Down the Hole (DTH) Drilling: An Introduction

Commonly known as a DTH, the down-the-hole hammer drill is a cylindrical-shaped percussion device with an integral bit that is placed directly into the bottom of a drill string. DTH is one of the most efficient ways of drilling rock, using rapid hammer action to break rock into sizable chips which are then blown out of the hole by air blown out of the DTH hammer. The technique provides fast drilling of precisely 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. Therefore, it is essential to spend time and effort to ensure that the hole size, overall blast design and payload are appropriate for the conditions, in order to achieve good fragmentation, optimum productivity, safe and 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, later including the manual jackhammer, wireline tool, rotary drill and hammer machines in air powered head

Jackhammers were used for jumping, stopping, and popping. Although they were lightweight, portable, easy to maintain, and inexpensive, they were essentially low-volume, labor-intensive producers, resulting in small holes and limited control, as well as presenting significant health and safety concerns.

The cable tool was primitive, using a heavy bar with a chisel, suspended from a metal cable which was then mechanically dropped onto the rock to crush it into the hole, using water to wash away the resulting 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 check. It required a regular water supply, two man operation and was really only useful for soft to medium rock formations.

Rotary drilling required large, heavy equipment to create weight behind the bit and provide fast rotational speeds with high-torque capabilities to penetrate rock. Although they were relatively fast, inexpensive to operate and maintain, and good in poor soils, they required a high capital outlay, were prone to hole deviation, and offered less impressive performance in hard rock.

The Top Hammer drill or perforator was a mechanical version of the manual jackhammer with the air-powered percussion 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 and vibration levels. Hole sizes were restricted to diameters of 70 to 100 mm.

In the 1950s, rock loading methods were sometimes primitive, using rope excavators or manually loading into rail tubs. Additionally, high vertical faces coupled with poor blast control increased the risk of rock falls resulting in injuries and fatalities.

In this context, the revolutionary DTH drilling technique appeared on the scene at the beginning of that decade, developed by the owner of a Belgian marble quarry, Andre Stenuick. Its many advantages were instantly obvious:-

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

For hole drilling, DTH meant straighter, cleaner and better aligned holes for easier charging, better blast control, better face, better floors, better fragmentation and yield, as well as lower operating costs. Perhaps most significantly, DTH meant greater overall security.

In the 1970s, DTH hammers used to work at 10 bar. With the system well established in blasting operations, the potential has also extended within the water well drilling sector. Advantages included lower capital outlay, all rock capacity with a wider range of hole sizes possible using the same hammer, deep hole capacity, easier insertion of casings, screens and pumps and, in particular, wells clean 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 are less effective in softer fractured ground. Since drift is still a risk in deep holes, they can also be extremely noisy unless soundproofed. Requiring high capital outlay and sometimes high cost consumables but low fuel usage, they are versatile and maneuverable and can be highly productive in short hole drilling applications, but due to high penetration and airflow restricted, the risk of gouging (spiraling in the hole) and dust left at the bottom of the holes can be a problem.

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

Cable Tool machines are still used for water drilling in underdeveloped countries, while the original principle of the manual jackhammer is thankfully relegated to the past.

In contrast, DTH is going from strength to strength. More effective button bits were introduced in the late 1960s, providing better cutting action, faster and smoother drilling, higher accuracy, longer life with less (if any) resharpening, thereby increasing productivity and reducing drilling costs. A variety of tungsten head and insert designs were made available to suit different formations and drilling conditions.

Today’s valveless DTH hammers are simpler, faster and more effective, especially at higher air pressures of 25 to 40 bar. When drilling with higher air pressure, penetration rates increase proportionally with increases in pressure, providing higher productivity at a lower cost per meter drilled.

Now fully automated, today’s DTH drilling rigs are designed for safe, all-weather operation, have air-conditioned soundproof cabs with multi-angle drilling capabilities, on-board compressors, dust collectors, angle indicators, logging facilities computer and rail mounted to accommodate all ground conditions. In short, they still provide the industry with straight, clean, precisely aligned holes for ease of loading, increased blast control, good fragmentation, safer faces and better floors, combined with excellent productivity levels and attractive overall cost advantages.

Having started as a blast hole system, applications now using DTH include water well drilling, geothermal drilling piles, ground consolidation, shallow mineral, oil and gas exploration, post hole insertion, directional drilling and even mine rescue. Using the DTH system the trapped Chilean miners were successfully located and provided with water, food, medicine and vital communications until a full rescue was made.

DTH hammers have precision parts manufactured to fine tolerances, which require 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 proper amounts to suit the environmental and ground conditions. Regular checks of the outer casing of the hammer should be carried out for any signs of damage; it is worth periodically disassembling and inspecting the internals of the hammer for signs of wear or damage and replacing such items. Under abrasive conditions, the outer casing of the hammer can wear more quickly than the internal components and, if cared for properly, these internal elements can be used to rebuild the hammer using an outer casing rebuild kit to extend its life. Often this can be done two or three times without any detrimental effect on the performance of the hammer in service. When not in use hammers should be oiled and stored flat in a dry place and before being put back into service a quantity of oil should be poured into the hammer to ensure internal parts are well lubricated.

Reputable brands of drill bits that are truly compatible with the hammer should always be selected; these should be checked regularly in service to ensure that the body is in good condition and not worn out; the bit (and chuck) flutes should be lubricated with hammer oil to ensure wear is kept to a minimum. Plastic foot valves should be checked to ensure they are not loose or worn or damaged and should be replaced if any damage is noted. Drill bits must be protected during storage and handling to prevent damage to the tungsten carbide inserts and plastic foot blades.

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 into even the hardest rock with a consistent velocity throughout the hole with the added benefits of accuracy and alignment to deliver clean, usable holes. DTH does not rely on heavy machines with fast rotational speeds or strong downthrusts; it only requires that the hammer be subject to sufficient thrust to initiate the percussion operation and that there is the appropriate amount of rotational speed, typically around 28 to 40 rpm, to relocate the bit to the rock face to produce the size optimal cut and minimum dust production. Performance, reliability, minimal use of spare parts, long life and overall cost effectiveness are the hallmarks of a well designed and manufactured “world class” DTH hammer that provides benefits to both the driller and the operating company: it is It is important to bet on quality. and not the price when making the decision to buy your DTH hammer.

The first DTH trials in the UK were carried out at the Dove Holes Quarry in Buxton and the Buckton Vale Quarry in Stalybridge. Early DTH hammers used a valved cycle system with a relatively small piston and liner operating at low air pressures (around 7 bar). Initially steel cross bits were used which were later converted to tungsten carbide tipped bits, providing better performance with longer life.