Down The Hole (DTH) Rock Drilling – An Introduction

Commonly known as DTH, the Down-the-Hole Hammer drill is a percussive device in cylindrical form with integral drill bit that is fitted directly onto the bottom of a drill string. DTH is one of the most efficient ways of drilling rock, using a rapid hammer action to break the rock into sizeable chips which are then evacuated from the bore hole by the air exhausted from the DTH Hammer. The technique provides fast drilling of accurately aligned, straight and clean holes in most ground conditions.

It is important to remember in rock extraction that blasting is the first phase of crushing. It is therefore vital that time and effort is spent on ensuring that the hole size, overall blast design and explosive loading is appropriate for the conditions, in order to achieve good fragmentation, optimum productivity, safe stable faces and level floors at minimal cost.

DTH is a relatively recent technique, introduced in the 1950s when the choice of drilling system for quarrying was limited, that then included the hand-held jack hammer, cable tool, and rotary drill and the air-driven top hammer machines.

Jack hammers were used for springing, stoping and popping. Although they were lightweight, portable, easily maintained and inexpensive, they were essentially labour intensive, low volume producers, resulting in small holes and limited control, as well as presenting significant health and safety issues.

The cable tool was primitive and used a heavy bar with a chisel, suspended on a wire rope that was then mechanically dropped on the rock to crush it within the hole, using water to bale clear the resultant cuttings and dust. Again easy to use, simple to maintain with low cost operation, minimal dust and effective in bad ground, but the cable tool was slow and limited to larger holes, with no alignment control. It required a regular water supply, two man operation and was really only of use for soft to medium rock formations.

Rotary drilling required large, heavy rigs to create weight behind the drill bit and provide fast rotational speeds with high torque capabilities to penetrate the rock. Although they were relatively fast, economical to run and maintain and good in poor ground, they required a high capital outlay, were prone to hole deviation and gave less impressive performance in hard rock.

The Top Hammer drill or drifter was a mechanical version of the hand-held jack hammer with the air driven percussive unit mast mounted on a simple three wheeled carriage. It was fast, portable, easy to maintain and good in hard rock. However, it was subject to hole deviation, reduced drilling speed at depth and produced high noise levels and vibration. Hole sizes were restricted to 70 to 100mm diameters.

Back in the 1950s, rock loading methods were sometimes primitive, using rope excavators or hand loading into railway tubs. In addition, high vertical faces coupled with poor control over blasting increased the risk of rock falls resulting in injury and death.

Against this background, the revolutionary DTH drilling technique arrived on the scene in the early part of that decade, developed by Belgian marble quarry owner Andre Stenuick. Its many advantages were instantly evident:-

  • Suitable for all types of rock (except clay)
  • Greater versatility in terms of hole sizes
  • Low capital outlay
  • Simple and easy to operate

For blast hole drilling DTH meant straighter, cleaner, better aligned holes for easier charging, improved blasting control, improved faces, better floors, improved fragmentation and throughput as well as lower operating costs. Perhaps most significantly, DTH meant greater overall safety.

By the 1970s, DTH Hammers were commonly working at 10 bar. With the system well established in blast hole operations, potential also extended within the water well drilling sector. Advantages included lower capital outlay, all rock capability with a wider range of hole sizes possible using the same hammer, deep hole capability, easier insertion of linings, screens and pumps and in particular clean holes for cleaner water were easily identifiable.

So in 2012, where are the other systems? Today’s Top Hammer machines are hydraulically operated with cabs and full automation, offering high energy output and productivity. They are ideal for short, small diameter holes in hard consolidated formations, but less effective in softer, broken ground. With deviation still a risk on deep holes, they can be also extremely noisy unless soundproofed. Requiring a high capital outlay and sometimes with high cost consumables but low fuel usage, they are versatile and maneuverable and can be highly productive on short hole drilling applications but because of the high penetration and restricted air flow, the risk of rifling (spiralling in the hole) and dust left in the bottom of holes can be a problem.

Rotary drills are now commonly used in big, open pit mines, producing larger holes from 165-300mm diameter, usually for short hole, multi row blasting in medium/soft rock. Steel tooth tricone bits are used in softer rock, with tungsten carbide types for harder rock. Internal bearings in tri-cone bits restrict the minimum hole size to 152mm, so rotary drag bits are normally used for drilling holes below 152mm but mainly in soft rock. The rotary drilling principle is still widely used in deep-hole water well drilling and for oil and gas wells.

