Lendlease’s work on Land Securities’ £170m mixed-use development saw contractors working yards from active tube lines and dealing with the tight logistics of one of central London’s busiest areas.
Project: Kings Gate & The Zig-Zag Building
Contract value: £170m
Contract type: JCT Design and build
Main contractor: Lendlease
Architect: Lynch Architects
Structural engineer: Pell Frischmann
Start date: October 2012
If a forest of cranes, contractor hoardings and redirected pedestrian traffic indicates the buoyancy of the local construction market, then the area around London’s Victoria station must be in rude health.
In common with many parts of the capital, there seems to be no end to the demand for commercial, retail and residential developments, particularly in those areas where contractors are working on travel hubs for Crossrail.
Victoria is no different and in the shadow of the offices for Westminster City Council, contractor Lend Lease is busy constructing a £170m mixed-use development that will provide 100 luxury apartments as well as more than 33,000 sq m of office and retail space.
Split into three main structures, the Kings Gate & The Zig-Zag Building development will also host a new electricity substation for distribution company UK Power Networks.
Mammoth project complexities
It is a complicated job that the Lend Lease team, with project director Ian Ronchetti at the helm, has been working on since it tendered for the scheme in March 2012.
“We have worked with the developer Land Securities on various projects in the past and have developed a good relationship,” he says.
“The new power station will serve the whole of Victoria. It was one of our key aims to get that handed over as soon as possible”
Ian Ronchetti, Lend Lease
“This scheme was taken under a single-stage design-and-build contract with the design at D+ stage. We started on site in October 2012, so there was plenty of work left to do.”
Split into three structures, the development features the Zig-Zag Building with four levels of basement, retail units and office space on levels 2 to 12; the Kings Gate building with its 14 storeys of residential space; and five levels of basement parking and retail units on the ground floor.
The new electricity sub-station sits between the two (see box). “The new power station will serve the whole of Victoria,” Mr Ronchetti explains.
“It was one of our key aims to get that handed over as soon as possible. It’s not due to go on line until after the buildings have been fully completed, but we needed to get it out of the way.”
Unsurprisingly for a central London project, the team has been hampered by the difficult location of the site.
Hemmed in by roads, existing offices, a school and London Underground’s Circle and District line, the team has been closely monitored during its work to construct and excavate the basements to the buildings.
Alongside the basement for the Zig-Zag building, the tube line runs through a brick arch tunnel built in 1868.
Working a metre from tube tunnels
With the site team excavating some 15 m below track level and within 1 m of the tunnel at pinch points, working methods and instrumentation monitoring arrangements were agreed with London Underground before work commenced.
Such was the importance of the scheme that LU had a resident engineer working on the project with the authority to halt work if it affected any of its lines or equipment.
“There are no bearing piles at all. The base is very heavily reinforced to resist heave”
Ian Ronchetti, Lend Lease
Temporary supports were placed alongside an existing retaining wall before a secant pile wall was installed around the whole site.
These 900 mm-diameter piles were driven to 25 m below ground through the made ground and river gravels into the underlying London Clay deposits.
Raft foundations support the buildings with a 1.7 m-thick base slab being cast in the Zig-Zag Building around a nest of 40 mm-diameter reinforcement steel at 100 mm centres both ways.
“There are no bearing piles at all,” Mr Ronchetti says. “The base is very heavily reinforced to resist heave.”
Groundwater level sits just below the LU track formation level and so the basement is waterproofed using water-retaining concrete liner walls, a delta drain system with a block wall constructed in front of that. It’s a belt and braces approach for which Mr Ronchetti is unapologetic.
“The last thing we need is to be coming back because there is some water ingress,” he says. “We looked at the design and the economies of any maintenance regime and decided this would be a better solution.”
Across the open excavation for the Zig-Zag building there are three levels of strutting supportting the sides of the huge excavation, but the work that has been put in on the Kings Gate building is very different.
Here, the team is using a top-down construction method to build its basement and spring the upper floors of the development at the same time, a method that has helped trim three months from Kings Gate’s overall construction time, according to Mr Ronchetti.
“The last thing we need is to be coming back because there is some water ingress”
Ian Ronchetti, Lend Lease
The team has again installed a 900 mm-diameter secant pile wall to a depth of 25 m, but it has also installed 2.4 m-diameter bearing piles to a depth of 40 m, through a steel casing and toeing into the chalk layers far below the London Clay deposits.
Concrete is poured into these bearing piles to a given depth before huge steel plunge column sections measuring up to 19 m and weighing as much as 18 tonnes are installed through a guide frame and set at the design depth.
The bored hole above top of concrete level is filled with pea shingle before the casing is removed, leaving the steel column section and bearing pile.
This enables the team to begin to cast the buildings slab at ground level and launch the frame for the upper levels, while excavating and casting the basement slabs and 1,200 mm-deep raft.
Heidi lends her long helping hand
Excavating the clay from Kings Gate basement is carried out by ‘Heidi’, a telescopic excavator with a digging reach of 21 m.
Across the whole development the team has been averaging 30 lorry loads of muck away each day, a difficult task to co-ordinate in the centre of London, but one the team has managed around the delivery of 150 cu m of concrete.
With more than 12 months to go before the project is handed over the team is focusing on bringing the frames to full height.
There’s a long way to go before the area can boast this newest addition to its property portfolio, but the Lend Lease team is making sure it is on sure footing when it does.
Powering up Victoria
As part of the development, the team is building a new substation for UK Power Networks that will house two 90-tonne 132 Kva transformers. These are set to serve the whole of Victoria and the team has worked hard to get them handed over to UKPN.
In common with the other sections of the project, the sub-station is set on a reinforced concrete raft foundation with a concrete-framed box built around the transformers.
The whole substation has been encapsulated in a steel box, which acts as a Faraday Cage to prevent electric current from passing from it.
The 15 mm-thick steel box was constructed using thinner 3 mm steel sheets placed across it and then tack-welded into position to build up the final thickness.
“There were manual handling advantages to using the thinner sections and the tack welds are quicker. It made it easier and safer to install,” Mr Ronchetti explains.
Bearings beat tube traffic vibrations
With both buildings being so close to the Circle and District underground line there was a strong likelihood that unwanted noise and vibrations might be transferred through to the building superstructure.
Obviously in a development boasting high-end residential and office space, any such noise transfer would be less than welcome, so the main sections of the two buildings sit on a series of bearings that will absorb noise and vibration.
In the Kings Gate residential block, these bearings are set at ground-floor level so that everything above that level is isolated, while on the commercial Zig-Zag building the offices above second-floor level are isolated.