Down-the-hole drill

A down-the-hole drill, usually called DTH by most professionals, is basically a mini jackhammer screwed on the bottom of a drill string. The fast hammer action breaks hard rock into small flakes and dust and is blown clear by the air exhaust from the DTH hammer. The DTH hammer is one of the fastest ways to drill hard rock. Now smaller portable drillcat drilling rigs with DTH hammers can drill as fast as much larger truck rigs with this newer technology. The system is thought to have been invented independently by Stenuick Frères in Belgium and Ingersoll Rand in the USA in the mid-1950s.

History

A pneumatic tool is first thought to have been used for rock drilling in 1844. Many quarries used hand held tools that required the driller to suspend himself from a rope over the quarry face in order to place the drill hole in the required position. This system used small diameter holes and was not only terribly inefficient, but very dangerous due to flying rock as a result of the inaccuracy of the drilled borehole.

Some quarries used primitive top hammer machines that carried the jack hammer on a mast - the slenderness of the drill rods working with a relatively large diameter drill bit caused bore holes to deviate which sometimes meant that a bore hole might finish dangerously close to its neighbour or indeed be closer to the face of the quarry than had been intended. In any event boreholes that are not aligned correctly which are then loaded with high explosive can be extremely dangerous, resulting in rock being projected beyond the intended site.

Larger quarries used big rotary machines that required huge amounts of down thrust and high rotation speeds to drive the tri-cone bit hard enough to crush the rock. This system could not be successfully used for holes below 6 inches (150mm) and the machines were very expensive to buy and to run. Another system in use was the very primitive cable tool machine (or bash and splash as it was known by the drillers) which caused a heavy bar and chisel to be lifted and dropped on the rock to crush it whilst water was introduced to create a slurry, which in the process, enabled the hole to be drilled. This system could not guarantee a finished hole size and only pure vertical holes could be drilled as the system basically relied on gravity. Debris from the hole was baled out using a baling tube with a clack valve, which was periodically dropped on a winch to capture the slurry, which was then brought to the top of the hole to be discharged.

It was only when the DTH system came along that many of the problems associated with the other systems were overcome - with the DTH system the energy source is constantly behind the drill bit, the drill tubes (or drill string) are rigid being only slightly less in diameter than the drill bit, copious amounts of air can be passed through the drill string to operate the DTH Hammer which is then used to efficiently flush the bore hole clean. DTH did not require heavy down thrusts or high rotational speeds and as such a light, cheap machine could be employed to carry out the drilling process - the machine could also be worked by one man, whereas some other systems required two operatives. The benefits that DTH brought to the industry were enormous - for the first time a drill hole could be placed where it was required because DTH gave a truly aligned, straight, accurately placed, clean bore hole that could be easily charged with explosive to provide good control over the blasting process that was safer and which provided good fragmentation of the rock. Holes could be drilled to increasing depths without the loss of performance since the energy source was always directly behind the drill bit. The system was able to drill in almost all rock conditions that other systems were unable to do. Quarry faces became safer, well profiled and quarry floors were level and easier for loading equipment to operate and move across.

The DTH system completely revolutionised the blast hole industry with many quarries embracing it with open arms. Eventually the larger DTH systems then found their way into other applications, such as water well drilling and construction work.

It still offers the same benefits to the operator that it initially brought to the quarry industry but it is now being used in many different applications such as gold exploration, ground consolidation, geo-thermal drilling, shallow oil and gas well, directional and piling. The advent of tungsten carbide for the drill bits (the first bits were all-steel) and the development of the button drill bit coupled with the introduction of high air pressures (25 bar plus) has meant that the DTH system can compete easily and efficiently with other drilling systems.

DTH tools were used to locate the trapped miners in Chile and enabled food, water, and medicine to be passed to them and communication systems to be set up that eventually led to their safe rescue.

Origin of the name

DTH is short for “down-the-hole”. Since the DTH method was originally developed to drill large-diameter holes downwards in surface-drilling applications, its name originated from the fact that the percussion mechanism followed the bit down into the hole. Applications were later found for the DTH method underground, where the direction of drilling is generally upwards instead of downwards.

Technical details

In DTH drilling, the percussion mechanism – commonly called the hammer – is located directly behind the drill bit. The drill pipes transmit the necessary feed force and rotation to hammer and bit plus compressed air or fluids for the hammer and flushing of cuttings. The drill pipes are added to the drill string successively behind the hammer as the hole gets deeper. The piston strikes the impact surface of the bit directly, while the hammer casing gives straight and stable guidance of the drill bit. This means that the impact energy does not have to pass through any joints at all. The impact energy therefore is not lost in joints allowing for much deeper percussion drilling. This is a great breakthrough for smaller portable water well drilling rigs, that before were limited. The DTH on smaller rigs now can get same results as large heavy truck rigs.

With recent advances in technology DTH hammers and bits can now be operated to run at up to 500Psi,[1] increasing the penetration speed.

Uses

DTH drilling is used in the construction industry to produce piles into rock, also water wells, and drilling bores for geothermal ground source heat pumps.

DTH products can be used in the following applications:

Mining- Drill & Blast holes in Open Pit mining, Where the drill operator will drill several holes, then fill with explosives and detonate to lift rock allowing access to ore body

RC- Exploration & Pit grade control

GW- Geothermal Bore Holes & Waterwells

Oil & Gas- Deepwell Bore Holes : Air hammers can be used as long as cutting uplift and borehole stability are ensured. For deeper wells, new DTH technologies including water hammer and mudhammer can be used to improve drilling rates in hard rocks.

Construction- Piling, Footings

References

  1. ↑ Bruce, Donald. The Evolution of Small Hole Drilling Methods for Geotechnical Construction Techniques (PDF) (Technical report). Geosystems. p. 4. Retrieved 31 May 2017.
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