The short answer
Burden is the distance from a row of holes to the free face. Spacing is the distance between holes along that row. In quarry bench blasting, burden usually starts at 25 to 40 times the hole diameter and spacing at 1.15 to 1.5 times the burden, with about 0.3 of a burden drilled below floor level and 0.7 to 1.0 of a burden left uncharged at the collar as stemming.
A drilling pattern is a drawing with five dimensions on it. Each one is a ratio of the hole diameter or of the burden, which is why a blaster can look at a rig, a bench and a rock face and say roughly what the pattern should be before anybody has opened a calculator. The ratios below are where a design starts. The rock then moves every one of them.
01The five dimensions on the drawing
| Dimension | What it is | Starting ratio | 110 mm hole |
|---|---|---|---|
| Burden (B) | Hole row to the free face | 25 to 40 x diameter | 2.8 to 4.4 m |
| Spacing (S) | Hole to hole along the row | 1.15 to 1.5 x B | 3.5 to 4.5 m at B = 3.0 |
| Sub-drill (J) | Drilled below floor level | 0.3 x B | 0.9 m |
| Stemming (T) | Uncharged length at the collar | 0.7 to 1.0 x B | 2.1 to 3.0 m |
| Bench height (H) | Floor to crest | 2 to 4 x B | 6 to 12 m |
Hard, massive rock sits at the tight end of each range. Soft or well-jointed rock tolerates the wide end.
02Why burden is the number that matters
Every other dimension hangs off the burden, and the burden is the one the rock punishes. Set it too wide and the explosive cannot push the rock forward. The energy goes somewhere else: down into the floor as vibration, back into the wall as cracking behind the last row, and up through the collar. What is left standing at the bottom is a toe, and what lies on the floor is boulders.
Set it too tight and the opposite happens. Rock leaves the face fast and travels. That is fly rock, and it is the fault that ends a quarry's permission to blast. A tight burden also spends explosive and drill metres on rock that would have broken anyway.
The burden that counts is the real one at the toe of the front row, not the one on the drawing. A face that leans back five degrees has more rock in front of the bottom of the hole than in front of the top. This is why a front row is often drilled at an angle to match the face, and why a face survey before drilling is cheap insurance.
03Square or staggered
In a square pattern each hole sits directly behind the one in the row ahead. In a staggered pattern the second row is shifted by half a spacing, so every hole sits behind a gap. Staggering puts each charge closer to the rock it has to break and leaves fewer dead patches between holes, which is why most production benches use it. The geometry that spreads energy most evenly is an equilateral triangle, and that works out to a spacing of 1.15 times the burden.
Wider spacing ratios, up to 1.5 and beyond, come into play with millisecond delays between holes in a row. The delay means each hole fires into a face its neighbour has just made, so the true burden at the moment of firing is no longer the drawn one.
04Bench height against burden
Divide the bench height by the burden and you get the stiffness ratio. It predicts how a shot will behave better than any other single figure on the sheet.
| H / B | Fragmentation | Fly rock and air blast | Verdict |
|---|---|---|---|
| 1 | Poor, with a hard toe | Severe | Redesign: smaller holes or a higher bench |
| 2 | Fair | Moderate | Workable with care |
| 3 | Good | Low | Where most quarries aim |
| 4 and over | Very good | Very low | Little gained beyond this |
This is the reason a 110 mm rig does badly on a 3 m bench. Its natural burden is about 3 m, the ratio comes out at 1, and the block behaves like a short thick beam that refuses to bend. Low benches want small holes. That is jackhammer work, covered in which rig drills what.
05Sub-drill and stemming
Rock breaks in a cone from the bottom of the charge, so a hole stopped at floor level leaves humps between holes. Sub-drill is the extra length that carries the charge below grade and cuts the floor flat. About 0.3 of a burden is usual. More than that wastes drilling and shatters the top of the bench underneath, which the next drilling shift then has to collar into. Where the rock has a natural parting at floor level, sub-drill can come down to nothing.
Stemming is the plug that keeps gas in the hole long enough to do work. Too short a column and the shot rifles out of the collar, taking stone with it. Too long and the top of the bench comes down as slabs. The material matters as much as the length: crushed angular chips of roughly 10 to 15 mm lock in the hole, while drill cuttings blow out like dust from a pipe.
06Working back from the crusher
A pattern is not designed to break rock. It is designed to make feed. The primary jaw sets the largest piece that can be swallowed, which is about 80 per cent of the gape, so a jaw with a 900 mm opening wants nothing over 700 mm or so. Every piece bigger than that waits for a breaker while the plant runs light.
Tightening a pattern costs more drill metres and more explosive per cubic metre. Opening it out saves both and hands the bill to the breaker and the crusher. The right pattern is the one with the lowest cost at the stockpile, which is why it helps when drilling and blasting answer to the same firm that runs the plant. The full argument is in oversize at the hopper.
07Drill metres per cubic metre
Once burden, spacing and bench height are fixed, the volume each hole breaks is fixed too, and so is the drilling needed per cubic metre. This is the figure a drilling rate is built on.
- Burden x spacing
- 3.0 m x 3.5 m
- Bench height
- 8.0 m
- Rock broken per hole
- 84 m3 in the solid
- Hole length with 0.9 m sub-drill
- 8.9 m
- Rock per hole at 2.7 t/m3
- 227 t
- Specific drilling
- 0.106 m per m3
Put the other way, every metre drilled yields about 25 tonnes of rock. Widen the pattern to 3.5 by 4.2 m in softer ground and the same metre yields 35 tonnes. That gap, multiplied over a month, is most of the difference between two drilling quotations for what looks like the same job.