Bachy expands Excel foundations for Olympics

Taking to the water for its largest UK marine project, Bachy Soletanche has stretched its groundwork expertise for an expansion project at London's ExCeL. James Stagg explains.

Located in the heart of East London's Royal Docks, ExCeL London is undergoing an expansion to accommodate boxing, judo, weightlifting and table tennis for London 2012. But this is no normal extension. The dock acts as partial flood relief for the Thames so its capacity cannot be altered, which means the additional room has to extend over a new marine deck.

Main Contractor Sir Robert McAlpine awarded Bachy Soletanche the 24-week marine deck subcontract, which included the installation of 148 marine piles, along with associated decking work, while Stent was awarded the contract for piling works on dry land (see box).

Ben Hardy, project manager at Bachy Solentache, says that while the firm has worked on many marine piling jobs internationally, its experience in the UK is limited. "We had to rent heavy-duty barges from Carillion Marine [which worked on the original building]," he explains. "They are simply flat-top barges with nothing on them but a hand rail."

From these barges, 63 piles are sunk at the rate of one per day, while a further 85 are installed from the quay. Each of the two barges is large enough to install five or six piles from one position, so the first job is to get them in place using tug boats and then anchor the barges using 'spuds' located at their back.

"There is a great deal of preparation time to consider," adds Hardy. "Getting the rigs onboard and keeping the barges stable during piling makes it a slow and delicate process. Even once the barge has been positioned and the spud legs are dropped, we have to survey to see if we're in the right place and, if not, repeat the process until we are."

The spud legs at the back of the barge are effectively large tubes that are dropped into the water and embedded in the dock bed to hold the barges steady while piling work takes place. They allow the barge to bob up and down, but not move from side to side.

"It's hard to get the position perfect because, with just two spuds at the back of the barge, when the barge rocks, the front moves in an arc," says Hardy. "However, there is still enough tolerance that if a pile is out of position by up to 75mm, the beams will still fit."

Moveable gates

Once the correct position has been established, specially designed moveable gates are used to hold the permanent pile casings in place before they are vibrated into position. Support boats, crawler cranes and a service barge assist in the transportation of the materials and provide access to the water's edge. When all of this is in place, the marine piling process can progress.

The 20m pile casings are vibrated into the dock bed, leaving about 2m above the water line. Water is then pumped out and replaced with super mud polymer, which acts as a support fluid when piling begins. "We use the polymer for environmental reasons," says Hardy. "We would normally use bentonite but, as any spillage into the dock would not break down, we have switched to the polymer."

The 900mm-1,050mm piles are then constructed using a traditional rotary bore large-diameter auger technique. This is performed with a Bauer BG-22, which bores down to the required depth ready for the insertion of a reinforcement cage. After a reinforcement cage is inserted, concrete is pumped into the pile through a tremie pipe, which displaces the support fluid. Each pile contains approximately 22m3 to 45m³ of concrete.

The cut-off level of piles is 200mm-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 88m pre-cast beams. These beams lay across the piles, which are flanked by pre-cast concrete planks to form a solid base of the deck. A topping slab will cover the initial base and the total decking area will cover 7,000m².

In addition, Bachy Soletanche will cut holes in the existing Phase 1 deck for the installation of 35 marine piles. When these are in place, 200m2 of the original deck will be demolished and replaced by the new concrete quay.

Hardy concludes: "From the offset, it was understood that the project would have its challenging elements, in particular piling from the barges into water. In spite of this, we have demonstrated that using our expertise in traditional LDA piling can be a benefit to a variety of project specifications, whether on land or water."

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Balfour stays on dry land

While Bachy Soletanche took to the water taking on the challenge of piling from barges, Balfour Beatty Ground Engineering (BBGE) was taking on the foundation work on dry land.

At the peak of the project, Penine and Stent had a total of six rigs on site for the £2.6m project.

Stent had three Casagrande rigs as well as Soilmec and Klemm rigs installing 1,982 continuous flight auger piles. Meanwhile, a 35t Pennine Stratacaster rig installed 310 vibro concrete columns (VCCs) to a maximum depth of 10.5m on the north-east section of the site using the bottom-feed technique.

As there were high-voltage cables nearby, BBGE used low-headroom CFA piles, sometimes called mini-piles. Stent's contracts engineer Alex Underwood explains: "With some of the works taking place at the face of the existing building and directly below high-voltage electric cables, we worked around a 9m exclusion zone surrounding the pylons. We also worked at a height restriction, so it was necessary to use low headroom CFA piles to negate the risk to the team."

The firm also had to be wary of unexploded ordnance from the Second World War and aircraft going to and from London City Airport. To guarantee the safety of the new hall and site operator, it carried out an investigation which included probing pile positions on the site to depths of 9m, while piling rigs were fitted with flashing beacons at the highest point of the mast.