Down the hole hammers: a guide for drillers and engineers

As urban redevelopment projects continue to rise, this poses an extra challenge for geotechnical engineers &#; how can these works be done without damaging surrounding buildings? To meet increasingly specific requirements for foundation works and drilling operations, specialized solutions have become ever more important. One of the most commonly used tools in the drilling industry to meet such challenging scenarios is the Down the Hole Hammer (DTH hammer).

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The Impact of DTH Hammers

DTH hammers are most used in hard rock formations and are designed to tackle complex tasks like drilling in populated areas without putting existing structures at risk. In these areas, they are ideal for drilling large diameter boreholes in hard rock formations, typically ranging from 3 inches (85mm) to 48 inches (mm) or more.

Another advantage is that DTH hammers can achieve greater depths drilling in hard rock than other methods. Depending on the size of the hammer and the rock formation being drilled, DTH hammers can achieve depths of up to 1,000 feet (305 meters) or more.

DTH hammers therefore offer drilling efficiencies with a high rate of penetration (ROP). This is down to the fact that the air-powered hammer consistently minimizes energy loss to the drill bit, allowing it to break down the rock efficiently. Additionally, the compressed air flushes out the cuttings, reducing the likelihood of clogging or binding the drill bit.

High pressure air is often necessary for efficient flushing of the bit &#; but caution is required when working with overburden ground where unconsolidation can compromise existing foundation stability.

Drilling in sensitive areas

Urban redevelopment projects present unique challenges and drilling in these sensitive areas can be particularly difficult. One of the key challenges faced by drillers is the need to minimize disturbance to the surrounding environment while drilling. This is where low pressure Down the Hole (DTH) hammers come in.

In urban development projects, it is essential to minimize noise and vibration levels to avoid disturbing nearby residents, buildings, and infrastructure. These hammers use a low air pressure range of around 12 to 18 bar, which is significantly lower than the pressure used by conventional high-pressure DTH hammers. This means that they produce less noise and vibration and cause less ground disturbance during drilling.

Using low pressure DTH hammers allows drillers to work in areas where conventional drilling techniques may not be suitable. This makes low pressure DTH hammers an excellent option for drilling in densely populated areas or areas where there is sensitive infrastructure, such as hospitals, schools, or historic buildings.

Another benefit of low pressure DTH hammers is their ability to drill through a wide range of soil types. Whether it is hard rock or soft soil, low pressure DTH hammers can provide efficient and effective drilling without causing significant ground disturbance. This enables drillers to use a single drilling method for all types of soil conditions, reducing the need to switch between drilling methods, and minimizing the time required for drilling.

Low-pressure DTH hammers have gained in popularity in recent years because they are &#;gentler&#; on the ring bit sets compared to high-pressure hammers. This is because low-pressure hammers operate at a lower frequency and produce less impact energy, resulting in reduced wear and tear on the ring bit sets. Being more efficient and requiring less air consumption, makes them cost-effective and environmentally friendly. Engineers should consider using low-pressure DTH hammers in their drilling operations to improve drilling performance and reduce maintenance costs.

Furthermore, low pressure DTH hammers can be used to drill deeper and wider holes, which can be advantageous for foundation and pile construction in urban developments. This is because they provide better penetration rates, ensuring that drilling is completed faster and more efficiently. This can be especially important in urban areas where construction schedules are often tight, and delays can be costly.

Terranox

TerraRoc provides a wide range of highly effective and efficient drilling equipment, including the most productive DTH hammers in the world.

Its Terranox revolutionary line of DTH hammers is engineered specifically for drilling in built-up urban settings. A range of hammers that deliver cost efficient, reliable geological drilling with outstanding performance in low pressure operations.

The Terranox hammer is based on reliable, trusted technology spanning three decades in the industry. They are renowned as rugged and can be easily replaced or rebuilt &#; perfect for urban areas with restricted drilling hours. Combined with TerraRoc&#;s global service network to ensure parts and support can be accessed quickly, this cost-effective solution will get the job done efficiently.

Terranox is part of a comprehensive, in-house turnkey package for geotechnical drilling in urban redevelopment projects. The DTH hammers are designed to work seamlessly with TerraRoc&#;s casing advancement systems Symmetrix, Elemex and Odex to ensure safe and reliable results.

With five sizes available, finding the right hammer is defined by hole size and the ground drilling conditions. Just match the hammer as closely as possible to the required hole size, leaving sufficient space to allow the cuttings to leave the hole.

Terranox hammers are designed to perform at peak efficiency in overburden drilling applications. To ensure that the drill runs optimally, select an air compressor with a capacity capable of meeting the maximum hammer consumption rate &#; ranging from 12-14 bars for 25 bar rated pressure operations.