Cable Tool machines are still used in drilling for water in under-developed countries, whilst the original hand-held jack hammer principle has thankfully been consigned to the past.

In contrast, DTH goes from strength to strength. More effective button bits were introduced in the late sixties, providing better cutting action, faster and smoother drilling, greater accuracy, longer life with less regrinding (if any), therefore increasing productivity and reducing drilling costs. A variety of head design and tungsten inserts became available to suit different formations and drilling conditions.

Today’s valveless DTH hammers are simpler, faster and more effective, especially at the higher air pressures of 25 to 40 bar. When drilling with higher air pressure, penetration rates increase pro-rata to increases in pressure, giving greater productivity at less cost per metre drilled.

Now fully automated, today’s DTH blast-hole machines are designed for safe, all weather operation, having air conditioned, soundproofed cabs with multi angle drilling capabilities, on-board compressors, dust collectors, angle indicators, computer logging facilities and track mounted for engaging all ground conditions. In short, they still provide the industry with straight, clean, accurately aligned holes for easy charging, greater blast control, good fragmentation, safer faces and better floors, combined with excellent levels of productivity and attractive overall cost advantages.

Having started out as a blast-hole system, applications now using DTH include water well drilling, geo-thermal drilling piling, ground consolidation, mineral exploration, shallow oil and gas, post hole insertion, directional drilling and even mine rescue – it was through the use of the DTH system that the trapped Chilean miners were successfully located and provided with vital water, food, medications and communications until a full rescue was made.

DTH Hammers have precision parts manufactured to fine tolerances, requiring some basic care to maintain the best results. Users should adhere to manufacturers’ recommendations, preventing dirt from entering the hammer and using the correct rock oil in the right amounts to suit ambient and ground conditions. Regular checks should be made of the outer case of the hammer for any sign of damage; it pays to periodically strip and inspect the internal parts of the hammer for signs of wear or damage and replace any such items. In abrasive conditions the outer case of the hammer may wear more quickly than the inner components and if properly cared for, these internal items can be used to re-build the hammer using an outer case re-build kit to prolong its life. This can often be carried out two or three times without any detrimental effect on the performance of the hammer in service. When not in use, the hammers should be oiled up and stored horizontally in a dry place and before returning to service, a quantity of oil should be poured into the hammer to ensure that internal parts are well lubricated.

Reputable makes of drill bit should always be selected that are truly compatible with the hammer; these should be regularly checked in service to make sure the body is in good condition and not worn – the splines of the bit (and of the chuck) should be lubricated by the oil from the hammer to ensure that wear is kept to a minimum. Plastic foot-valves should be checked to make sure that they are not loose or in any way worn or damaged and should be replaced if any such damage is observed. Drill bits should be protected during storage and handling to prevent damage to the tungsten carbide inserts and the plastic foot vales.

The DTH Hammer method provides one of the most efficient and versatile ways of drilling in a wide variety of rock conditions for many different applications. It provides fast penetration even in the hardest rock with constant speed throughout the length of the bore hole with the added benefits of accuracy and alignment to give good clean usable holes. DTH does not rely on heavy machines with fast rotational speeds or heavy down-thrusts; it only requires that the hammer be subjected to sufficient thrust to initiate the percussive operation and that there is the appropriate amount of rotational speed, usually around 28 to 40rpm, to relocate the drill bit on the rock face to produce the optimum size of cutting and minimal dust production. Performance, reliability, minimal spares usage, long service life and overall cost effectiveness are the hallmarks of a well-designed and manufactured ‘World Class’ DTH hammer that brings benefits to both the driller and the operating company – it is important to go for quality and not price when making the choice to purchase your DTH hammer.

The first DTH tests in the UK were carried out at Dove Holes Quarry in Buxton and at Buckton Vale Quarry in Stalybridge. The early DTH Hammers used a valved cycle system with a relatively small piston and liner which operated at low air pressures (around 7 bar). Steel cruciform drill bits were used initially that later evolved into tungsten carbide tipped bits, providing better performance with longer life.