Sisk is slashing embodied carbon and optimising offsite-manufactured components in its latest job at Wembley Park
Project client: Quintain
Total cost: £230m (fixed-price contract)
Contract type: JCT design-and-build
Main contractor: John Sisk
Architect: Haworth Tompkins
Structural engineer: Elliott Wood/CampbellReith
Groundworks: Harringtons Builders
Demolition: Goody Demolition
Concrete cores and frames: Byrne Group
MEP: Halsion (NE03)/HE Simm (NE02)
Facade contractor: Techrete
Construction start: November 2021
Expected handover: May 2025
The poet Sir John Betjeman had a curious obsession with Metroland, the suburban areas of north west London that emerged in the early 20th century amid the expansion of the Metropolitan Line. “The lure of Metroland was remoteness and quiet,” Betjeman wrote, describing “the trees, the fairy dingles, and a hundred and one things in which Dame Nature’s fingers have lingered long in setting out this beautiful array of wooden slope, trout stream, meadow and hilltop sites”.
It’s safe to say this bucolic description didn’t fit the stadium built in Wembley for the 1923 British Empire Exhibition. Nor does it apply to the modern Wembley Park area, with its growing array of high-rise residential blocks, such as the NE02/NE03 scheme, which is under construction on a former car park and office site.
“We’ve come up with a system whereby we don’t leave any holes
in the facade and the balconies literally clip on”
Daniel Mackell, Sisk
Two high-rise blocks on each plot are being constructed by John Sisk & Son for Quintain, which is building out 85 acres of land near the stadium for residential, leisure and commercial uses. When completed in 2025, two structures on the NE02 plot (17 storeys and 27 storeys) will comprise a total of 487 flats. Two more blocks on NE03 (10 storeys and 21 storeys) will deliver a total of 282 flats.
The total scheme will have a footprint of 9,593 square metres including green space/landscaping.
Sisk is no stranger to working with Quintain on projects at Wembley Park, but the £230m design-and-build deal for NE02/NE03, announced in September 2022 is the biggest it has worked on to date. By mid-September this year, NE02 and NE03 were respectively 44 and 52 per cent completed and fit-out work was underway.
Sisk took possession of the site in November 2021, engaging subcontractors to demolish the car park and office buildings in the preconstruction phase. Subcontractor Keller drove 1,080 continuous flight auger piles before the single cores on each block were slipformed by Byrne Brothers, which also built the reinforced concrete frames for the new buildings. All concrete for the project is being batched offsite and poured onsite.
“We’ve finished the frames on NE03 and we’ve topped out three of the four frames on NE02,” says Sisk project manager Daniel Mackell. The time it takes to pour the floor slabs depends on the size of the floor plates and weather conditions, so can range from between six to 14 days.
CantiDeck crane loading platforms are installed floor by floor next to the facade before it is clad. The platforms are positioned to clear the back propping and then load out prefabricated bathroom pods, closely followed by the facade. “While the facade is progressively installed up the building, we can get everything weathertight and release the fit-out below,” says Mackell. “When we get a clear enough space – generally about 10 floors – below the frame contractor’s props, fans, CantiDecks and general logistics, we start on the precast [facade] underneath. As the frame moves up and out of the way, we start to clad the building beneath it.”
Three heavy-lifting Terex cranes – capable of lifting up to 14 tonnes at 45 metres-reach – are being used to clad NE02 and NE03 simultaneously. The facade panels are manufactured offsite by Techrete in Lincolnshire and weigh up to 10 tonnes. The largest panels are 7.5 metres wide.
“Generally [on other schemes] we try to top the frames out and then start the cladding. But on NE02 we can’t wait for level 27 to be topped out and then do the cladding, otherwise we’ll never finish the project!
On this job, the team put recesses in the screen and used a Bomecon I-Boom counter-balance lifting rig to pick up the precast panels and cantilever them underneath the formwork. It means we can start wrapping these buildings as close as we can to the formwork above. We can clad a floor a week.”
By using precast facades, the new buildings differ from the nearby seven-block Canada Gardens, completed in 2021 with brickslip facades. “One of the reasons to engineer out the brick was to reduce embodied carbon,” says Matt Voyce, executive director at client Quintain. “Behind that brick [at Canada Gardens] you’ve got concrete, so you’re almost doubling up on your embodied carbon. We love brick – it stands the test of time and looks fantastic. But we also want to give different parts of the estate their own personality.”
Mackell adds: “We’re using concrete facades with fluted articulation, different colours [such as terracotta] and different finishes.” The panels are all concrete, with articulation achieved using very detailed moulds in the casting process. “Acid etch and grit blasting is carried out after the moulds are stuck,” he explains. “Once this is complete, the windows and doors are fitted offsite.”
