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Sonic sampling

M&J Drilling recently completed a site investigation project for the Environment Agency, through its consultant SKM Enviros.

The contract included ground investigation and the installation of monitoring wells on a former landfill site on the banks of the River Mersey, near Warrington, UK.

The project required 30 boreholes to be drilled to an average depth of 17m in alluvium and glacial till, with deeper holes into the bedrock at around 30m depth.

In total, some 500m of drilling was carried out at a cost of £65,000. Work commenced in early February and was completed in just over two weeks.

Steve Rule, director of M&J Drilling, tells GDI: “Sonic drilling was chosen for the project due to its ability to progress holes rapidly without the use of air or water flushing, while obtaining continuous samples of material, even through the former landfill. Boreholes were sequentially cased as necessary to maintain their integrity, isolate the various horizons and preclude cross-contamination.”

M&J used a SDC-550-18 Sonic rig, which runs on a Caterpillar 312B base unit, with a Sonicor 50K drilling head.

The machine was the first of its kind, on tracks, to be operated in the UK when it was imported from Canada in 2006. UK distribution agent Sonic Drilling Supplies has found that, since M&J Drilling took delivery of this model, the rig has been increasingly requested by clients for its ‘dry’ casing and sampling capabilities.

Choosing the rig

M&J Drilling, founded in 1982, operates a fleet of more than 20 rigs throughout the UK, most of which are Klemm Bohrtechnik 802-2 models. The company is based in the West Midlands, and its drilling services range from rotary investigation to slope stabilisation by soil nailing, rock anchoring, and the stabilisation of abandoned mine shafts and mine workings.

M&J Drilling originally purchased its sonic rig with a view to focusing a significant amount of time on drilling holes for GSHP installations. While the rig is still used for this occasionally, it is more often employed to investigate potentially contaminated sites.

Mr Rule explains: “From the outset, when rigs of this type were first viewed operating in Canada, we at M&J were impressed with the production rates achievable on large-diameter, cased boreholes.

The ability of the sonic drilling method to take continuous core samples of superficial material allows engineers to identify both the geology and any discrete zones of contamination that may be present. This allows bespoke remediation schemes to be developed, while passing on potential cost savings to clients.”

Conventional drilling

Many of the contamination surveys that M&J has undertaken have been carried out in conjunction with the installation of steam/vacuum extraction wells. SKM Enviros has worked with M&J on a number of landfill sites recently where the Sonic rig has been utilised.

The benefits of speed and continuity of sampling, while casing a bore-hole ready for subsequent monitoring well installation, have proven extremely useful and have enabled the work to be completed very quickly when compared with other more conventional methods.

For geological investigations, M&J has used sonic drilling to complement other drilling methods, such as wireline coring, and enable rapid initial evaluation with targeted follow-on works. A recent example of this was along the proposed route of the Thames Tideway Tunnel, where M&J’s sonic rig was used to identify lenses of sand and gravel within the Thanet Group and London Clay, and facilitate the positioning of subsequent wireline boreholes.

Although more expensive than cable-percussive drilling, sonic offers significantly faster results. M&J has found that some clients, especially remediation contractors, place greater emphasis on achieving time reductions, which are offset against increased rate costs to reduce the overall project time and cost.

The time saved by the use of sonic drilling may give engineers and contractors greater scope for more investigation works, and/or more time spent on the interpretation of results, with resultant cost savings in the longer term.

The high penetration rate associated with sonic drilling gives contractors the ability to quickly resolve issues that can delay construction; for example, compensation claims for pipeline routes through potentially exploitable resources, such as coal tips where the coal may be economically recoverable.

M&J has also found that sonic drilling gives a cleaner, better quality result than either auger or cable percussive drilling.

The samples obtained are generally less disturbed and result in fewer anomalies. The cores are bagged as they leave the core barrel, resulting in less exposure of the core and minimal chance of contamination.

The firm has investigated and implemented the use of an alternative type of short sampling barrel, similar to a piston sampler, incorporating a semi-rigid liner, which encases the core sample within the barrel.

This can be used in conjunction with the standard, sonic core barrel where a less disturbed, higher-quality sample is required from discrete zones.

“We believe that sonic drilling in the UK has yet to be used to its full potential, but we are confident that, with increased awareness of the benefits now being experienced by consultants, the demand is set to increase dramatically in the future,” adds Mr Rule

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