Delivering a new basement in London’s suburban zone 4 is no less challenging than one in zone 1, just different.
Construction of a single storey basement and piled foundations for a residential development in
London Clay sounds simple, doesn’t it? But when the basement covers an area of 15,000m2 and is in the shadow of Wembley Stadium, it is certainly not straightforward.
Contractor O’Keefe is currently constructing the below ground elements for Quintain’s latest Wembley Park development, which will include four towers up to 18 storeys high with a single basement spanning the whole site. The location means that it is under close scrutiny from Spurs supporters, X-Factor hopefuls and music fans – as well as existing residents and shoppers at the London Designer Outlet – on a daily basis.
Quintain is taking a different approach to the housing market, with a focus on the rental market at
Wembley Park (see box). It is also taking a different approach to procuring the work. “We prefer a collaborative approach rather than tendering everything,” says Quintain executive director of construction Matt Voyce. “Money is always a factor, but the complexity with our stakeholders with Wembley Stadium, and Brent Council’s offices on site and residents means that the partnership approach gives us greater confidence in the contractors representing us.”
As a result of this approach, O’Keefe is one of only seven contractors on site and is working on both of the two basement projects within the development – the Southlands and Eastlands schemes.
“We have given contractors some tough programmes to work with, but everything we do has to fit around the events,” says Voyce. “With Spurs in the Champions League, we have gone from 35 events a year to more than one a week, so the challenge has increased.”
Another factor that helped secure the £16M contract for Eastlands was the way it dealt with an asbestos issue at Southlands.
“The same problems exist at Eastlands and we knew we could trust O’Keefe to manage the issue sensitively again,” says Voyce.
For O’Keefe the programme at Eastlands is centred around handover of part of the site to main contractor Wates six months after ground was first broken.
“On a project of this scale in central London, we would normally have the first handover after 12 months, so there is a challenging critical path here,” explains O’Keefe managing director Lee Horsley.
Work started on site in June with installation of sheet piled retaining walls for the basement and the first phase of continuous flight auger (CFA) piles to support the piled raft foundation.
O’Keefe has worked with Ramboll and independent consultant David Puller on the design of the piles that will support the raft, while the piles outside this area were designed by Rock & Alluvium. Geotechnical Experts is also providing technical advice for the piling works.
In total O’Keefe has installed 520 linear metres of sheet piled wall – using silent driving due to the proximity of the stadium – to 12m depth, which will mostly act in cantilever for the 3.5m deep basement excavation.
There are two exceptions close to the stadium, where the sheet pile wall will be tied back to a second sheet piled wall section.
“The cantilever approach was a key part of meeting the programme here as it meant the basement walls will be free-standing and not require any raked shoring, so will be faster to excavate and give Wates an open area at handover,” says Horsley. “The London Clay was stiffer than expected, but we still completed the work with no difficulty.”
The cantilever design meant that some of the bearing piles had to be undertaken by Rock & Alluvium ahead of the sheet piling. The 40 piles installed in the first phase were 600mm diameter CFAs constructed to 26m depth.
“The non-raft pile design has benefited from reduced partial factors as a result of preliminary pile load tests,” says Puller.
Horsley adds: “There are a couple of areas of poor ground, but once we’re through the made ground, the ground conditions are mostly good London Clay.”
Rock & Alluvium started work on the second phase of piling in early September and is expected to complete installation of more than 550 piles in mid-November.
“The piled raft designed by Ramboll uses a sophisticated soil-structure numerical model,” adds
Puller. “This means that the piles are working much harder, as the load is shared between the piles and underside of the raft itself. The resulting solution has enabled the buildings to be supported by a greatly reduced number of piles with obvious cost and programme benefits.”
Once the piling is complete, excavation of 30,000m3 of spoil from the basement will start and that – along with welding of the sheet piles to provide waterproofing – is programmed to take 12 weeks.
O’Keefe’s contract also includes construction of the 1m deep, heavily reinforced raft.
“There is a lot of muck to take out and a lot of concrete to get in,” says Horsley. “The Champions League and Spurs home games limit our working times. While there is more room here than on a standard London basement and the depth is smaller, there are still lots of risks that need to be carefully managed.”
Horsley says the muck away is the element that gives him sleepless nights, simply because of the volume of material to be moved and the site’s prime location at Wembley.
Nonetheless, he seems confident of that O’Keefe will meet the first handover date with Wates in March next year, clearing the way for the first residents to move into the 660 apartments in summer 2020.
Developer Quintain is working to develop Wembley Park as a new 35ha residential area that will eventually be home to 20,000 people. It will have a 3ha park and river feature that hugs around the iconic Wembley Stadium.
Construction at the site started in 2006, but acquisition of Quintain by private equity company Lone Star has accelerated the pace of work at the site.
Currently Quintain is spending £1M a day on contractors with 1,300 workers on site and 3,000 units under development but this is expected to ramp up to 3,500 on site when work reaches its peak.
“It’s a very attractive location with good infrastructure links,” says Quintain executive director of construction Matt Voyce. “Tube services mean it only takes 12 minutes to reach Baker Street and we’re served by the Night Tube too.”
Unlike many other residential developments in the UK, the homes will mostly be for rental under Quintain’s Tipi brand, which is described as a lifestyle focused rental company. The remainder will fall into the affordable housing bracket.
According to Voyce, Tipi offers a zone 1 or 2 product in zone 4 at Wembley Park with its hotel standard facilities and all-in rent.
“Zone 4 developments are typically eight to 10 storeys but here will go up to 24,” says Voyce.
“The stadium is a massive building so the height is in context here.”