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Norwegian compass guides Swedish drill

NORWEGIAN PIPELINE Drilling has just completed two hard rock microtunnels on a sewer renewal project in southern Stockholm for main contractor Skanska.

The specialist firm used directional rock drilling methods developed in the oil industry, combined with back reaming to form tunnels in Scandinvai's hard rock. Conventional microtunnelling is unworkable.

The two tunnels demanded high accuracy, most of all on the 400m long Skarpnõck-Bagarmossen tunnel. The 660mm diameter bore had to follow a gentle curve to avoid private property, to fall at 4.7% and to intercept the roof of an existing 2m by 2m drainage tunnel, 30m below ground. Vertical tolerance was 300mm.

A second tunnel was almost twice the diameter at 1,060mm though shorter at just 185m.

Vertical alignment was less demanding but it still had to hit a 'target', the gap between two old oak trees with a large fine payable if the trees were damaged. Both were done successfully.

NPD worked with Sandvik Rock Tools engineers on the project and used a Sandvik rotary drill head on an 0.8inch drill string to create a 300m diameter pilot hole. The bit rotates independently driven by a turbine mounted directly behind it, using high-pressure flushing water for a power source. Spent water removes cuttings.

An electronic sonde emitter and a magnetic gyropscopic compass system developed for the oil industry were mounted on the drill string for guidance. NPD has refined these and the control software for greater short distance accuracy. The compass is mounted 13m behind the drill head to avoid interference effects and NPD has to use accumulated engineering experience to overcome the lag effect this produces on direction measurement.

An eccentric mounting for the drill head and motor, combined with a loading on the drill string allowed 360infinity direction changes.

To achieve straight lines the drill string was rotated.

Once an accurate pilot bore was in place, backreaming followed with special heads developed by Sandvik for horizontal and near horizontal work. These have scrapers to throw back the cuttings, allowing water flushing back to the receiving pit.

Production rates through Stockhom granite were about 2.5m/h for pilot holes and 0.9m/h-1m/h reaming. NPD contracts manager Roland Kõllvik was impressed with cutter life.

'Even the head used for the 400m Skarpnõck-Bagarmossen tunnel still had 200m to 300m of service life remaining.'

Client Stockholm Vatten is now considering extending use of the system to more environmentally sensitive areas.

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