In south London Galliford Try is approaching its eighth year on a residential development by the Thames that now faces the trickiest phase of work yet.
Project: Building 6B, Riverside Quarter, Wandsworth
Client: Frasers Property UK
Contract value: £63m
Main contractor: Galliford Try
Concrete frame subcontractor: O’Halloran & O’Brien
Start date: January 2017
Completion date: January 2020
If practice makes perfect, then the Galliford Try team working on a residential scheme in Wandsworth should have few worries about hitting the client’s ‘zero-defect’ target.
After more than seven years, its work to deliver badly needed homes in the London borough is finally coming to a close. Not that the team has been dragging its heels; rather, it’s the sheer number of phases involved that has kept Galliford Try firmly anchored to the Riverside Quarter site, which will provide almost 550 homes on the South Bank where the River Wandle empties into the Thames.
Delivered over four separate phases, three of which have been handed over, the team is currently focused on the final structure: Building 6B. This 14-storey block will contain 172 apartments plus retail / leisure space for developer-client Frasers Property UK, with the apartments split 50:50 between private purchase and affordable homes.
Most awkward of the lot
It is a difficult site, hemmed in by the Wandle, the Thames and the development’s previous phases, leaving the project team with its work cut out to manage the construction.
“This is probably the most awkward plot to build so far,” says Galliford Try senior project manager Tim Stables. “It is right alongside the rivers and is the last section to be developed.”
“There were hydrocarbons present across the site. We had the muck-shift closely monitored for contaminants before transportation from site”
Tim Stables, Galliford Try
That riverside location and the site’s history as an oil terminal and Victorian milliners factory has been reflected in the groundwork that been carried out on the project. Around 6,000 cu m of contaminated ground has been removed to landfill across the entire scheme, with this final section accounting for some 1,400 cu m of this material.
“There were hydrocarbons present across the site,” Mr Stables says. “We had the muck-shift closely monitored for contaminants before transportation from site. It was either sent to licensed landfill or treated at a waste transfer facility in Surrey.”
Client Frasers Property UK has incorporated a single community heating network in the scheme that will supply the entire site through a combined heat and power system. There is a ground-source heat pump array that is connected to open-loop boreholes, which provide groundwater to internal heat pumps and exchangers, then on into the apartments.
The apartments themselves are heated through a wet underfloor heating system that is regulated by a unit fitted in each apartment, offering some localised control in a building that is designed to have a steady ambient temperature throughout. “There are no other heat sources for the apartments and so they might take a little getting used to for some residents. The apartments are very well insulated,” Mr Stables says.
Built to meet Code of Sustainable Homes Level 3 standards, the development features ‘brown’ roofs – the latest generation of ‘green’ roofs with bird perches and insect stations. “The roofs are slightly different to ‘green’ roofs in that they provide areas to encourage wildlife and bird colonisation,” Mr Stables explains. “These aren’t just flat roofs covered in plants the substrate has to be sculpted at different thicknesses to encourage the development of different plant species.”
In common with the earlier phases of the project, there are secant pile walls that form the basement car park. These walls have been installed progressively across the four phases to depths of 13 m. Every successive phase is launched from a female pile installed during the previous.
“Each of the buildings has a secant piled wall basement box,” Mr Stables explains. “As each stage is built, the basement is extended by launching the new wall from a female pile in the existing wall. Once the extension has been fully completed, we come back and remove the temporary wall so that the basement continues across the whole footprint of the site.”
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The two-storey basements were propped using 1.2 m-diameter Groundforce Shorco props spanning as much as 30 m across the excavation for Building 6B. In earlier phases these spans have measured up to 49.5 m.
There are a further 60 cased CFA loadbearing piles of 900 mm-diameter installed through the layers of made ground, riverbed gravels and ending in the London Clay layers beneath.
The main basement slabs and the podium slab of the building’s reinforced concrete frame are 325 mm thick, but the team has used post-tensioning techniques across the upper floors of the structure. These 225 mm-thick PT slabs are in position from levels 1-14 before reverting to a 325 mm-thick traditional reinforced concrete roof slab.
“It was always going to be a concrete frame on this project. The client prefers concrete because it performs so well acoustically”
Tim Stables, Galliford Try
“Our frame subcontractor O’Halloran & O’Brien uses a team from Structural Systems, which specialises in post-tensioning,” Mr Stables adds. “It was always going to be a concrete frame on this project. The client prefers concrete because it performs so well acoustically.”
The post-tensioning work is difficult at the slab edge, Mr Stables concedes, particularly around the balcony brackets. But it offers time savings over traditional methods. “The post-tensioning worked really well. It is very quick to install once the team has everything set up,” he says.
The positioning of the tensioning straps has been marked on the underside of the slab. Lines of blue paint that criss-cross the soffit at every level offer guidance to those carrying out any future work.
Rarely spotted technique
There are two lift and stair cores in the building. Rather than jump or slip-formed, these have been brought up through the building on a floor-by-floor basis, O’Halloran & O’Brien electing to use a traditional building technique that is not often seen on site today.
“It wasn’t jump-formed; the core was brought up as the columns were brought up,” Mr Stables explains. “O’Halloran & O’Brien preferred to do it that way and it has worked well.”
The columns themselves continue through the building largely on the same grid plan from floors 1-14, with only a few minor deviations from that grid. The bulk of these columns are 250 x 100 mm ‘fins’ cast so that the columns can be hidden within the stud walls of the apartments. “It makes sense and when complete it looks great because the fins fit into the drylining – but it does mean you have to be accurate when you are pouring them,” Mr Stables smiles.
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The drylining details specify a galvanised metal stud. Here at Riverside the team has used a Knauf system, complemented with 15 mm-thick acoustic plasterboard on each internal wall, which is doubled up to 30 mm each side with mineral wool insulation fill on the party walls.
Many of the apartments will feature a ‘winter garden’. These are fully enclosed glazed balconies with horizontal louvered windows that can be opened from their mid-point to ceiling.
Elsewhere the cladding is a combination of rainscreen panels, rendered panels and curtain walling. The sections of aluminium rainscreen panels feature cold-rolled steel frames with mineral wool insulation, and are lifted into position before being fixed to the concrete frame.
With just a little over 12 months left, the project team is nearing the final handover that will bring nearly a decade of Galliford Try work to a close and deliver the final piece in the Riverside development jigsaw.
Thames fails to deliver
With views over one of the world’s most famous rivers and direct frontage onto it, the team unsurprisingly looked at using the Thames when delivering bulk materials to the Riverside scheme.
But despite Galliford’s best efforts, river deliveries proved too complicated. Tide times, heights, materials quantities and delivery flexibility meant that the team had to use conventional methods to bring materials to site.
The development’s proximity to the Thames has nevertheless had a significant impact on the team. Licences that allow cranes to oversail the river can be expensive. “We looked at having two cranes on this scheme but licences are difficult to get hold of and expensive if you are oversailing the Thames,” Mr Stables says.
“To be honest, having two cranes only really looked viable if we could gain access to the site from the riverside, but the client wasn’t keen on that. Instead we took a licence to oversail the River Wandle and use mobile cranes to complement the one saddle tower crane we have had whenever necessary. We can just about cover the site from that.”
All deliveries to the project were carefully directed through the development. During concrete casting, material was brought to site and pumped into position using a spider boom arrangement.