The Royal Hotel in St Peter Port, Guernsey, was the headquarters of the German occupation during the Second World War. It was destroyed by fire in the 1990s and in its place three new harbourside developments are taking shape.
The first phase of construction by Long Port Developments gave rise to the new five storey Royal Bank of Scotland building.
Long Port also has offices here and a show flat for the second development phase. This will comprise a smart residential apartment building fronting onto the esplanade, with two levels of underground car parking. The third phase comprises another office complex to the rear of the bank.
The site slopes steeply to the seafront, with ground comprising made ground over medium dense to dense sand. This in turn overlies weathered Gabbro which becomes more competent with depth.
Ground works design calls for a retaining and load bearing wall, constructed as a combi-pile system. This consists of 457mm diameter tubes at 2.75m centres with intermediate sheet piles tied back with walings and anchors.
There was no piling rig or equipment stationed on the island, so piling contractor Dawson WAM arranged for its ABI Leader rig to be delivered.
The rig is one of only four of its type in the world and is capable of tube boring, auger boring, counter-rotating boring, pitching hammer or hydro press installation.
When installation of the retaining walls got under way, it soon became apparent that there was a hidden rock profile on the site. Piles only 3m apart were hitting the bedrock at up to 2m difference in level.
Pile depths range from 9m to 14m with all the tubular piles socketing 1m into sound rock.
To minimise noise and vibration, steel tubes were screwed into the ground with the aid of sacrificial rock cutting teeth.
Tubes were reamed out using the rig's counter rotating auger.
Tubular piles are fitted with steel clutches into which the sheet piles key. The walls are anchored back with forces spread by two to three waling beams. Sheet piles are extended 1m below the excavation level.
If extra load bearing capacity is required in future, the tube cores can be grouted. Design for the combi wall was by Doran Consulting of Belfast. Foundation and enabling works contracts are worth roughly £2M.