Blast Hole Drilling: DTH vs Top Hammer vs Rotary, Hole Size and Depth

What are the three ways to drill a blast hole?

A blast hole is drilled in one of three ways: top hammer, DTH or rotary. The first two break rock by hammering it. Rotary crushes and cuts it with a turning bit pressed down hard.

In top-hammer drilling the hammer, called a rock drill or drifter, rides on the rig’s mast (the tall frame that guides the drill) above the hole. It strikes the top of a string of drill rods and the blow travels down to the bit, while the rig turns the rods so the bit hits fresh rock each time. Older manuals call the small crawler version an air track drill.

In DTH (down-the-hole) drilling the hammer goes into the hole, fixed right behind the bit. The rig only turns the drill pipes and pushes them down. Compressed air runs down the pipes, drives the hammer’s piston and blows the rock chips out.

Rotary drilling has no hammer. A rotary head turns the pipe while the rig pushes down hard on it (pulldown), and a tricone bit, three toothed cones that roll as it turns, crushes and scrapes the rock. Air, or sometimes drilling mud, lifts the cuttings.

Table 1: Top-hammer, DTH and rotary blast hole drilling compared

Source: the US Federal Highway Administration’s Rock Blasting and Overbreak Control manual (FHWA-HI-92-001), section 9.2 and Tables 9.1 and 9.2. Hole sizes are the ranges that manual gives.

What is the difference between DTH and top-hammer drilling?

In DTH drilling the hammer works at the bottom of the hole, right behind the bit; in top-hammer drilling it works at the top, and every blow travels down the rods first.

Each rod joint soaks up a little of the blow. On a short hole that hardly matters and top hammer drills fast. On a long hole the bit gets less with every rod added, and the thin rods bend and wander in seamy rock. A DTH hammer gives the same blow at 3 m as at 20 m, because the energy is always delivered at the bit. Its pipes are only a little narrower than the hole, which keeps the bit centred and the hole straight.

Figure 1: Where the breaking force is made in each drilling method

The orange box marks where the rock-breaking force is made.

Top hammer still wins for angled holes and quick work on low benches. A DTH hammer runs on high-pressure air, so the rig needs a compressor matched to it, and it tilts less. Wikipedia’s history of the method sums up why quarries took to DTH: a hole could at last be placed where it was required, straight, clean and easy to charge.

How is the blast hole diameter chosen?

The blast hole diameter is chosen to suit the bench height, the size of the machines that will dig and crush the broken rock, and the rigs the site already owns. For benching, a Penn State mining course gives hole diameters from about 76 mm (3 inches) at the low end to 381 mm (15 inches) at the high end, with surface mines in the middle to upper part of that range.

A taller bench takes a bigger hole. The powder factor guide sets out the Dyno Nobel rule that links the two: bench height in metres should be at least the hole diameter in mm divided by 15. A 115 mm hole wants a bench of about 7.7 m or more. How high benches are allowed to go in India is covered in the guide to bench height in opencast mining.

A bigger hole is spaced wider, so each hole breaks a bigger block and the patterns tend to leave bigger boulders. If the primary crusher has a small mouth, or the excavator a small bucket, smaller holes on a tighter pattern usually give smaller rock in the muckpile (the heap of broken rock).

The explosive sets the smallest hole. IS 4081:2013, India’s safety code for blasting and related drilling, says the hole must be wider than the outside of the explosive cartridge (clause 8.8). The Coal Mines Regulations, 2017 (regulation 192(1)) and the Metalliferous Mines Regulations, 1961 (regulation 162(1)) both go further: the drill must leave at least 0.3 cm (3 mm) of clearance over the cartridge diameter.

How deep is a blast hole drilled?

A blast hole is drilled to the full height of the bench plus a little more, called subdrill, which goes below floor level so the bench breaks cleanly at the bottom instead of leaving a hard hump. The powder factor guide gives the rule of thumb, 3 to 15 times the hole diameter.

