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Grounded

30 Jul 14 Balfour Beatty Ground Engineering has joined forces with Cementation Skanska to install foundations for a £768m development in Victoria. Meanwhile, at the other end of the scale, soil mixing techniques have helped keep a small drainage project on programme

London’s appetite for swanky high-rise developments shows no sign of being sated. The latest landmark development – right next door to Victoria Station – is the £768m first phase of an ambitious mixed-use development by Land Securities and the Canada Pension Plan Investment Board.

Known as Victoria Nova, the development consists of six buildings totalling 897,000ft2 of office, retail and living space. The main contractor is Mace. After initial site clearance by demolition contractor Keltbray, the piling joint venture of Balfour Beatty Ground Engineering (BBGE) and Cementation Skanska started foundation work in July last year.

Their task was to construct a double basement structure with a 445m long secant piled retaining wall around the perimeter of the site, followed by a series of 336 rotary bored piles, including over 270 with steel plunge columns. BBGE and Cementation carried out the work in 50:50 joint venture, both parties contributing to each element of the foundation works.

Ian Lovett, director with Cementation Skanska and project director for the joint venture, said: “The team working on this project has built up a strong reputation with Mace by delivering successful projects like London Bridge Place and the Shard.”

The construction method for the double basement was a top-down design, with the ground floor slab cast in-situ before commencement of the basement excavation.

Once the secant-piled diaphragm wall was completed, the piling joint venture started installing the bearing piles for the main building frames.

The piles range in diameter from 1.2m to 2.1m. Plunge columns were installed in 274 of them to provide internal support for the two basement slabs. Piles were driven to a depth of between 20m and 40m but those designed to accept the plunge columns were cut off 14m below ground level. The heavy steel plunge columns provide the main structural support for the building’s superstructure and must be placed with a high degree of accuracy. On the Nova project the columns, despite ranging in length from 14m to 16m and weighing up to 24 tonnes each, had to be placed to a tolerance of +/- 10mm.

With the bearing piles and plunge columns installed, construction could then continue both upwards as well as down. 

As the basement excavation progresses, the floor slab and intermediate basement slabs are tied-in to the perimeter secant piled wall with specially-designed horizontal couplers while also supported within the excavation by the steel plunge columns. Throughout the piling phase, a maximum of five rigs (two from Cementation; three from BBGE) were working on site. Despite the close proximity of Victoria Underground station, the piling team had few worries about encountering Tube tunnels or underground services.

None of the underground tunnels actually cross the Nova site, although one – a pedestrian access tunnel linking the Tube station concourse to the new Crossrail platform – will eventually be driven beneath. As work started on site last November Jane Towse, project director with BBGE commented: “The close proximity of the secant wall to the adjacent Victoria Station Upgrade works and the future Crossrail 2 tunnel has introduced some unique challenges for our design and construction teams on this project. “We have an excellent track record of working in joint venture with Cementation Skanska and everyone in the joint venture has worked really hard during the preparatory stages. We look forward to delivering an exemplary scheme.”

Tony Palgrave, project director with Mace, said: “Appointing the Cementation Skanska and Balfour Beatty Ground Engineering joint venture … was a natural decision considering the size and complexity of the project. “The joint venture has a record of working well and we are seeing the benefits of this relationship already with their approach to overcoming some of the interesting challenges we are facing on the design and implementation of the project.”

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Piling work was completed in late January.

In the mix

Scots contractor Ground Developments recently deployed its ‘deep soil mixing’ technique to strengthen the ground in an excavation for Scottish Water.

Main contractor Barhale was required to install a combined sewer overflow (CSO) chamber for a new sewer at Potter Place in Glasgow. The project stalled, however, when it was found that the base of the excavation could not provide sufficient bearing pressure to support the installation. Finding and delivering a quick on-site solution was vital to keeping the project on track, and Barhale found it in Ground Developments.

“The excavation couldn’t be taken any deeper due to restriction of the plant capability on site,” explains Ground Developments commercial director Kevin Mackenzie.

“Therefore our capability to deliver cost-effective solutions for the difficult soil conditions that we often come across in Scotland, and the specialist apparatus that we own and operate, meant that we could respond really quickly to solve this problem,” he adds.

Deep soil mixing involves mixing binder, either in a dry or liquid form, to poor soils to dry it or raise its bearing capacity. The company generally uses cement, lime and pulverised fuel ash (PFA) binders.

Ground Developments says its deep soil mixing method differs from traditional stabilisation techniques because it allows the binder to be used in very soft or contaminated soils in situ to depths of up to 25m.

Besides stiffening poor soils to increase bearing pressures, deep soil mixing is also used to create slurry cut-off walls for landfill sites, make improvements to flood defence systems, and dry out soils prior to excavation.

“We can either construct individual soil-mixed columns or undertake the process en masse to create an entire block of improved material,” says Mackenzie.

[This article first appeared in the June 2014 issue of The Construction Index magazine, which can be viewed in full at: http://epublishing.theconstructionindex.co.uk/magazine/june2014/]

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