Piled high

A dual-use retaining wall will help transform a corner of south London into an enticing residential development. Alexandra Wynne reports.

The south of the UK’s capital may be no stranger to high-rise accommodation, but a new project at Stockwell Green is under way to provide a mixture of accommodation reaching up to nine storeys high in a modern take on a 1960s’ classic.

In addition to the 271 apartments it will hold, the building, designed by architect Hawkins Brown, will have a retail level on the ground floor as well as a basement car park.

To take care of all of these requirements, the site is being subjected to a major piling operation consisting of about 1000 continuous flight auger (CFA) piles. These are for a horseshoe shaped retaining wall, which is partially load bearing, as well as for the foundations within the footprint to support the new structure.

Subcontractor May Gurney is installing all of these for main contractor Higgins Construction for nearly £600,000. The design meets a number of different needs, with a medley of piles differing in length and diameter, as well as steel reinforcement bars that range in length and number.

But the fact that this is such a large and varied job, which needs to be completed by the piling contractor in two phases, gives cause for celebration to its contracts manager Neil Willis and foreman Justin Chapman.

“It’s nice for the design team to work on longer jobs, which can be quite rare,” says Willis. “With each new job there’s a learning curve and here we’ve got time to change the way things are done if we need to.”

“We’re also lucky on this job to have the room to come back once the area has been dug out to about 400mm to 500mm above formation level, because one of the problems we’ve got is that the bearing piles are at 3m below ground level [before excavating],” Willis says.

He explains that if the team had instead opted for creating longer piles than necessary, the costs could have doubled due to the wasted materials and effort of breaking down the top 3m of the bearing piles.

In the first phase, which began in September last year and ran until Christmas, site workers installed 380 CFA piles in a contiguous formation at 550mm centres to retain the soil surrounding the car park.

But this wall has a second purpose – pockets of piles along its length will also provide load-bearing support to the between five- and nine-storey building above, which is why the piles are as deep as 18m in places.

“A 3m retaining height with piles down to about 8m or 9m would normally be fine for this kind of wall and this kind of building,” says Willis. “But because of the extra load requirements we’re having to go much further down with some of them.”

This length also accommodates a contingency of 0.5m, which Willis says helps if there is any over zealous work during the digging-out phase.

This excavation followed on from the first piling phase for the wall, before May Gurney returned to site in January to install the foundation piles for the apartments. These were due to be complete by late February, as GE went to press.

The wall piles work in cantilever to retain the surrounding soil and are self-supporting during this digging-out phase. For the longer term, subcontractor Mann fixed in a concrete ground beam that ties in to the steel reinforcement within each pile and stops them working independently, making the wall a stronger unit.

The ground profile comprises 4m of made ground overlying firm to stiff clays, which Willis says is ideal for creating a perfect-looking wall. “Call it vanity, but in softer ground than this the piles don’t look so perfect,” he says.

May Gurney has an engineer on site and uses marker pins in the ground to help with ensuring the pile locations are exact. Willis says if the team is working on a deep wall then it tends to use a guide wall. But despite the length of the piles this wall is only supporting a 3m excavation, so the engineer and markers are sufficient in ensuring the piles go in straight.

For the foundations, site workers are installing a combination of 297, 450mm diameter and 302, 600mm diameter CFA piles, again down to a maximum of 18m. To help cope with variations in pile design, May Gurney has chosen to subcontract a team of steel fixers, which has been preparing all the cages on site.

The wall piles are reinforced with 6T20 or 6T25 steel cages down to a maximum of 10m within the pile to cope with tension loads of between 25kN and 225kN, and compression loads up to 900kN. The foundation piles require 6T16, 6T20, 6T25 or 7T25 bars to cope with compression loads of between 800kN and 2075kN and up to 1075kN tension and 125kN horizontal loads.

Willis says some needed extra steel in the form of tension bars inside the cages to cope with the higher tension loads. Steel fixers attach spacers at up to 3m intervals along the cages which allow site workers to ensure the pile integrity is not breached by steel resting too close to the edge.

Site workers then attach lengths of bonding foam to the top end of the steel reinforcement, which helps with breaking down the top of the pile when tying it into the ground beams for the building, or capping beam along the wall. Willis says it simply prevents the concrete from attaching to the steel and therefore crumbles away with ease.

The building, expected to be complete by late 2009, will have solar panels installed. These are designed to provide 40% of the hot water for the building’s residents, as well as offset their heating requirements. These moves are being taken to help the new building meet its sustainability brief and lower its carbon footprint on what is an already densely populated city.