The Coal Mines Regulations, 2017 define deep-hole drilling and blasting as holes more than 3 m deep used for blasting in an opencast mine (regulation 2(1)(o)), and regulation 194 leaves the conditions for such blasting to be set by the Chief Inspector of Mines.

Depth also decides the rig. The FHWA manual says a top-hammer rig may do for holes up to about 12 m (40 feet), but at 30 m (100 feet) a DTH or rotary rig is better for both straightness and cost. Top-hammer holes can start to lose accuracy past about 6 m (20 feet), while large DTH hammers hold accuracy down to about 18 to 21 m (60 to 70 feet).

Before charging, IS 4081 clause 8.7 asks for every hole to be checked for length, water and dust with a wooden tamping pole or measuring tape, and cleared of debris.

Why do blast holes go crooked, and what does it do to the blast?

Blast holes go crooked for five reasons the FHWA manual lists: the operator, the hole diameter, the hole depth, the rig’s alignment devices, and the local rock. A crooked hole puts its charge in the wrong place, and a small angle grows into a large miss by the bottom of the hole.

Table 2: How far the bottom of a hole drifts for a given angle error

Our own arithmetic: drift = hole depth x tan(angle error). It assumes a straight hole set at the wrong angle; a hole that bends as it goes can drift further.

Put that next to the pattern. For a 115 mm hole the burden (the distance from a row of holes to the open face) is usually about 2.9 to 4.6 m, which is 25 to 40 times the hole diameter, the rule given in the powder factor guide. A 10 m hole that is 5 degrees off has moved nearly 0.9 m, a fifth to a third of that burden. If it leans towards the face, the rock in front of the charge is thinner than planned, which is one way fly rock happens. If it leans away, the toe is left unbroken and the excavator has to fight it.

The FHWA manual names the operator as the biggest single influence: holes lined up by eye, alignment gauges left broken, and holes not restarted (recollared) after the bit crosses a seam. Its practical answer is to let each operator drill only as deep as their holes stay straight. Small rigs wander more than large DTH and rotary rigs, and joints, cavities, mud seams and changes in hardness push any bit sideways. The traces of the holes left on the new face show how straight they really were.

Too many boulders after every blast? Check the drilling first, then compare rock breakers for excavators for the oversize you cannot avoid.

Should blast holes be drilled vertical or at an angle?

Most production blast holes are drilled vertical because that is easier and more accurate, but angled holes break the toe better and leave less damage behind. The FHWA manual lists the trade-off.

Table 3: Angled blast holes, for and against

Angled holes are common for presplitting, a line of closely spaced holes fired first to leave a clean final wall, which suits a top-hammer rig.

Which drill bits are used for blast holes?

Top-hammer and DTH rigs use button bits, which are steel bits studded with rounded tungsten carbide buttons that chip the rock with each blow. Rotary rigs use tricone roller bits, or drag bits in soft ground. The FHWA manual shows rotary bit designs from soft to extremely hard formations.

Wikipedia’s account of DTH drilling credits the carbide button bit, in place of the first all-steel bits, and air pressures of 25 bar and more for making the method competitive. A blunt bit drills slowly, and the Coal Mines Regulations, 2017 make bit care a dust rule too: regulation 143(13)(b) says dust must be reduced by using bits that are sharp and of proper shape, keeping suitable pressure on the bits and keeping the holes clear of cuttings.

How is a blast hole drilled on site, step by step?

On site the face is checked, the holes are marked, drilled, measured and logged, and only then handed over for charging. The steps follow IS 4081:2013 and the mine regulations.

