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Olympic excellence

Expansion of London’s ExCeL centre, another key 2012 venue, is also calling for some complex work below ground.

London’s 2012 Olympic preparations are now in full flow across the capital, with expansion of the Royal Victoria Dock-based ExCeL Exhibition Centre one of the more significant. The expansion is approximately half the size of the current building and when construction works complete, it will cover an area equivalent to 14 football pitches.

The extended ExCeL will be ready in time to play its central role in hosting seven Olympic and five Paralympics events at the London 2012 Olympics. However, the job is not straightforward as the extension is only possible by building out over the dock. And as the dock acts as a flood store for the River Thames and must stay unaltered, a marine deck is a necessary requirement.

Contractor Sir Robert McAlpine called in piling specialist Bachy Soletanche for the job, which will be the largest marine project in the firm’s history. The 24-week sub contract includes the installation of 148 marine piles and associated decking work, with piling taking place from quayside and aboard two barges.

From the quay 85 piles will be installed and 63 will be completed from the barges, with the piles varying in diameter: 136 of 900mm diameter and the remaining 12 of 1.05m. All are to be drilled to depths ranging from 34m to 44m, with 2m to 10m of this through water. To install the piles, 20m casings are vibrated into the ground with approximately 1.9m of the casing above water level. The water within the casing is pumped out and replaced with Super Mud Polymer which acts as support fluid when piling begins.

Bachy is using the large diameter rotary auger technique to construct the piles within the permanent casings. The cut-off level of piles is 200mm to 300mm above the water level and equally spaced over six parallel rows. The top of the pile is then broken down and replaced with a mortar top ready for the installation of 88 pre-cast beams. These beams lie across the piles which are flanked by pre-cast concrete planks to form a solid base for the deck. A topping slab will cover the initial base and the total decking area will cover 7,000m2. In addition, Bachy will cut holes in the existing deck to install another 35 marine piles. When these are in place, 200m2 of the original deck will be demolished and replaced by a new concrete quay.

The logistics of piling from the barges is one of the most challenging aspects of the project. As such, getting the rigs onboard and keeping the barges stable during piling makes it a slow and delicate process. "There is a great deal of preparation time to consider, especially when piling from the barges, especially as it takes a day to install just one pile," says Bachy project manager Ben Hardy. Each barge is large enough to install five or six piles from one position so the first job is to anchor it using "spuds" located at the back of the barge.

Specially built, in-house designed, movable "gates" at the front of the barge, are used to hold the casings in place until they are vibrated into position. Support boats, crawler cranes and a service barge assist in the transportation of the rigs and provide access to the water’s edge. Once everything is in place piling begins. "This work requires a great deal of forward planning, skill and perseverance," says Hardy.

Bachy will complete the marine deck project at the end of January.

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