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The difference down the hole

Environmental Drilling  Construction Drilling  Ground Source and Waterwell Drilling 
Site Investigation  Exploration and Production Drilling  

For ‘conventional’ air-powered hammers and bits, very sophisticated computer programs and simulations are now being used to optimise performance, reliability, and durability, and to develop systems with particular characteristics, such as short hammers for restricted conditions.

The typical 1970s air pressures of 160-250psi have been replaced by pressures of up to 500psi; and penetration speeds have increased accordingly. Productivity has also increased dramatically through better understanding of the metallurgy of hammers and bits, and the use of sophisticated concepts such as polycrystalline diamond coatings on the carbide bit inserts. Reverse-circulation hammers are being developed with dual-wall rods. These use air and, if required, can accommodate water misting.

Dual-fluid system (DFS) hammers feature conventional air activation but incorporate a central tube that permits water to be used for hole flushing. This has not had widespread application so far in the US.

Water power
Water-powered hammers, used in underground mining in Scandinavia since 1986, have become accepted in the last 10 years as the DTH method of choice for grout-hole drilling in rock (where air flush or ‘misted’ flush is not permitted). They tend to satisfy the smaller end of the DTH range with typical hammer diameters of 31/8in (79mm), 4in (102mm), 4¾in (121mm) and 6in (152mm). However, they can also be used in percussive duplex mode to install casings through overburden of 4½-85/8in (114-219mm) diameter.

Water-powered DTH hammers – the most popular being built by Wassara – use filtered water at up to 2,500psi (17.2MPa) and flow rates of 50-75gpm (190-285L/min) to activate the hammers and flush the hole. They are reportedly more energy-efficient than air-powered hammers. Because the uphole velocity of the water flush is significantly lower than that of air, more stabilisers can be placed on and around the drill string, promoting a straighter hole.

Sonic drilling
A final development for rock drilling is the rotary vibratory (or sonic) method, although it has so far seen most use in overburden drilling. This technique was researched separately in the US and the USSR in the 1940s, and developed commercially in the 1960s by the oil-well drilling industry. One of its developers, Ray Roussy, considered it “to be the only true innovation to come to the drilling industry since the Chinese invented cable-tool drilling 3,000 years ago”.

Sonic drilling uses minimal water flush and so is especially valuable in environmentally sensitive areas (where flush may be contaminated) and when drilling in structures such as dams (to avoid hydrofracture). The dual-cased system uses high-frequency mechanical vibration to provide continuous (but disturbed) core samples or to advance a temporary casing. Slow rotation is used in rock formations to even wear tendencies. Sonic drilling is typically commercially viable only in certain circumstances. However, it may indeed be ‘the wave of the future’ in rock and overburden drilling for many applications and especially those of a remedial nature in difficult site and ground conditions.

This is an extract from ‘The Evolution of Small Hole Drilling Methods for Geotechnical Construction Techniques’ by Dr Donald A Bruce, president of Geosystems,
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