Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Rigged up

Plant - An exotic piling rig has been pressed into service on the island of Guernsey.

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.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.