A blast design is geometry. So many kilograms of explosive, so many metres from the face, so many from the next hole. Blast hole deviation is what happens to that geometry between the drawing and the rock, and a shot can only be as good as the holes it is loaded into.
Our own drilling page says it plainly. A hole a metre off pattern, or two degrees off vertical, does not announce itself. It shows up as oversize at the hopper, a toe left standing at the bench, or flyrock going somewhere it should not. By then the charge is already spent.
What blast hole deviation is
Blast hole deviation is the distance between the designed position of a hole and its real position, measured at the collar, at any depth along it and at the toe. It is usually quoted as a distance at the bottom of the hole or as a percentage of hole length. A 12 m hole whose toe is 0.6 m from where it should be has deviated 5 per cent.
The drilling literature splits it into four errors. They are worth keeping apart because they have different causes and different cures.
Drill hole deviation causes: the four errors
| Error | What it is | Usual causes | How it grows |
|---|---|---|---|
| Collaring | The hole starts in the wrong place | Unmarked or lost collar marks, rubble on the bench, a bit that skates on a sloping surface, worn pins and bushes in the boom | A fixed offset, the same at every depth |
| Alignment | The hole starts at the wrong angle or bearing | Mast set by eye, rig not level, no angle instrument or one out of calibration, a feed that shifts during collaring | In proportion to depth: about 17 mm a metre for each degree |
| In-hole deflection | The hole bends as it is drilled | A long, slender rod string, too much feed force, a worn or unsuitable bit, bedding, joints and hard and soft bands met at an angle | Faster than depth; worst in long, small-diameter holes |
| Depth | The hole stops short or runs deep | One depth used for every hole on an uneven bench, miscounted rods, cuttings falling back, a hole at the wrong angle drilled to the right length | Hole by hole; it shows as an uneven floor |
Depth is often a consequence of the other three as much as an error of its own.
The first two errors happen before the bit is a metre into the rock, and both belong entirely to the driller. One widely cited study attributes more than half of all hole path inaccuracy to the operator. Geology gets the blame more often than it earns it.
The third error is the one that separates drilling methods. A top hammer string is a long, thin column struck at the top and pushed from the top, and it bows. Sandvik treats deviation above 8 per cent of hole length as a problem on top hammer benches in poor ground, and reports 3 to 5 per cent with a guide adapter in the string. A down-the-hole hammer nearly fills the hole and is struck at the bottom, which is why it is the usual answer on deep benches. The comparison is laid out in blast hole drilling methods.
Offset at the toe = hole length × tan(error angle)
1° on a 10 m hole = 10 m × 0.0175 = 0.17 m
2° on a 10 m hole = 10 m × 0.0349 = 0.35 m
2° on a 15 m hole = 15 m × 0.0349 = 0.52 m
3° on a 15 m hole = 15 m × 0.0524 = 0.79 m
On a 3.0 m burden, 0.52 m is 17 per cent of the design before the hole has bent at all
What a crooked hole does to the shot
- Thin burden, and flyrock. A front-row hole that drifts towards the face, or a face that is undercut where nobody looked, leaves less rock in front of the charge than the design assumed. The gas breaks out early at that point and throws stone. It is among the commonest causes of flyrock from a bench, and air blast comes with it.
- Thick burden: toe, oversize and vibration. A hole that drifts back into the solid has too much rock to move. The toe stays as a hump on the floor, the rock in front comes out blocky, and energy that could not break rock goes into the ground. Where a monitor stands at a house, the over-confined hole is the one that pushes the reading towards the vibration limit.
- Holes that converge or spread. Two holes meeting at depth put a double charge in one place and none in another. Close enough together, one charge can set off or deaden its neighbour out of sequence, and the timing the blast was designed around is lost.
- Back-break. Back-row holes leaning into the new face crack it. The next shot inherits a loose crest and an irregular front-row burden, so one bad pattern makes the next harder to drill well.
- Wrong depth. Short holes leave high floor that has to be broken out afterwards. Deep ones shatter the bench below, where the next collars must be drilled.
All of it reaches the plant. Boulders mean a breaker working at the hopper and a jaw waiting on it, which is the argument of fragmentation and crusher output.
How deviation is measured
Depth is the easy part: a weighted tape down every hole before charging, written against the hole number. Collar position is checked against the marked pattern by tape or by survey. Inclination at the collar is read with a clinometer on the mast or the rod, or from the rig's own angle instrument.
The path of the hole below the collar needs a probe. A borehole survey tool is lowered or pushed down the hole, on a cable or on rods, and records inclination and bearing at intervals, from which the hole is plotted in three dimensions. The face is surveyed separately with a laser profiler or by drone photogrammetry. Put the two together and the burden in front of each front-row hole is known at every depth, which is the number the shotfirer needs.
Not every bench justifies a probe down every hole. The front row beside anything that can be hit, deep holes, and any pattern where the last shot threw rock or left toe are the ones to survey first. A hole found to have wandered is not scrapped. It is charged for the burden it really has: a lighter column or a deck of stemming where the burden is thin, a re-drill beside it where the burden is too heavy.
How hole deviation is controlled
- Bench preparation. A level, cleaned bench. Collaring through a metre of broken rock left by the last shot's sub-drill is where many holes go wrong, and the cure is a dozer or an excavator, not a drill.
- A surveyed pattern. Every collar marked and numbered from a base line, with the front row set out from the face as it actually stands and not from where the last drawing left it.
- Set-up. Rig levelled, feed set with an angle instrument in both planes and held firmly against the rock before collaring. Collar slowly, at reduced percussion and feed, until the bit is seated in solid ground.
- A stiff string. On top hammer rigs, a guide tube or guide rod behind the bit and the largest rod the hole allows. The closer the string is to the size of the hole, the less it can bow.
- DTH for deep holes. Past about 15 m, or in strongly bedded or jointed rock, a down-the-hole hammer holds a line that a rod string will not.
- Feed force. Enough to keep the bit against the rock and no more. Excess feed bends the string and steers the bit along the first joint it meets.
- Bit condition. Worn gauge buttons and uneven wear pull a hole off line. Regrinding on time is a straightness measure as well as a cost one, and is covered in compressor air and bit selection.
- A record. Depth, voids, seams and anything odd logged by hole number and handed to the shotfirer, who loads from the log and not from the drawing.
Who checks the hole
Where drilling and blasting are let to different firms, deviation falls into the gap between them. The driller is paid by the metre and the blaster inherits the result. Our drilling page says deviation is measured, not assumed, because a hole nobody checked is a hole that will surprise the shotfirer. We drill collar position, depth, diameter and inclination to the blast design. If you are comparing drilling quotes, ask each bidder what is measured before charging and who signs for it. The service is described under drilling services, and where the same crew fires the shot, under controlled blasting.