In short
- Burden is the distance from a hole to the free face, and spacing is the distance between holes in a row. Textbook starting points are 25 to 40 hole diameters of burden, and 1.15 times the burden for spacing on a staggered pattern.
- Hole depth is the bench height plus sub-drilling, the extra length below floor level that lets the rock break to grade at the toe. A first estimate is 3 to 15 hole diameters.
- These ratios are first estimates. Trial blasts in the actual rock refine them, and the design for each bench belongs to the licensed shotfirer.
- The pattern is a specification. A hole drilled off its mark changes the burden somewhere, and that shows up as oversize, a toe or flyrock.
A blast pattern is a set of distances, and most arguments about fragmentation come down to one of them being wrong. Before a hole is drilled somebody has decided how far it sits from the face, how far from its neighbour, how deep it goes and how much of it runs below the floor.
The short version: burden is the distance from a row of holes to the free face in front of it, and spacing is the distance between holes along the row. Depth is the bench height plus a little extra, called sub-drilling. Published rules of thumb scale all of them from the hole diameter. They are where a design starts, not where it finishes.
The terms, one sentence each
- Bench height: the vertical distance from the floor the excavator loads from to the floor the drill stands on.
- Burden: the distance from a hole to the nearest free face, measured square to the row, or to the row in front for holes further back.
- Spacing: the distance between neighbouring holes in the same row.
- Hole depth: the drilled length, which on a vertical hole is bench height plus sub-drilling and on an inclined hole is longer.
- Sub-drilling: the length drilled below floor level, so that the rock breaks to grade and does not leave a hump.
- Stemming length: the top part of the hole that carries inert material in place of explosive, to hold the energy in the rock.
- Collar: the mouth of the hole at the bench surface, which is the point the surveyor marks.
- Toe: the bottom of the face where it meets the floor, and the unbroken rock left there after a poor blast.
Starting ratios and what moves them
The figures below come from an explosives manufacturer's quick reference guide, which offers them as a first estimate in the absence of better data. No regulation prescribes them, and two quarries in the same district can finish on different numbers.
| Dimension | Starting ratio | What moves it |
|---|---|---|
| Bench height against hole diameter | Bench height in metres at least hole diameter in mm ÷ 15 | Low benches take smaller holes |
| Burden | 25 to 40 × hole diameter | Hard, massive rock pulls it in. Softer or closely jointed rock lets it out |
| Spacing | 1.15 × burden on a staggered pattern | Joint direction, the fragment size wanted, the pattern shape |
| Sub-drilling | 3 to 15 × hole diameter | How hard the toe is. A natural parting at floor level may need very little |
| Stemming length | At least 20 × hole diameter, or 0.7 to 1.2 × burden | What stands nearby, and how blocky the collar rock is |
| Stiffness ratio (bench height ÷ burden) | 2 to 3.5 good fragmentation, above 3.5 very good | Mostly the bench height you were handed |
Source: Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010). Starting points only. The design for a given bench is the licensed shotfirer's, adjusted from trial blasts.
Square and staggered patterns
In a square pattern the holes of each row sit directly behind the holes of the row in front, and spacing equals burden. In a staggered pattern every second row is shifted sideways by half a spacing, so each hole sits behind the gap between two holes ahead of it.
Staggering puts every part of the rock nearer to a hole, which is why it tends to fragment more evenly. The 1.15 figure is geometry, not folklore: when three neighbouring holes form an equilateral triangle, the spacing works out at 1.15 times the distance between rows. Square patterns are quicker to mark and easier to check by eye.
Vertical and inclined holes
A vertical hole is the easiest to set up and to check. Its weakness shows on a face that leans back, which most do: the burden at the toe is then larger than at the crest, so the bottom of the hole is asked to move more rock than the top.
A hole inclined parallel to the face keeps the burden the same from collar to toe, and tends to leave a cleaner toe and a sounder new face. The price is set-up. An angle and a direction have to be held on every hole, and an error in either grows with depth. How hole deviation turns into oversize puts figures on that.
Why the pattern is a specification
A hole a metre off its mark does not announce itself. Where two holes have wandered apart, a block of rock sits too far from both and comes out as oversize. Where the bottom of a front-row hole is further from the face than designed, a toe is left standing. Where it is closer, the charge has too little rock in front of it and the energy leaves as noise and flyrock.
By then the explosive is spent. So on our drilling work the pattern is treated as a specification and not a suggestion: collar position, depth, diameter and inclination are drilled to the blast design.
What the driller records for each hole
The shotfirer cannot see inside the rock. The driller has just felt every metre of it through the machine, and the log is the only account of what was there.
- Whether the collar is on its mark, and if not, how far off and why.
- Depth drilled against design depth, and the angle the mast was set to.
- Anything felt on the way down: a soft seam, a cavity, a sudden loss of flushing air.
- Water in the hole, and whether it stayed open after the steel came out.
Cavities, mud seams and weak planes are recognised causes of flyrock, because the design assumed solid rock. A void that was felt and not reported is the worst kind.
Working back from the crusher
A pattern is not designed to break rock. It is designed to break rock to a size. Our three-stage plants have the reduction ratio to take a 700 mm boulder down to 10 mm chips, and rock that will not enter the primary jaw has to be broken a second time on the quarry floor.
A tighter pattern gives finer rock and costs more drilled metres and more explosive in every cubic metre. A wider one is cheaper at the face and coarser in the muck pile. The right pattern is the widest that still feeds the jaw without a queue at the breaker. Fragmentation and crusher output follows the rock from there, and the charge side of the design sits with the controlled blasting crew.
Standards and sources
- Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010): rules of thumb for bench blasting, given as a first estimate in the absence of better data
- Published studies of flyrock in opencast blasting: cavities, mud seams, weak planes and inaccurate drilling among the recognised causes
- Plant reduction ratio and drilling 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.