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New wave drilling

While geotechnical construction work has benefitted from modern techniques, ground investigation work has tended to rely on conventional methods. But rising demand for sonic techniques suggests a step change is in progress.

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For years site investigation work has relied on percussive shell and auger rigs or rotary techniques to construct boreholes with few site investigations bearing little difference to those undertaken 50 years ago.

The main reason for the lack of change may be due to a combination of low margins restricting investment in new techniques or clients driving down the money set aside for the work. Nonetheless, one UK drilling contractor is seeing a change in demand and the recent buyout of the company by a private investor suggests that there is confidence that this is the start of a new trend.

Boart Longyear’s UK drilling services business was acquired by the Mulraney Group in January and rebranded as Geosonic Drilling. General manager David Dennis says demand is such that he hasn’t got a rig to spare until late April.

“We focus solely on sonic drilling work, which I believe offers a much higher quality result in terms of both the environment and geotechnical information,” says Dennis.

Sonic drilling works by using high-frequency, resonant energy to advance a core barrel or casing into subsurface formations. During drilling, the resonant energy is transferred down the drill string to the bit face at various sonic frequencies. Simultaneously rotating the drill string evenly distributes the energy and impact at the bit face.

When the resonant sonic energy coincides with the natural frequency of the drill string, resonance occurs which results in the maximum amount of energy being delivered to the face. At the same time, friction of the soil immediately adjacent to the entire drill string is substantially minimised, resulting in very fast penetration.

“Sonic drilling is good for rapid profiling of a site,” adds Dennis. “It is cleaner and more controlled, and it can cope with obstructions so the boreholes can be constructed exactly where they have been planned. The technique can cope with challenging geology and underground obstructions on urban redevelopment sites - we have never had a refusal on any borehole.

“The ability to use minimal water or dry drilling means that it is ideal for sensitive sites. We can also telescope the casing to create a seal when drilling through an aquifer and capture the waste too.”

Dennis says that many reject sonic drilling on the belief that sampling is compromised by the technique and blames misrepresentation in Eurocode 7 for some of this misconception.

“A good sample class is achievable,” he says. “It may not always be a class 1 sample but many techniques struggle to produce that anyway. We can carry out any conventional insitu testing and take U100 samples using normal boring methods and then switch back to sonic drilling.”

As an example of the benefits of sonic drilling, Dennis points to work that is currently being undertaken on a rail project in Scotland. “On this project a number of boreholes needed to be undertaken in roads and the time for closing them was limited so our client wanted a reliable technique that could get the boreholes constructed to the right depth first time, without any extra delays,” he says. “We are installing boreholes to 30 to 40m through glacial tills with frequent boulders. The sonic drilling can core through these and recover the core so that the engineers will have an accurate record of the stratigraphy to build the ground model from, resulting in less risk for the designer.”

Sonic drilling was first introduced to the UK in 2006 and take up of the method was slow but, given Geosonic Drilling’s current workload, Dennis believes that more consultants and clients are seeing the benefits of it. “We are seeing the need for sonic drilling starting to be included in tender documents more and more frequently,” he says. “There is greater focus being placed on safety and quality, which conventional techniques cannot always deliver.

“In the past the focus has always been on the cost when it comes to site investigation but there needs to be a shift towards the money that is invested delivering quality in terms of the work on site and the data it produces. Some clients are beginning to understand this but government agencies need to takea stand to drive standards forward.”

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