May Gurney's Greenwich piling project: Focus on Piling & Ground Engineering

The eventual residents of Greenwich's newest development lucky enough to have a north-facing window will enjoy one of the capital's most dramatic skylines, taking in the City of London, Canary Wharf and the O2.

It seems astonishing that this substantial 6ha brownfield site - formerly wharfs and a commercial boatyard - has taken this long to be snapped up by developers, in this case the London and Regional (L&R) development group.

Durkan won the initial £45m phase of the project to build 267 new affordable homes, comprised of four separate buildings rising to six and seven storeys in height, providing affordable 237 flats and 30 four-bedroom townhouses for private ownership. This is part of a wider development, to be known as Greenwich Wharf, which will include private apartments, a 100-room hotel, an office building, retail units and a health club. The overall project is due to complete in 2012, with the current phase in place by 2010.

May Gurney is undertaking the piling work for the first phase of the job, valued at £1.1m, and has been on site since late January. The first block - split into the town houses and a concrete frame for apartments - will take four weeks to pile. The second to be built, called block seven for the purposes of the job, will take eight weeks, and the third, block six, another four weeks.

A mixture of 400mm diameter and 600mm piles are being used, with maximum load capacities of 1,390kN compression, 80kN horizontal and 2,585kN compression, 175kN horizontal respectively. The maximum pile depth is expected to be about 20m, although piles will be shorter if they take a lighter load. Five preliminary sacrificial test piles have also been sunk.

Continuous flight auger (CFA) piling is the predominant technique, as May Gurney Southern area manager Mike Cowan explains. "Initially we drill down to the correct designed depth. The auger is then withdrawn and at the same time the concrete is pumped through the hollow stem of the auger until it reaches ground level. Then the steel reinforcement cage is inserted into the fresh concrete.

"The process is fairly standard - from our point of view the really unusual thing about this site is the sheer scale - it's vast."

Underground car park

Block one will require 500 piles block six 200 piles, while block seven will need 400 normal bearing piles, plus another 600 piles for a secant retaining wall - another unusual element of the project for May Gurney. These will form a solid square wall, allowing the centre to be later excavated as part of the construction of an underground car park for residents.

The ground conditions are fill and rubble to 3m, then dense sand and gravel to a depth of 8m. Between 8m and 16m is the very stiff clay typical of London, and below 16m is dense sand. Less typical is the presence of limestone bands. "We had to make sure we had a powerful rig on site to get through these areas where necessary," says Cowan. May Gurney chose a tough Spanish Llamada P140TT rig.

The constant hammering that persists throughout this interview is coming from demolition contractor Erith, which is busy removing the remaining spoil from the site and installing sheet piles through the limestone to create coffer dams, an important defence this close to the tidal Thames and a technique that has been used since the original Victorian Thames wall was built.

Durkan site manager Pat Phillips works closely with Erith, a fellow main contractor for L&R, and is keeping a close eye on the water table. "We're meeting a lot of groundwater now," he says. "The high tide is 2m above the ground level of block seven of the development. If there is a lot of water we have to dewater by pumping, and at the moment there are wells throughout the whole development. Water levels have been monitored over the past six to eight months. The water will be kept away from the finished buildings by the tank and the damp proofing works that will be done."

Erith is sending its spoil away on barges up the river, and has a licence to remove up to 50,000m3. Part of the council's planning agreement was for Durkan to do the same, but there is a snag that means the spoil cannot be removed. "I can't remove the spoil because under the planning conditions everything is supposed to go out by the river," says Phillips. "But Durkan tendered the job for these two phases to take it out by road because there is a 15m strip of land behind the riverside hoarding that doesn't belong to the client, so we can't access the river. The spoil is also a long way from the river, making it difficult to load onto barges. The client is dealing with the council now to allow us to take it out by road."

Phillips hopes the situation will be resolved. "It's all to do with the residents, we don't want to fill up the roads with lorries. Avoiding this was key to the project's planning permission. Also, the roads just won't be able to take it. Once this development gets up and running, there will be 500 to 800 guys here every day.

"If Durkan wins the next phase, and we think we have a good chance, everything we dig out for that will go out by river. We will put a batching plant on site and all the aggregate for the concrete is going to come on by river - Hanson has a plant next to the O2. If May Gurney is also lucky with that phase, that's also where its concrete will come from."

Restricted hours

Durkan has set up a one-way traffic route round the site to lessen the impact on local residents, and works restricted hours - between 8am and 5pm. "We hold residents meeting every two or three months, send out newsletters, and have a resident liaison officer," says Phillips.

This is normal on Durkan's central London developments. What is less common is the amount of green technology being used. To meet the government's renewable energy requirements, it will install ground source heat pumps to provide heating and hot water to all homes, and the buildings will have a combination of green and brown roofs to attract wildlife to the area. "It's a big learning curve for us," says Phillips. "Standards are being tightened all the times to make housing more eco-friendly. Using the river rather than the roads to transport materials will be a great addition for us and the subcontractors."