High-end office space is a rare commodity in London’s St James’s but a new development promises to deliver just that while bringing an architectural gem back to life.
Project: 7-8 St James’s Square
Client: Green Property
Contract value: £29m
Main contractor: Galliford Try
Project manager: GVA Second London Wall
Architect: Eric Parry Architects
Engineer: Price & Myers
M&E subcontractor: Mecserve
Start date: March 2012
It’s not surprising that developers make a beeline for St James’s in central London.
With its sky-high commercial rent putting the area well and truly into the real estate premier league, any opportunity to provide office space is highly prized.
Of all the addresses in the district, St James’s Square is probably the most keenly sought.
It is home to the headquarters of BP and Rio Tinto as well as think-tank Chatham House and gentlemen’s clubs the East India Club and Naval and Military Club.
It is also home to number 7 St James’s Square: an elegant Grade II-listed townhouse designed by Sir Edwin Lutyens. who went on to plan The Cenotaph and Thiepval Memorial to the Missing, alongside countless other First World War monuments.
It is on the site of this and number 8 next door that developer Green Property has called on Galliford Try to demolish existing buildings and provide new category A office space, while retaining the listed Lutyens building and constructing a two-storey basement beneath it.
The site itself fronts onto St James’s Square at its southern end, Duke of York Street on its eastern elevation and Apple Tree Yard to the north.
The Lutyens building sits in the south-western corner of the site and has left the site team with the complex challenge of constructing a six-storey L-shaped building and the basement alongside and indeed beneath it.
“This is one of the most complicated schemes I have worked on in 25 years”
Steve Simpson, Galliford try
Senior project manager Steve Simpson is all too aware of the project’s complexities.
“Here we have a very deep excavation, working on a tight site alongside another ongoing development, with a listed building that needs to be retained and underpinned while a two-storey basement is created beneath it,” he says.
“This is one of the most complicated schemes I have worked on in 25 years.”
Work on the site began in March 2012 after the Galliford Try team won the £29m design-and-build contract, taking the design from stage D to stage E.
The client had instructed JF Hunt to carry out demolition of the existing buildings down to the stepped ground-floor slab level, leaving the slab in place to help support the development on the site next door and the surrounding roads and properties.
That demolition contract was novated to Galliford Try when it took on the project and it brought in specialist contractor Keltbray to complete the balance of the demolition and install the first stage of the temporary works design.
“We have such a deep excavation compared with the development next door,” Galliford Try project manager Steve Brown explains.
“Ours is a two-storey piled solution, theirs is a raft. Our temporary works design needed to take into account the progression of their works as well as ours.”
Geotechnical consultant CGL carried out exercises modelling the ground conditions throughout the various stages of the development.
“The ground conditions meant we had to re-engineer in real time. It was all hands to the pump”
Steve Simpson, Galliford Try
But it was during this stage that the site team realised the ground conditions were markedly different from those they had initially anticipated.
“Post-demolition, we found that the level of the clays, gravels and silts varied enormously from those that we expected,” Mr Simpson says.
“We won this contract on the back of an excellent scheme for the sub-structure, but the ground conditions meant we had to re-engineer in real time. It was all hands to the pump.”
That complete rethink meant changing the pile design and array across the site.
Wall piles along Duke of York Street became secant piles of 1,000 and 900 mm-diameter, with males at 1,250 mm centres and female panels at 750 mm diameter.
“Generally the sizes were increased and strengthened – more rebar, more concrete”
Steve Simpson, Galliford Try
Those along Apple Tree Yard are a mixture of 880 and 780 mm-diameter piles and 1,000 and 900 mm-diameter male contiguous piles at 1,250 mm centres.
Temporary piles around the new UKPN substation being installed on the site were beefed up to 750 mm-diameter from 600 mm.
“Generally the sizes were increased and strengthened – more rebar, more concrete. The bearing piles were lengthened by around 3 m,” Mr Simpson explains.
The main building features a reinforced concrete substructure to ground-floor slab, where a steel frame with two main cores at the north and south ends of the project jumps from it.
To help push the project forward and regain some of the time lost during the groundwork redesign, the team rethought the construction of the two cores, deciding to slip form the north core and use a hybrid of jump form and precast twin-wall techniques to form the building’s south core.
“The rear wall is so tight against the Lutyens building that we needed to use the twin-wall system and then standard jump form the outside three walls,” Mr Simpson says.
Spans across the floor plate measure as long as 15 m with pre-cambered plate girders featuring flange thicknesses of 50 mm – manufactured offsite by Norfolk-based specialist SCWS – spreading the load of the composite slab system that has been installed.
Cladding the BREEAM Excellent steel frame building is a difficult mixture of glazed curtain walling on the sixth floor, aluminium cladding at fifth floor level and handmade bricks specifically made for the project to tie in with the surrounding buildings.
There are also elements of granite, lime render with limestone reveals and a complicated solid limestone projection at the Apple Tree Yard/Duke of York Street elevation.
The detailing in some of these areas is awkward and drainage has been installed behind the final façade with no surface fixing. This will add to the clean look of the building when it is finally completed.
“There are lots of deep digs in London but the complexities we have experienced on this scheme set it apart”
Steve Simpson, Galliford Try
“There are some difficult details on the scheme,” Mr Brown says. “The limestone projections and the handmade bricks need to be finished with lime mortar.
“It needs longer to set than conventional mortar and so we worked hard to get it off the critical path so we could press ahead elsewhere.”
The team aims to be off the project and handing the newly completed office over to the client by early autumn. It’s been a taxing scheme for the team but one they have revelled in.
“There are lots of deep digs in London but the complexities we have experienced on this scheme set it apart,” Mr Simpson says.
“It has been a challenge but these are the type of schemes we are good at. We pride ourselves in getting around these sorts of issues.”
Ship in a bottle underpinning
Working alongside a Grade II-listed townhouse is never an easy task. Working underneath one to create a car park and basement area for the eventual occupier can be even more hair-raising.
With no room for any sort of misjudgement, the site team needed to underpin across the footprint of the Lutyens building at number 7 before creating the car lift, parking basement and upper basement level that will eventually house a swimming pool.
In the construction equivalent of building a ship in a bottle, the Galliford Try site team sank a series of 7 m-deep underpin access shafts around the perimeter of the building.
This enabled them to install a series of piles beneath at the final basement level. Steel columns were then plunged into these piles, allowing the sub-structure to be built.
“These were big mining shafts that we needed to install so we could get beneath the building and start underpinning around the perimeter,” Mr Simpson explains.
“We are adding an extra one-and-a-half storeys to the building. We needed to pre-jack the building to compensate for any potential movement and install a concrete ring beam around the existing building.”
Substation sparks fast-track solution
Thanks to planning issues and the geotechnical challenges discovered during the initial stages of the project, the Galliford Try team needed to rethink some of its construction methods.
With the installation of a new electricity substation directly on the scheme’s critical path, the team used a top-down construction solution to help push its progress along.
The substation itself sits at ground level on the project’s Apple Tree Yard elevation, with three levels below it.
The site had already been excavated to two levels below the substation chamber level, so the team installed the piles and poured the floor slab before excavating beneath and building the subsequent upper floors of the substation structure from this lower basement level.
“Effectively we created a piled cofferdam and then built the section of the concrete frame to house the substation,” Mr Brown says.
“It meant the new sub-station was built and watertight before we had even started on the main basement excavation.”