Quintain has tasked Wates with delivering hundreds of homes to high standards as a neglected corner of London is turned into a thriving community.
Project: Wembley Park Block NW07 and NW08
Contract value: £104m
Contract type: JCT Design and build
Main contractor: Wates
Concrete frame subcontractor: J Reddington
Start date: June 2016
Completion date: December 2018
While Wembley Stadium has been treasured as a national icon for nearly a century, until now the neighbourhood surrounding it has been afforded no such attention.
Starved of any real investment since the 1960s, the Wembley area had become severely run-down.
Backyard businesses and pockets of heavy industry had scarred the landscape, while the potential problems caused by 90,000 spectators regularly inundating the neighbourhood had made property investors reluctant to commit funds.
However, as real estate prices have rocketed across the capital in recent years, investors have been emboldened. Property developer Quintain saw that the area was ripe for investment and has been steadily working on Wembley Park’s transformation for the past decade.
Quintain has planning permission for more than 5,000 homes on land around the stadium, overseeing a host of large residential projects either recently delivered, under construction or poised to put spades in the ground.
Wates is delivering several of these schemes for Quintain under its build-to-rent arm, Tipi.
It has just completed NW06 – a block of 362 homes comprising PRS, housing association and private sale units – and is currently focused on the delivery of two more blocks – NW07 and NW08 – for the client. These will be a similar mixture of 361 PRS and residencies operated by the Network Homes housing association, as well as 14 retail units across the ground floor.
Wates Wembley Park 8
And while more developments are due to start imminently, the Wates project team is concentrating on the delivery of these NW07 and NW08 blocks, which are being delivered under a £104m design-and-build contract.
“We have a fantastic relationship with the client, but they are tough”
Gareth Davies, Wates
Wates was brought onto the project under a preconstruction services agreement (PCSA, see box) as part of a two-stage tender that saw the team move onto site to carry out enabling works in June 2016. These works featured some low-rise demolition including the old ‘Yellow’ car park that served the Wembley Stadium complex.
“There was quite a lot of work that needed to be carried out before we could get on with the construction work proper,” says Wates’ project director Robert Vigar. “The ground on the car park had just been levelled, so beneath that there was a wave of uncharted material we had to deal with.”
Concrete sensors speed up stripping
The team has used several techniques to help speed up the delivery of the cast in-situ reinforced concrete frame. One of the most innovative is the use of thermo-sensors in the concrete that help the team gauge when the material has reached the correct strength to start stripping the formwork.
Traditionally this is done by little more than a blend of intuition, experience and sample concrete cube tests carried out seven or 24 days after the pour. It can be little more than the equivalent of a wet finger in the air to judge wind speed and direction.
But by using the sensors to plot the concrete’s thermal gain from the heat of hydration, the team can ascertain the strength of the concrete with much greater accuracy. This means the team can start to strip formwork as soon as the concrete hits its required strength, speeding up the whole frame delivery process.
“It is difficult to judge just how much time we have saved against standard delivery,” Mr Vigar admits. “But we managed to complete the frame four weeks ahead of schedule.”
That uncharted material included the threat of unexploded Second World War bombs, but also the more mundane high-voltage electricity cables and deep drainage that ran across the site. “German Second World War bombs had been found on a nearby site so we needed to make sure that all was clear here,” he says.
As part of the enabling works the team installed sheet piling and capping beams around the basement and the lift cores. These sheet piles are generally driven to 11 m, with those around the lift core extending 14 m below ground level.
The bulk dig from the basement area saw some 9,000 cu m of earth removed to depths of 4 m. Across the site this total rises to almost 14,500 cu m, with the scheme recycling / diverting to landfill more than 98 per cent of its waste.
There are four cores across the two blocks. Cores A and B in block NW07 are 16 and 12 storeys high, while cores C and D in NW08 rise to 12 and 14 storeys. The basement level spans both blocks.
Slip-forming wins out
The team used slip-forming techniques to bring the cores up to height rather than jump-forming. It was a decision based largely on the speed of work that can be achieved and its impact on the programme, as well as safety aspects such as the working environment for the core team.
“We feel slip-forming offers our teams a much safer work area than jump-forming,” Mr Vigar says. “It is also much quicker: the forms were advancing at 100 mm every hour – we could get a full storey height in within a day.”
Wates Wembley Park 2
The slip-forms also allowed the team to bring in Stair Master prefabricated stairs almost immediately behind the cores.
These are delivered with handrails and tread reinforcement already fixed and so can be concreted to finished level at the same time as the core is being slip-formed. Once the concrete has hardened, these stairs are finished and provide access for the construction teams, reducing any dependence on temporary stairs.
“We find that cast in-situ concrete slabs offer us the greatest efficiency”
Robert Vigar, Wates
The structural frame itself is based around cast in-situ reinforced concrete slabs and precast concrete columns, with the team gaining advantages through the marriage of both.
Wates estimates that, coupled with a table formwork system and the sensors used to measure strength gain and speed stripping time (see box), that it has finished the frame four weeks ahead of schedule.
“We find that cast in-situ concrete slabs offer us the greatest efficiency,” Mr Vigar explains. “The precast concrete columns too. Our concrete frame subcontractor J Reddington has a table formwork system that includes edge protection as soon as they are installed. These are placed around the perimeter and then the rest filled in with standard table forms. It is a very good, quick, safe and efficient system.”
Wates Wembley Park 4
The heavyweight frame is being supported on a raft of CFA piles that have been installed to depths of up to 24 m. There are 429 in total across the site, the bulk of which are 750 mm in diameter – although there are some 900 mm-diameter piles, mainly under the cores. The pile caps are traditional, with 350 mm-thick raft foundations throughout.
Further up the structure the team has installed preassembled and fitted bathroom pods throughout the building. Delivered from provider Bathsystem from its Italian manufacturing base, on-time delivery is crucial to the smooth running of the overall project.
“Their delivery needs to keep pace with the frame,” Mr Vigar explains. “The team did well to manage the flow of that. If anything was late then we would have to leave a ‘zip’ in the frame so that we could get the late arrivals into position. Fortunately that hasn’t happened.”
Already some of the apartments are going through completion stages; those on the lower levels of the buildings have central heating in place and running water.
With contract completion due by Christmas, it’s looking good for Wates down Wembley Way.
’Mission-critical’ risk sharing
Wates won its Wembley Park jobs as part of Quintain’s framework to regenerate the area.
Work on this section of the scheme was assigned to the team under a preconstruction services agreement (PCSA). It saw Wates and its supply chain rally together to get the design and construction method developed.
“It was an extremely testing procedure; we have a fantastic relationship with the client, but they are tough,” says Wates framework director Gareth Davies. “They know what they want and if they think we and our supply chain is not competitive – on price, speed or quality, among many deciding factors – then they have no hesitation in saying so.”
The team runs design / development workshops with the supply chain. Everything is open for comment from day one and input from every part of the supply chain is encouraged. There is a high level of engagement throughout.
“Risk is driven straight through the team,” Mr Davies says. “This is mission-critical stuff. One work package failure could collapse the whole engagement.”
He admits the pressure is on from the word go, but is confident the team’s efforts will ensure final delivery is exactly as required by the client. “We want the shortest period to final contract signing as is possible, but there is an awful lot to do during that period,” he says. “The tempo is very fast and it is competitive. We have to declare three quotes from our supply chain for each package.
“Ultimately there is escape from the framework for the client – if it doesn’t like what we put to them, it can walk away,” he adds. “The client wants cost certainty, which is difficult because the design still has a certain amount of flux in it. That’s where our supply chain helps us out.”