1. Check the face for unfired explosive; no drilling is done where it is suspected (IS 4081 clause 8.5). The Metalliferous Mines Regulations, 1961 (regulation 168(1)) ask for loose rock to be removed and the ground within 2 m of each proposed hole to be cleaned or washed down and examined for misfires and sockets (the leftover bottoms of old holes).
2. Flush the earlier holes in the blasted area with air and water before drilling starts (clause 8.2).
3. Mark every hole position in white paint (clause 8.6), using the burden and spacing from the blast design.
4. Level the rig, set the mast to the planned angle and start (collar) the hole at the painted mark. The collar is the mouth of the hole.
5. Drill wet or catch the dust. With pneumatic drilling, water goes on before the compressed air (Coal Mines Regulations 2017, regulation 143(13)(b)), and IS 4081 clause 8.25 asks for approved respirators in silica dust. The guide to dust suppression in mines covers wet drilling and dust collectors.
6. Measure each hole (clause 8.7) and log its number, row, depth and any seams, mud or soft layers hit, so the blaster knows where to change the charge.
7. Never drill and load at the same time in the same area (clause 8.10).

Two rules are absolute. Nobody may deepen a hole that has held explosive, or put a drill into the butt of an old hole, even if nothing is found in it (clause 8.23). And after a blast, drilling only resumes once the area has been checked for unexploded charges in old butts that the drill could strike (clause 8.24).

What do Indian mine rules say about blast hole drill machines?

Indian mine rules say a blast hole must be drilled with a blast hole drill made for mining, not a borewell rig. Regulation 216(3) of the Coal Mines Regulations, 2017 bars truck-mounted drill machines designed for tube well drilling and allows only proper blast hole drill machines especially designed for mining.

The rules also cover drilling after a misfire (a charged hole that did not go off). Under regulation 204(6) of the Coal Mines Regulations, 2017 and regulation 167(5) of the Metalliferous Mines Regulations, 1961, a relieving hole must be drilled so that at no point is it nearer than 30 cm to the misfired hole, in the presence of the shot-firer or blaster. IS 4081 clause 12.5(c) gives the method for other blasting work: a new hole 600 mm away, parallel to the old one and about 300 mm shallower, charged and fired.

These regulations were made under the Mines Act, 1952. Mine safety law in India is moving to the Occupational Safety, Health and Working Conditions Code, 2020, so check the current DGMS rules and your mine’s own standing orders before relying on any clause quoted here. Who may charge and fire the holes is explained in the powder factor guide.

Which machines depend on how well the holes are drilled?

Every machine after the drill works on the rock the blast leaves. Straight holes on a true pattern give an even muckpile; crooked or short holes leave boulders and a hard toe.

Figure 2: A crawler excavator loading broken rock into a tipper at a quarry face

The loading excavator feels it first. A mining excavator such as the Tata Hitachi EX 1200V, SANY SY580C-10HD or Komatsu PC2000-8 loads a well-broken heap in steady cycles, and slows down and wears faster digging into an unbroken toe. Compare the full range of mining excavators to match the bucket to your bench.

Oversize boulders go to an excavator-mounted hydraulic rock breaker, and the crusher is last in line; the jaw and cone crusher guide explains how feed size limits a plant. Drilling rigs, crushers and compressors are not listed on DesiMachines, so no rig makers or models are named here.

The quarry sequence from drill to stockpile is in the guide to quarry equipment, the wider fleet in mining equipment in India, and clearing the cover above the rock in overburden removal. Explosives makers are listed in the post on explosives companies in India, and more guides are on the mining guide hub.

Setting up a quarry or mine loading fleet? Browse mining excavators and connect with a dealer near your site.

Where the figures in this guide come from. Drilling methods, hole sizes, depth limits, drilling error, angled holes and bits are from the US Federal Highway Administration’s Rock Blasting and Overbreak Control manual (FHWA-HI-92-001, 1991), section 9.2. The 76 to 381 mm benching range is from Penn State’s mining engineering course notes on blast design, and the DTH history from Wikipedia’s article on the down-the-hole drill. Site steps are from IS 4081:2013, Blasting and Related Drilling Operations, Code of Safety, clauses 8.2 to 8.25 and 12.5. Indian mine rules are from the Coal Mines Regulations, 2017, regulations 2, 143, 192, 194, 204 and 216, and the Metalliferous Mines Regulations, 1961, regulations 162, 167 and 168, both read in full from the published text. Table 2 is our own arithmetic.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.