Buro Happold is carrying out operational and embodied-carbon assessments on NE02 and NE03. Voyce claims that embodied carbon will be 80 per cent lower than in its 2021 completed projects and 20 per cent lower than its 2022 completed projects.
This is achieved not only on the facade but also with ground granulated blast-furnace slag (GGBS) as a substitute for cement in the floor slabs, columns and core walls. “We can get up to 40 per cent [GGBS] in our mixes for the slabs in sunny conditions but it becomes a little bit more limited in the winter months,” Mackell notes. “Right now [before autumn] is the time where we want to intensify, especially in areas like the roofs and the podiums, where we can leave the formwork in for a bit longer and allow the concrete a bit more time to cure.”
Sisk worked with Reading-based manufacturer Sapphire Balconies and Techrete to develop a non-combustible, lightweight (350kg) aluminium clip-on balcony system for NE02/NE03. “It’s all about optimising thermal efficiency – trying to reduce any thermal breaks through the facade – but also safety by making sure we’re reducing any risk of fire,” Voyce says of the system.
Sisk also wanted to make the balconies weathertight for drylining to start. “So we’ve come up with a system whereby we don’t leave any holes in the facade and the balconies literally clip on,” says Mackell. “That’s something quite unique that hasn’t been applied to any other project that I’m aware of.”
He adds that this method aids efficiency by removing the need to hang cradles over the edge of the building. “All the load is transferred – instead of going directly into the structure, the load is transferred through the facade panel.”
When CN visits, the first batch of offsite-manufactured 3D risers has arrived for installation. “One thing we’ve done slightly differently with this project [compared to other jobs for Quintain] is split the MEP [mechanical, electrical and plumbing] between HE Simm and Halsion,” Mackell remarks. This decision was taken partly because of the sheer number of flats being built across the two buildings, he adds.
Halsion makes its own 2D risers in Kent to install in NE03, whereas HE Simm is installing larger 3D risers made by Pinnacle in NE02. “It allows us to see which option is more effective,” Mackell says. “This information will be great for lessons learned when looking at future projects.”
The hotel-style risers are located on the corridor-side of the apartments. The 3D ones are tall enough to cover three floors, and pass through a glass-reinforced plastic (GRP) floor-grating at each level. These are loaded out with cranes through holes in the slab.
The 2D risers are brought in to the floors via a goods hoist and fitted to the riser walls, with a GRP grating cast into the floor slab by the frame contractor. Each 3D module can be brought on site and dropped into the building in the space of a couple of hours, Mackell says, whereas before it would have taken more time for several workers to install risers across multiple storeys.
Innovative thinking at NE02/NE03 extends to drylining, with Sisk aiming to use prefabricated plasterboard partitions from Platt Reilly. The offsite-manufactured partitions are designed to slot into floors. “But as you can imagine, just from a fire-safety perspective, a huge amount of testing is going on, to get approval from certification bodies and buy-in from consultants, contractors and the client,” Mackell says. “There are several wall types. These are fully tested and walls that require a fire rating do achieve this.”
The new buildings taking shape may well feature prefabricated drylining partitions “in six months’ time in the upper floors”, he says. “We’re looking to use the prefabricated walls on this project. And if it’s a success, I suspect we’ll be using them on all the floor plates in our next project.”
Cutting facade reporting times
Sisk says it has been using a digital tool with BIM 3D-modelling data to cut the NE02/NE03 precast-facade reporting process from “days to minutes”. A dashboard tracks the progress of offsite fabrication, delivery and installation.
The contractor found that limitations in software and file formats hindered the effective use of the 3D model in reporting on previous jobs. This led to inefficiencies in monitoring offsite manufacturing, requiring teams to rely on updates from multiple sources, including non-standardised Excel trackers. Real-time visibility of the production status of each facade panel was lacking, causing time-consuming tasks.
Previously, the project team would spend up to 72 hours compiling, updating and reporting on more than 1,700 individual precast facade panels. The new approach is up to 72 times faster and delivers time savings of up to 98 per cent compared with the traditional process, Sisk calculates.
The firm adds that the success of the reporting tool has resulted in “substantial interest” from undisclosed sources on implementing additional asset-tracking capabilities for various building components, including offsite-manufactured windows and their installation.
“Moreover, this has sparked curiosity in establishing a tracking system for riser modules’ installation,” the firm adds.
The design and construction of NE02/NE03 has a distinct security dimension. Because their location – 400 metres from Wembley Stadium – is a potential target for terrorism (as shown by the November 2015 terrorist attack at the Stade de France in Paris), the envelope design was undertaken in consultation with the Metropolitan Police’s architectural liaison officer.
Planning documents from Brent Council in August 2016 stated that the new buildings must be “suitably resilient against terrorism” to withstand an explosive blast, the exact intensity of which is classified on national security grounds.