The short answer
Every degree a blast hole is off line moves its toe about 17 mm for each metre drilled. On a 10 m hole a two-degree error puts the bottom of the hole 0.35 m from where the pattern needs it, which is more than a tenth of a 3 m burden. Holes that converge leave boulders between them, and holes that drift towards the face throw rock.
A pattern on paper is a grid of straight lines. What a rig actually leaves in the bench is a set of holes that start a little off their marks, lean a little off their angle, and bend a little as they go down. Nobody sees any of it until the shot is fired.
01Three errors, added together
- Collaring error. The bit starts somewhere other than the peg. Uneven benches, loose rock at the collar and pegs knocked out by the previous shift all cause it. It is the same at the top and bottom of the hole.
- Alignment error. The mast is set at the wrong angle or the wrong bearing. This one grows with depth, and it is the largest of the three on most sites.
- In-hole deflection. The string bends as it meets joints, bedding or a hard band at a shallow angle. It grows faster than depth, and thin rods bend more than thick tubes.
02What an angle costs at the toe
| Hole depth | 1 degree | 2 degrees | 3 degrees |
|---|---|---|---|
| 6 m | 0.10 | 0.21 | 0.31 |
| 10 m | 0.17 | 0.35 | 0.52 |
| 15 m | 0.26 | 0.52 | 0.79 |
| 20 m | 0.35 | 0.70 | 1.05 |
Depth multiplied by the tangent of the angle. In-hole bending adds to these figures.
Set those numbers against a burden of 3 m and a spacing of 3.5 m. At 15 m and two degrees, one hole has moved half a metre. If its neighbour moved half a metre the other way, the two toes are now a metre further apart than designed, or a metre closer.
03What the shot does about it
Two holes that converge at depth share the same rock. The charge is doubled where it is not needed, and the block on either side, now further from any explosive, comes out as boulders or stays behind as toe.
A front-row hole that drifts towards the face is the dangerous case. Its burden at the toe may be half what the blaster believes, and half a burden with a full charge behind it is how stone ends up outside the danger zone. The reverse, a hole drifting back into the bench, leaves a buttress the shot cannot move.
Further down the line the cost becomes ordinary and daily. Oversize goes to a breaker, the excavator digs a hard floor slowly, the tippers queue, and the jaw runs half empty. None of that is logged as a drilling fault.
04Keeping holes where the pattern put them
- Prepare the bench. A level, cleaned floor lets the rig sit square and the bit start on sound rock instead of on loose stone.
- Mark with a survey instrument, not a tape pulled from the last hole. Tape errors accumulate along a row.
- Set inclination and bearing with an instrument on the mast, and write both down for every hole.
- Collar slowly. The first half metre decides the line of the rest.
- Use the stiffest string the hole allows. Guide tubes on a top hammer, or a DTH hammer, where the bench is high.
- Tape every hole before the rig moves. Short holes get redrilled then, while the machine is standing over them.
On benches where the stakes are high, such as shots beside a road or near houses, the holes can be surveyed after drilling and the charge adjusted hole by hole. That takes time, and it is a good deal cheaper than the alternative. The design side of that is covered in controlled blasting near structures.