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Marine piling - Iron-like boulders and bedrock have required a combination of three piling techniques for a marine job in the South Pacific.

Contractor Seacore is tackling one of the world's most technically and environmentally challenging foundation drilling projects in the Bay of Prony at the southern end of New Caledonia.

The company is installing piles for two berthing wharves with approach jetties, part of the new port facilities for the massive $1.88bn (£989M) Goro Nickel Development Project.

Work involves piling and specialist foundation drilling techniques to install tubular steel piles in the foreshore through highly variable ground.

It is randomly peppered with exceptionally large and hard igneous Gabbro rocks, with a compressive strength up to 250MPa.

Raking and vertical compression piles are 914mm diameter, ranging from 16m to 48m long. The 102 vertical bearing piles are installed individually in rows. The 24 rock socketed rakers are in clusters.

All have to accommodate an ultimate compressive load of 1,130t and tension load of 835t. The piles are interlinked with precast concrete crosshead beams, which are spanned with precast concrete deck panels and overlaid with a final cast in-situ concrete deck slab.

The port and nickel mine are expected to be operational by late summer next year.

'The drilling conditions have been very challenging, ' says Seacore specialist marine services divisional manager Jason Clark.

'Piles have had to be installed through sand, clays and Gabbro Saprolite, which is a friable and decomposed Gabbro, also known as rotten granite. This is randomly intermingled with extremely hard Gabbro core rocks, some the size of a pickup truck.

'Stringent nvironmental protection measures have to be adhered to, involving the use of marine silt screens during drilling operations, to prevent suffocation of the coral reefs in the surrounding lagoon - the area's a breeding and calving ground for Humpback Whales, ' says Clark.

To install the piles, Seacore is using a vibratory pile driver and a 20t hydraulic drop hammer combined with a reverse circulation, under-reaming hammer and its own reverse circulation drill rig. All the equipment is operated from a Dutch jack up barge supplied to the main civil works contractor, a joint venture of McConnell Dowell Constructors, Vinci Grands Projects and local New Caledonia contractors Socalmo and Dumez DTH. Civils work is worth £35M.

After assessing ground conditions, Seacore selected a down the hole hammer as the best solution to tackle the very hard Gabbro, as the penetration rates would be signicantly greater than conventional reverse circulation drilling.

But Seacore also considered a reverse circulation system was essential in this environmentally sensitive area.

Conventional ushing systems would typically discharge into the sea.

Installation involved driving the piles to refusal with the vibrating hammer, then using the drop hammer to take them further and toe them into bedrock. Drilling was then used to create 4.5m deep rock sockets.

Seacore moved onto site in late autumn 2005 and started on the frst hole in January 2006.

Initially the toes of the thin walled steel tubes buckled and deformed when hitting the Gabbro core stones or bedrock and the site team had to adapt their piling operations to the extreme ground conditions.

This involved welding onto the bottom of the piles a heattreated shoe with a strength of 1,500MPa. This resolved the deformation problem.

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