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If a pile has to go into hard granite or basalt, a DTH hammer will usually get through it faster than a rotary piling rig; if the bore stays in clay, sand or silt, a rotary rig with a kelly bar and casing or bentonite is the right tool. Most Indian sites sit between the two, with a few metres of soil over weathered rock, and that mixed ground is where the choice is really made.
DTH piling means drilling the pile hole with a down-the-hole (DTH) hammer: an air-driven hammer that sits at the bottom of the drill string, right behind the bit, and chips the rock with fast blows while the same air blows the chips out. Rotary piling means turning a cutting tool (an auger, a drilling bucket or a core barrel) on a long telescopic bar called a kelly, and lifting the soil out in loads.
This guide is for a contractor, a site engineer or an owner who has a soil report in hand and two piling offers on the table. It explains how each method works, which ground suits which rig, how deep a pile has to go into rock, and what else each method needs on site. The rig families themselves are described in the guide to types of drilling rigs. No prices are quoted here.
What is DTH piling?
DTH piling is a way of making pile holes in rock with a hammer that works inside the hole. A compressor on the surface sends high-pressure air down hollow drill pipes. The air drives a piston in the hammer, the piston strikes the bit, and the carbide buttons on the bit chip the rock. The spent air then rushes back up the outside of the pipes and carries the chips out with it.
Because the hammer follows the bit down, it does not lose force as the hole gets deeper. Wikipedia’s article on the down-the-hole drill calls it one of the fastest ways to drill hard rock, and notes that it does not need heavy downward push or high turning speed. A DTH rig can therefore be lighter than a rotary rig that has to crush the same rock by weight and torque alone.
On Indian sites a DTH machine is used for pile holes and rock sockets in hard ground, and in rocky country for the short foundation holes of solar plant mounting structures and similar light frames. It is the same hammer used for blast holes in quarries, covered in the guide to blast hole drilling, only aimed at a hole that will be filled with concrete instead of explosive.
DTH has a weak side too. In loose sand or soft clay there is nothing for the hammer to chip, the air can wash out the walls, and the hole may collapse. That ground needs a different tool.
What is rotary piling?
Rotary piling bores the hole by turning a tool into the ground and lifting the cuttings out, load after load. The rig’s rotary head turns the kelly bar, and the kelly carries the tool: an auger (a screw that holds soil on its flights), a drilling bucket for wet or loose soil, or a core barrel (a toothed steel tube) that cuts a ring into rock.
IS 2911 (Part 1/Sec 2):2010, the Indian code for bored cast in-situ concrete piles, defines this kind of pile as one formed by boring a hole “by percussive or rotary method”, with a temporary or permanent casing or drilling mud holding the hole open, and then filling it with reinforced concrete. The code asks for at least two metres of temporary casing at the top of every bored pile, and more where the strata or ground water call for it.
Drilling mud is a bentonite clay slurry. It fills the hole and presses back on the walls so that sand and silt below the water table do not run in. Rotary rigs are at their best in exactly that kind of ground, and in stiff clay that stands up on its own. The rig’s alignment and position tolerances under IS 2911 are covered in the guide to the drilling rig machine; this page is about which method to pick.
The older third method is still common on smaller Indian jobs: bailer and chisel. A heavy chisel is lifted by a winch and dropped to break the ground, and a bailer (a tube with a flap valve at the bottom) brings the broken material up. IS 2911 warns that in soft layers and loose sand the bailer and chisel must be used with caution, because pulling them out can suck the ground in behind them.
DTH vs rotary piling: what is the difference?
