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Foundations and retaining walls for a series of new developments on one of the Channel Islands required the use of innovative techniques.

Marcus Brierley takes a look.

The Royal Hotel in St Peter Port, on the Island of Guernsey, is infamously remembered by the locals for its use as the headquarters of the German occupiers during the Second World War. Destroyed by fire in the 1990s, the site has now given way to a series of three developments facing onto the harbour and its marina where piling contractor Dawson Wam has been preparing the foundations.

The first phase of the St Peter Port development by Long Port Developments was a new five storey Royal Bank of Scotland. The piling contractor has now fi nished work for the second stage - a U-shaped residential apartment building fronting the main coastal road with an open swimming pool inside the forecourt, also with five storeys and two levels of underground car parking.

The final phase, which Dawson Wam is still working on, is an office complex to the rear of the bank.

The site slopes steeply downwards to the seafront with the strata comprising made ground overlaying medium dense to dense sand overlaying weathered gabbro becoming more competent with depth.

Enabling work designs called for a retaining and load bearing wall which has been built as a combi-pile system consisting of 457mm diameter tubes at 2.75m centres with AU16 intermediate sheet piles, all of which are tied back with walings and anchors.

Since there was no piling rig or equipment stationed on the island, Dawson Wam arranged for its new ABI Leader rig - a TM 18-22 telescopic fl exible format machine and one of only four in the world - to be delivered directly to Guernsey. The rig is capable of both tube and auger boring, counter-rotating boring as well as pitching hammer or hydro press installation dependent upon the tool attached.

However, as site workers began installing the largest retaining wall on the west side of the site it soon became obvious that the hidden rock profie was mimicking the uneven exposed rock outcrops along the Guernsey coastline.

This meant piles only 3m apart were hitting the bedrock at up to 2m difference in level. Pile depths ranged from 9m to 14m with all the tubular piles socketing 1m into sound gabbro.

As the bank built in phase one is now open for business, the minimisation of noise and vibration was important. The solution was to screw in the steel tubes - each having its own sacrificial rock cutting teeth - and at the same time, reaming out the centre of each tube as it was being installed, using the rig's counter rotating double head auger drive.

These piles are fitted with steel clutches to either side into which the sheet piles key. Site workers weld all the clutches for water tightness and walings are bolted onto the excavation sides. The sheet piles extend 1m below the excavation level.

If additional load bearing capacity is required, Dawson Wam has the option of grouting the tube cores.

Dywidag bar ground anchors and waling bars, which have been installed as a temporary support mechanism, will become redundant once the internal floors are installed as the building takes shape.

The ongoing phase three ground enablement works began immediately following the end of those for phase two last July. In this, the combi wall technique is being replicated along the road boundary.

However, near to a terrace of housing - in extremely tight installation conditions - a secant piled wall of 457mm diameter piles on 400mm centres is being built, with every second pile socketed into rock. Reinforcement is with 'H' beams.

An automatic monitoring regime is in place comprising vibration gauges, tiltmeters, crack monitors and inclinometers along this boundary, to prevent structural damage occurring along the party wall.

Design for the combi wall was undertaken by Doran Consulting of Belfast with which Dawson Wam has had a long working relationship.

The client is the Jubilee Group, part of the Longport Development group, which is the major developer in Guernsey.

The foundation and enablement works contracts are worth about £2M and Dawson Wam expects to finish its work on phase three by next month.

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