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Drill and casing system eases Dorset anchors Highly faulted rock in the Swanage cliffs made the building of a massive retaining wall a fraught affair until a new drilling system was brought in.

Swanage in Dorset is renowned as one of the loveliest stretches of coastal landscape in England. But, as rock anchoring contractor Ritchies discovered, stabilising its areas of highly faulted rock was fraught with problems until a new drilling system from Sandvik came to the rescue.

The resort's population of about 11,000 nearly doubles in the holiday season. This puts considerable pressure on the town sewers which have to cope with rainwater as well as domestic sewage.

Wessex Water is investing more than £28M in a new sewerage system and treatment works. The pumping station and treatment work will be on the foreshore, on the site of a now demolished hotel between the cliffs and the entrance to the town's historic pier. Wessex Water has designed the scheme in-house with Dr Brian Hawkins acting as geotechnical consultant.

Because the treatment works will back directly on to the cliffs, a massive retaining wall is under construction to allow excavation. The cliffs form part of the Purbeck beds and consist of clays and intermittent mudstone and limestone well folded and faulted. The site slopes west to east about 12 and the water in the mudstone means there is danger of slides if the ground is not restrained. Individual rocks lying around the area of excavation in front of the retaining wall look like baulks of rotten timber and are completely friable, presenting problems for Ritchies.

The company constructed a section of the 260m long retaining wall and is installing rock anchors through the massive waling beams both as a main contractor and under a separate subcontract to contractor TJ Brent.

The 16m deep retaining wall is stepped, the lower section set some 3m forward of the upper section. Construction comprises 280mm diameter piles installed at 0.5m centres. Four massive waling beams have been cast along the upper section of its 260m length and are anchored back into the cliff. The lower section of wall is 9m deep and has two walers and two rows of anchors now being installed.

The two step wall was necessary because early in the contract Wessex Water opted for secondary treatment, so it became necessary to increase the site area and construct a return to the retaining wall.

The treatment works will employ membrane technology and will be the largest using this form of treatment anywhere in the world by a factor of five. Once completed, people using the park area overlooking the site, the pier and the bay beyond will see very little apart from the buildings housing the plant as much of it will be housed underground.

Faulted rock made installing the anchors on Ritchies' first contract a 'real headache' according to the company's agent John Littler. The anchors are fairly standard multi-bond four strand type installed at a 14 inclination and are unusual only for their length, which averaged 29.5m. Once installed, they are stressed to 55t.

On the initial contract, Ritchies tried using eccentric systems but there were problem with lost bits, which then had to be retrieved, and drill rods tightening up.

Here, however, the scenario is very different thanks to Ritchies' investment in Sandvik's Centrex drilling system which Littler is using on the 10% or so of really problematic installations.

The Centrex system allows simultaneous drilling and casing through bad ground. Once good rock is reached, the driller simply gives the bit a one third reverse turn and the pilot bit disengages from the ring bit assembly. The driller then carries on open hole drilling without the need to trip back and change to a conventional down the hole drill bit.

Should a further patch of bad ground be hit, the casing can be extended by relocking the ring bit and drill and case continued until good rock is reached. Again, there is no need to withdraw the drill to make change.

Littler is enthusiastic about the system which has played a significant part in ensuring that this latest contract will be finished on programme. 'It's easier on the rig too,' he says. 'There's no jerking and no damage to the threads on the drill rods.'

Littler says the system proved a worthwhile investment in terms of the elimination of lost drill bits, damage to rod and rig, not to mention the time saved. 'And you can could also use a smaller rig, because you don't need the extra torque to drive this system.'

Ritchies also intends to use the Centrex system on a large contract in Ireland.

While anchoring and excavation of the retaining wall continues, the first pours for the floor of the treatment plant are under way.

When completed by late 1999, the fully disinfected effluent will be discharged through the existing outlet at Peveril Point while the sludge will be pumped to a remote treatment plant to be built as part of the project. Here the sludge will be thickened and used on farms as a soil conditioner.

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