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Under the river

Powermole International recently used its new dry-rock technology to install a section of a new high pressure (7 bar) gas main to supply northern areas of Scotland.

Working for Underground Moling Services, which in turn was subcontractor to Turriff Contractors (BGTransco), Powermole's project involved directionally drilling under the River Deveron, a prized and protected salmon river.

Various ground strata were encountered during the drilling including sandy silty loam and medium-dense sandy coarse to fine gravel with cobbles, but it was the weathered and fresh Dalradian quartz schist that presented the main challenge. Until recently no attempts would have been made to directionally drill at this small scale through such hard rock (measured at UCS 133MPa) - directional drilling in rock has previously been carried out at larger diameters using mud motors.

But Powermole's system employs a patented pneumatic/ percussive head unit which is automatically activated once it comes into contact with hard ground conditions. Having simultaneous drilling and percussion ensures effective and consistent penetration even in the conditions found here.

Powermole's system uses compressed air with a small quantity of biodegradable additive as lubricant, which also produces a filter cake around the bore, increasing its stability. As a dry system it meansthat no mud motors, mixing or recycling plant are necessary, significantly reducing the cost of the operation, and in this case providing a solution that was acceptable to the Scottish Environmental Protection Agency, which had barred the use of bentonite.

Powermole completed the 154m pilot bore in only seven hours. After 14 hours pre-reaming and only six hours of pullback the 180mm HDPE gas pipe was installed - to the surprise of the other parties in the project, including the client.

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