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Torque to the animals

TRENCHLESS TECHNOLOGY

Directional drilling is being used to protect wildlife habitats during construction of a new sewer pipeline near Southampton. Max Soudain reports.

Trenchless techniques are being increasingly accepted in the UK as a technically viable and cost-effective alternative to conventional construction methods. Using directional drilling to install new services or replace existing ones can save time, money and reduce disruption to the public and businesses.

But the methods also have environmental benefits, as shown by a recent contract for Southern Water on the south coast of England.

At Downes Park in Totton, west of Southampton, directional drilling contractor Allen Watson recently finished installing a large diameter sewer line through a site of special scientific interest (SSSI) where important habitats of birds and fish had to be protected.

The 710mm diameter sewer replaces the line from a pumping station to a treatment works.

Running southward at up to 14m below ground level, the 450m long new pipe has to pass beneath gardens, a small stream called Jacob's Gutter, a wooded marshy area and a river called Bartley Water before rising back to the surface.

Costain Geotechnical Services carried out site investigations.

Seven cable tool boreholes were put down along the route to identify ground conditions and help consultant WS Atkins design the pipeline. Main contractor was Clancy Docwra.

The site was underlain by thick deposits of dense silty and clayey estuarine sand, with a number of gravel layers between 1m and 3m depth.

Allen Watson used its 50t drilling rig from German manufacturer Prime Drilling to form the bore for the pipeline. The variable ground did not present any problems during drilling, according to Allen Watson business development manager Vukan Andjelkovic.

'This is a midi-rig, ' he explains. 'Although it is has a 50t rating, it is actually capable of 75t of pull back and had enough thrust and power to cut through all the material.'

All drilling was carried out from the northern end of the site.

The 110mm diameter pilot hole was formed using a drill bit with bentonite cutting jets, followed by a series of back reamers to enlarge the bore to allow it to accept the large diameter pipe.

Bentonite slurry was used not only to cut the hole on the pilot bore but also as a lubricant to reduce drag on the tools so that much less torque was needed, Andjelkovic explains.

'The viscosity and composition of the drilling fluid depends on the ground conditions.

Because here we were in sands and silts, we used a bentonite slurry with a viscosity of between 60s and 70s, ' he says.

No additives were needed at Downes Park, with the slurry just a simple mix of bentonite and water, he adds.

The bore was filled with bentonite at hydrostatic pressure throughout the work, to keep the hole open and to transport excavated material out of either end of the borehole, depending on the position of the drill head or reamer. A bentonite cake also formed on the borehole walls, further helping stability.

The bentonite slurry and cut material was collected in two pits, excavated at either end of the line, before high pressure pumps conveyed it to two large bentonite recycling plants mounted on lorry trailers. These cleaned the slurry, allowing it to be reused in drilling.

'Both plants did the same job, ' Andjelkovic explains. 'The slurry was passed over two shakers to remove the sand and the silt sized particles. 'The remaining mixture then went into a hydrocyclone where it was rotated at high speed and centrifugal force used to separate the finer material.' Finally the slurry passed into a settling tank to remove any remaining soil particles and the clean bentonite slurry pumped off.

'The recycled slurry has less than 0.3% natural material in it, ' Andjelkovic says. The soil was then disposed of at a licensed landfill.

Often only one recycling plant is used, but at Downes Park one was needed at the exit end to make it easier to pump back to the drill rig, he explains. 'It is a considerable distance.' In plan, the sewer is a straight line but it is on a 400m vertical radius, with the pilot bore entering the ground at 16° before flattening out at 14m below the entrance point and then rising to the surface, emerging exactly as planned: 'The stake marking the exit was actually knocked out of the ground by the cutting head, ' says Andjelkovic.

Back-reaming was then carried out to enlarge the bore. 'The reaming process is decided by the results from the pilot bore, ' he says.

A continual drill string was used, with the 5m drill rods added at the entry point during drilling and at the exit point during reaming. This meant that rods were in the ground throughout construction, ensuring that the line of the bore was not lost at any time, he explains.

Rods were transported to and from the entry and exit points on a tractor-pulled trailer, which had to travel around the site as no construction traffic was allowed on the SSSI area.

A series of ever-larger reaming tools, starting at 475mm diameter, were used to increase the diameter of the bore. These comprised a fly cutter - a hollow ring with teeth - followed by a barrel.

'The cutters are very good for clay on smaller jobs, ' explains Andjelkovic, 'and are also good in sand on larger ones.' These tools were therefore ideal for the mixed estuarine material at Downes Park.

The barrel behind the fly cutter opened up the hole and forced bentonite into the surrounding ground to improve stability.

'The final barrel had a 30% overcut [ie its diameter was 30% larger than the pipe diameter] to allow the pipe to be installed, ' says Andjelkovic.

The entire 450m long pipe was pulled through in one continuous process so pipe sections had to be welded on site and curled around the exit area. Rollers were placed under it at regular intervals to allow it to move easily as it was pulled into the bore behind the last barrel.

Settlement was kept to between 3mm and 5mm, which Andjelkovic says was acceptable as work was carried out at a sufficient distance from any buildings so as not to cause any damage.

Allen Watson began drilling in November and finished installation of the pipeline at the beginning of December 2001.

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