The contractor is redeveloping the site of one of the world’s first ever psychiatric facilities, which sits above the Metropolitan line, adjacent to a Grade II-registered garden and near one of Crossrail’s largest operations. Madness, surely?
Project: 8 Finsbury Circus redevelopment
Client: Mitsubishi Estate Company
Contract value: £65m
Contract type: Bespoke
Project manager: Stanhope
Main contractor: Lendlease
Demolition subcontractor: Cantillon
Concrete subcontractor: AJ Morrisroe
Steelwork subcontractor: William Hare
The thirst for bigger, better, more energy efficient office space in the City of London seems unquenchable.
As the financial services sector throws off the shackles of recession, the scramble to develop top-notch space to accommodate expanding companies has reached fever pitch.
The area of the city sandwiched between Moorgate and Liverpool Street stations is proving popular with developers.
Contractor Lendlease is involved in the redevelopment of one such office at 8 Finsbury Circus, overlooking the largest public open space in the City of London.
Being built for client Mitsubishi Estate Company, the office will replace a 1980s-designed block, the layout of which had become obsolete over the years.
Method in madness
It is located on the former site of the Bethlem Royal Hospital, which was one of the world’s first psychiatric facilities once notoriously known as ‘Bedlam’.
This project, however, has avoided such chaos by planning its logistics with exceptional care.
“It is a complicated project with difficult access,” says Lendlease project director John Chesters.
“We are hemmed in on two sides with a Crossrail site directly opposite.
“That site has meant that we haven’t been able to use Finsbury Circus itself for most of our deliveries with Crossrail taking priority.
“We have had to use South Place to bring in most of our materials.
“It is a much busier road so staying on top of deliveries and managing those interfaces between us, pedestrians and road users has been challenging.”
The old 1980s ‘River Plate Building’ covered the wedge-shaped site and featured a central atrium through the centre of the building.
This created a doughnut effect through the floor plate and with office rental prices so high in the City the client was keen to make the most of its footprint.
With Lendlease on site at the end of October 2013 specialist contractor Cantillon was brought in to carry out the hard demolition in January 2014.
“Normally a client will have the demolition under way by the time we arrive on site, but here it was felt that the location warranted a demolition manager too,” Mr Chesters says.
Some 23,500 cu m of demolition arisings and muck away have been removed from the site, most of it managed through the small pit lane established on South Place.
“This would have required extensive propping which would have interfered with demolition, excavation and new construction. There was too much risk in that”
John Chesters, Lendlease
“We haven’t been allowed to close the footpath so it has been difficult to manage the heavy footfall from Liverpool Street station,” Mr Chesters explains.
The new building will feature 11 levels overall with two reinforced concrete basements, a ground floor, then eight storeys above.
Driving out risk
Initially there were plans to retain one section of the north elevation, jacking three floors of the retained façade to win an extra 3.5 m of storey height, but the difficulty in propping and jacking the façade effectively and safely forced the team to find an alternative.
“The original brief was to retain four floors of the façade and then jack the top three floors to insert two courses of stone,” Mr Chesters says.
“This would have required extensive propping which would have interfered with demolition, excavation and new construction.
“There was too much risk in that. We proposed removing the top three floors of the north elevation, storing it and rebuilding with it as the new façade rises.”
Thankfully the planning authority agreed and allowed the Lendlease team to remove the section of wall and work unhindered, getting the basements built and concrete cores slipped.
“We looked at reusing existing piles and adding to them or installing a completely new piled solution, but in the end opted for the reinforced concrete raft. There was less risk involved that way”
John Chesters, Lendlease
The basement has been excavated to depths of between 9.5 and 10.5 m below street level with the existing basement retaining walls left in position.
The perimeter to the southern section of the plot is supported by 14 to 16 m-long sheet piles which stretch through the overlying gravel deposits and into the London Clay at around 7 m.
To the north and west, props helped support the work. A new 1.5 m-thick heavily reinforced concrete raft has been cast at the base.
With no piling work other than the steel sheets, this is the only foundation for the building which continues as a concrete frame to ground level.
Here the construction method shifts to a concrete core and structural steel solution.
“It was going to be piled,” Mr Chesters says. “We looked at reusing existing piles and adding to them or installing a completely new piled solution, but in the end opted for the reinforced concrete raft. There was less risk involved that way.
“We didn’t need to survey and justify the existing piles. The concrete raft gave us more surety in the programme.”
Crossrail’s concrete pact
Pours of up to 400 cu m were pumped from street level.
Fortunately the Lendlease team was able to negotiate a limited access agreement with Crossrail so that it could pump the bulk of the concrete from Finsbury Circus itself.
Specialist contractor AJ Morrisroe carried out all the reinforced concrete sub and superstructure work including the slip-formed main core and remote staircase core that feeds the building (see box).
The team also installed the thrust blocks that were needed to hold the series of heavyweight props which held the basement walls during construction.
From ground floor level and above steel takes over with steel contractor William Hare installing the beams and columns including the 16 m span beams that are hung at one pinch-point.
These allow the services to be threaded through the web of the beams.
Work on the 110-week contract is due to complete at the end of 2015 and for his own sanity Mr Chesters is focused on bringing home the scheme without any hiccups. No one wants to see Bedlam returning to the site.
Slip formed cores make perfect sense
The Morrisroe team that undertook all the reinforced concrete work on the new block at 8 Finsbury Circus has used a slip form technique to complete the stair and elevator cores that stretch to a final height of 50 m.
Slip forming became the preferred solution thanks to its speed over other methods such as jump forming, despite the intricacies of the shape of the core.
“It was a programme decision to use slip forming techniques to bring the cores up,” Mr Chesters says.
“It was quite an awkward slip. There is the main core which is fine, but then there is also a remote staircase that needed to come up at the same time so that the steelwork contractor could follow on behind.
“It was tracked a few metres before the Morrisroe team was able to bring in the trailing work platform.”
And the decision must clearly have worked in Lendlease’s favour. The team was able to bring in steelwork contractor William Hare shortly after Christmas, helping keep the project on track for completion.
Supply chain trust
It’s not just the retained section of listed façade that the project team has to work on.
Across the envelope of the building there will be a total of 15 different cladding types including Portland stone, precast concrete, glazed curtain walling, zinc and slate.
All of the interfaces between each type needs to be carefully managed and controlled. It is a task which Mr Chesters says underlines the strength of the relationships between each specialist installation team.
“It is about balancing the package split and the interfaces between them,” he says.
“We like to get our people in as early as we can, certainly on this project, so that they can look at those interfaces and deal with them accordingly.
“We have meetings with these specialist contractors to talk through any issues that may arise.
“It means we develop a tremendous amount of trust within our supply chain.”
The Lendlease team also has an in-house façade specialist who can help track any potential issues with the packages.
“Wilkinson Eyre, the architect and ourselves have design teams that are fully integrated with the façade delivery, but it is still vitally important that you trust the supply chain and its specialisms,” he says.