The difference is how the ground is broken. DTH breaks rock by hammering it; rotary cuts soil by turning a tool and lifts it out. Everything else follows from that, as the table shows.
| Point | DTH piling | Rotary piling (kelly) | Bailer and chisel |
|---|---|---|---|
| How the ground is broken | Air hammer at the bit, fast blows | Turning auger, bucket or core barrel | Heavy chisel dropped by a winch |
| Ground it suits | Hard and medium rock, boulders | Clay, silt, sand, gravel, soft and weathered rock | Mixed ground and boulders, on small jobs |
| Weak in | Loose sand, soft clay, water-bearing soil | Hard, unweathered rock (slow wear on teeth) | Loose sand (suction), deep hard rock (slow) |
| How the hole stays open | Rock stands on its own; casing through any soil on top | Casing or bentonite slurry | Casing or bentonite slurry |
| How cuttings come out | Blown up by the hammer’s exhaust air | Lifted out in the tool, load by load | Lifted out with the bailer |
| Main power need | A large high-pressure air compressor | High torque at the rotary head and pulldown force | A winch |
| Rig weight for the job | Lighter (no heavy push needed) | Heavy, so the mast stays steady under torque | Light tripod or crane-mounted winch |
The table is a general guide drawn from IS 2911, IS 14593 and the way each tool works. Your soil report and the rig maker’s figures decide the real case.
Which piling method is used for which ground?
Read the soil investigation report first, then match each layer to a tool. The report is a borehole log: it lists every layer from the surface down, its thickness, the water level and the standard penetration test (SPT) N value, which counts the hammer blows needed to drive a sampler 300 mm (12 inches). A higher N means firmer ground.
Figure 1: Which tool suits which layer of ground
- Site A, soft soil above the water table (blue dashed line): rotary auger, with casing at the top.
- Site A, loose sand or silt below the water table: rotary drilling bucket with bentonite slurry or full-length casing. Not DTH.
- Site B, soil overburden: rotary auger inside temporary casing.
- Site B, weathered rock: rotary rock auger or core barrel; chiselling where it refuses.
- Site B, hard rock socket: core barrel on a heavy rotary rig, or a DTH hammer where the rock is hard and dry enough.
- Site C, a thin skin of soil on hard rock: DTH from just below the surface, with a short casing through the soil.
IS 2911 gives two lines that help sort the layers. In its design annex, ground with an N value above 60 is to be treated as weathered rock rather than soil. And rock whose crushing strength is higher than that of the pile concrete is to be deemed hard rock. Hard rock is where a DTH hammer earns its place; weathered rock is usually still rotary ground.
How deep should a pile go into rock?
For preliminary design, between one and four pile diameters, depending on the rock. That length is the rock socket: the part of the pile shaft that goes into rock below the soil. IS 14593:1998 (reaffirmed 2003), the Indian guideline for bored cast in-situ piles founded on rock, gives these starting figures for piles up to 1,500 mm across.
| Rock type | Minimum socket length | For a 600 mm (about 24 inch) pile |
|---|---|---|
| Sound, fairly uniform rock such as granite or gneiss | 1 to 2 times the pile diameter | 0.6 to 1.2 m (2 to 4 feet) |
| Moderately weathered, closely jointed rock such as schist or slate | 2 to 3 times the pile diameter | 1.2 to 1.8 m (4 to 6 feet) |
| Soft and sedimentary rock: hard shale, sandstone, siltstone, mudstone | 3 to 4 times the pile diameter | 1.8 to 2.4 m (6 to 8 feet) |
The 600 mm column is our own arithmetic from the standard’s ratios. The guideline is clear that these are only a starting point: the socket length should be worked out from the design methods in the standard and preferably checked by a load test on a trial pile. For a pile that has to resist pull-out (uplift), it sets the socket at no less than twice its diameter in every case.
Three other lines in IS 14593 matter to the person choosing the rig. The cutting tool should not be smaller than the pile diameter by more than 75 mm, so a 600 mm pile needs a tool of at least 525 mm. In rock, chiselling may be used to reach the required depth. And the bottom of the bore must be cleaned of rock debris before concreting, by a rope-operated grab, a kelly-mounted grab or an air lift.
A harder rock needs a shorter socket, but each metre of it is slower to drill. That trade-off is why the method matters most in the socket, not in the soil above it.