Choosing the Right DTH Hammer

Choosing the right Terranox DTH hammer for a drilling project requires careful consideration of several factors. The following are some of the factors that should be considered when selecting a hammer:

Borehole Size: The size of the borehole required for the project will determine the size of the DTH hammer needed. As discussed earlier, DTH hammers are available in many sizes, ranging from 3 1/8 inches (79mm) to 7 1/8 inches (181mm) outside diameter or more. Selecting the appropriate hammer size ensures efficient drilling and reduces the likelihood of equipment failure or downtime.

Rock Formation: The type of rock formation being drilled is also an important consideration. Different rock formations have different densities and hardness, which can impact the drilling efficiency and the wear and tear on the hammer and drill bit. For instance, harder rock formations, such as granite or basalt, require larger and more powerful hammers to achieve the desired drilling depth and speed.

Depth of the Borehole: The depth of the borehole required for the project is also important in selecting the appropriate DTH hammer. Deeper boreholes require larger and more powerful hammers that can withstand the high pressure and temperature of deeper drilling.

Availability of Compressed Air: DTH hammers are powered by compressed air, and the availability of compressed air on the job site may impact the choice of hammer. Larger hammers require higher volumes of compressed air, which may not be available on some job sites. In such cases, selecting a smaller hammer may be more appropriate.

Type of Drill Rig: The type of drill rig being used for the project may also impact the choice of DTH hammer. Different drill rigs have different capabilities and power outputs, and selecting a hammer that is compatible with the drill rig ensures optimal drilling efficiency and safety.

Cost: Finally, the cost of the DTH hammer is also an important consideration. DTH hammers are available in different price ranges, and selecting a hammer that is cost-effective and within the project budget is crucial.

Conclusion

DTH hammers are an essential tool for drilling large diameter boreholes in hard rock formations. When choosing a DTH hammer for challenging urban areas where there are so many restrictions to contend with, remember TerraRoc offers a wide range of high-performance drilling equipment designed for percussive drilling. Its range of Terranox DTH hammers are designed for low pressure operations that engineers and drillers can count on to ensure that their drilling operations are efficient, safe, and cost-effective.

Jackhammer-like tip for drilling deep into rock

A down-the-hole drill, usually called DTH by most professionals, is basically a jackhammer screwed on the bottom of a drill string. The fast hammer action breaks hard rock into small cuttings and dust that are evacuated by a fluid (air, water or drilling mud). The DTH hammer is one of the fastest ways to drill hard rock. The system is thought to have been invented independently by Stenuick Frères in Belgium and Ingersoll Rand in the USA in the mid-s.

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 above the drill bit. The drill pipes transmit the necessary feed force and rotation to the hammer and the bit, along with the fluid (air, water or drilling mud) used to actuate the hammer and flush the cuttings. The drill pipes are added to the drill string successively behind the hammer as the hole gets deeper.

The hammer is fully fluid actuated. It is composed of two mobile parts: a valve, controlling the flow and a piston that strikes on an impact surface directly linked to the bit. The hammer body gives straight and stable guidance of the drill bit.

There are three types of hammers, depending on the actuation fluid:

• Air hammers have first been developed to drill hard rock but are limited to shallow holes (< 200m);
• Water hammers have been developed by a swedish company called Wassara.

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  A lost water flow (between 55 and 744 L/min, depending on the hammer size) is necessary to actuate the hammer. It allows better performance (borehole control) and can drill deeper than air hammers;
• Hammers operated with drilling mud are currently developed by a French company called Drillstar Industries.

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  They present the advantage of being able to drill on every existing rig using classical mud composition. Their ability to drill deep makes it a good technology to drill hard rock encountered in deep geothermal projects. An undergoing research project called ORCHYD,

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  funded by the European Union (EU) under the Horizon programme will develop a new drilling technique, fully fluid driven, that combines the DTH mud hammer with a high pressure water jet system

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  that cuts grooves in the rock along with the percussive action. That will help cut the rocks at greater depths in an efficient and safe manner, reducing the operational time for geothermal exploration and thus the total cost to utilize the geothermal energy.

History

A pneumatic tool is first thought to have been used for rock drilling in . 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 jackhammer 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. Much higher penetration rates could be achieved by using DTH hammers which decreased the drilling Cost per Meter in smaller hole diameters.

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.[5]

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.

Uses

DTH products can be used in the following applications:

• Mining- drill and 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- mineral exploration and pit grade control (needs specialized reversed circulation hammers)
• RAB - rotary air blast - mineral exploration using a conventional DTH hammer

• GW - geothermal bore holes and waterwells

• Oil and 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 industry - Piling, footings, soil nailing

See also

• Deep hole drilling in manufacturing

References