbags for example, to hold fragments down. DGMS Circular 2 of 2003 lists adequately muffled holes among the accepted controls. Controlled blasting methods puts it beside the other techniques.
Behind the design stands the danger zone. Regulation 164 of the Metalliferous Mines Regulations, 1961, which DGMS has applied to stone quarries for decades and which are being replaced by regulations under the OSH Code, 2020, requires warning signals over the whole area within 300 metres of the place of firing and everybody in it under proper shelter. Where a public road or railway lies inside, two guards are posted, one in each direction.
The 2003 circular advises 500 m and substantially built shelters. Check the regulation currently in force. The limits that apply near buildings are in blasting near houses, roads and structures.
Near-miss signs on the bench
- A crater around a hole collar after the shot, wider than its neighbours.
- Stemming ejection: a jet from one or more collars at the instant of firing, easiest to see on a video of the shot.
- Rock landing beyond the expected throw, even when it is well inside the danger zone.
- A burst from one part of the face while the rest of the shot moved normally.
Why the contractor's records matter
A flyrock incident is investigated from paper. If the blast design, the driller's log, the measured burdens, the charge in each hole and the sequence were recorded, the cause can usually be found and the next shot changed. If they were not, the next shot is the same shot.
So before appointing anybody, ask to see the records of three recent shots, including what was observed afterwards. Our blasting crews design every shot for the bench in front of them instead of repeating the last one, which only means something if the last one was written down.
Standards and sources
- Metalliferous Mines Regulations, 1961, Regulation 164 (taking shelter, danger zone, guards on roads and railways), being replaced by regulations under the Occupational Safety, Health and Working Conditions Code, 2020
- DGMS (SOMA)/(Tech) Circular No. 2 of 2003, 31 January 2003, 'Dangers due to blasting projectiles'
- Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010): stemming length and stemming material first estimates
- Published technical papers on flyrock causes and mechanisms in opencast blasting (rifling, cratering, face burst)
- Drilling and blasting practice: SM Infra's own operating record
Your contract and its technical schedules override anything written here. Check the clause before you build to it.