What happens when there is soil over rock?
This is the common case on Indian bridge, metro and plant sites, and it usually means using more than one tool on the same pile. The soil part is bored first, with casing pushed down to stop it caving in. When the tool reaches weathered rock it is changed, and the socket is cut with a rock auger, a core barrel or a chisel.
Where the rock is too hard for those tools to make steady progress, a DTH hammer sized to the socket can cut it, as long as the rig and the compressor can run it. For a wide pile, a heavy rotary rig with a core barrel is the more common answer.
Watch for boulders in the soil. A boulder can stop an auger dead and send a bore off line. IS 14593 warns that over-cutting at a change of strata should be done with caution, and IS 2911 asks that the depth bored in soft and hard rock and the time spent chiselling be recorded for every pile. Those records are what the bill is built on, as the post on piling rate per meter explains.
What machines does each piling method need on site?
Neither rig works alone. A DTH piling crew needs a compressor big enough for its hammer, and air demand rises fast with hammer size and pressure; the figures are worked through in the guide to air compressor CFM calculation. Which compressor classes suit site work is in air compressors for construction. Expect dust at the collar of a dry hole, and plan for it.
A rotary piling crew needs a bentonite mixing and storage set-up if the bore is wet, a tremie pipe for concreting under slurry, and a crane to lower the steel cage and pull the casing. The ACE FP 210 Piling Crane is one machine built for that side of the work: a pick-and-carry crane listed with a 20-tonne maximum lifting capacity, a 101 HP (74 kW) Tata 497 engine and four-wheel drive. How it lines up beside the ACE F170 is shown in the ACE F170 vs ACE FP 210 comparison.
Both crews need a level, firm working platform and somewhere to put the spoil, which is usually an excavator’s job. Browse the range of excavators for that part of the site. Where a boulder or rock lip has to be broken at the surface before the rig can start, a hydraulic rock breaker on an excavator does it faster than a chisel.
How do you choose between DTH and rotary piling?
Let the ground and the pile size decide, in that order. Ask these questions before you accept a method:
- How deep is rock, what kind of rock is it, and how strong is it? The soil report should give depth to rock, rock type and core recovery.
- What is the pile diameter, and does the hammer or tool on offer match it within 75 mm?
- How long is the rock socket, and is that figure from IS 14593’s starting ratios or from a design calculation?
- Which tool goes in each layer, and how will the hole be kept open through the soil and below the water table?
- How will the socket base be cleaned before concreting?
- How will chiselling or hammering hours be logged, and who signs the log each day?
A contractor who names a different tool for each layer has read the soil report. One who quotes a single method for the whole pile, through sand and granite alike, has not. The companies that build rotary and hydraulic piling rigs for India are listed in piling rig manufacturers in India, and the machines a bridge job uses around the rig are on the bridges and tunnels page.
Need a crane beside the rig to lower cages and pull casing? Compare crane models and talk to a verified dealer. More on rigs, from rock drills to piling rigs, sits in the drill rig guides.
Sources: the DTH method, its speed in hard rock and its low thrust need are from Wikipedia’s article on the down-the-hole drill. Bored pile definitions, the two-metre casing rule, bailer-and-chisel cautions, the N above 60 and hard-rock lines and the site records are from IS 2911 (Part 1/Sec 2):2010, clauses 3.4, 5.3, 8.1.3 and 8.6, and annex B-7 and B-8. Socket lengths, the 1,500 mm scope, the 75 mm tool rule, chiselling, cleaning and the uplift rule are from IS 14593:1998 (reaffirmed 2003), clauses 1, 5.1, 5.2, 5.3, 5.5 and 6.8.2.2 and Table 1. All were read on 6 November 2026. The ACE FP 210 figures are from the machine’s listed specifications. The 600 mm socket lengths and the 525 mm tool size are our own arithmetic from the standard’s ratios.
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.