A massive, complex basement has been constructed in record time beneath one of London’s largest new residential developments
Thirty years after redevelopment first started in London’s Docklands, construction in the area is as busy as it has ever been. A mass of tower cranes festoons the skyline, but this time they are not being used to build office blocks but high rise, luxury residential developments.
Among these is Harbour Central, a 39ha development consisting of five residential blocks and a leisure centre, being built by Galliard Homes. The development, close to Canary Wharf, will eventually have more than 900 apartments spread over five towers, the highest of which – Maine Tower – will be 41 storeys tall.
Beneath them all is a very large single storey reinforced concrete basement, which will house car parking for the entire development using a car stacker system. This basement is approximately rectangular, measuring 150m long and 65m wide.
Central Piling installed secant piles to form the basement walls early last year, during a contract that also included all the piles for the main towers. The piling contractor left the site with the basement wall in place topped with a concrete capping beam, ready for contractor Modebest Builders to move onto the site in June 2016.
Modebest has a £40M contract with Galliard Homes that includes the basement construction and the reinforced concrete cores and frames for the towers, which range from 24 to 41 storeys. The contract also includes a 2m deep suspended second floor transfer slab; three low rise reinforced concrete cores and frames; and floors throughout all of the buildings. The contractor is also supplying and installing the five tower cranes that will be used during construction.
When Modebest arrived on site, the first challenge was to find a way to support the massive basement excavation as the dig went down to base slab level and below. “It is a single storey basement, but within that are excavations for the car stackers, and some very deep excavations for the pile caps underneath the towers,” explains Modebest contracts manager Michael Brennan. The excavation reaches 5.5m below slab level, equating to a total depth of 9.5m of excavation below the propping level.
The contractor investigated different support options with structural consultant Coyle Kennedy.
“One was to install isolated sheet piling around each of the rafts, and install raking props off them, but we didn’t want any restrictions – we wanted an open dig,” recalls Brennan. “Coyle Kennedy suggested flying props and developed the design and prop layout to suit, meaning we could get excavators in underneath and do an open dig, rather than use sheet piles and raking props.”
Modebest went to Groundforce Shorco to provide the support. The result was a relatively simple layout that consists of eight supports spanning the full 65m width of the excavation, and a further 10 props measuring between 5.3m and 41.7m in length across the corners. The props are spaced to avoid the locations of the cores and the five tower cranes.
At 65m, the main props are considerably longer than any the company had supplied before, as Groundforce major projects manager
Ajay Nagah explains: “The longest free-spanning prop we had previously achieved was 49.5m. The props Modebest required were longer than anything previously undertaken.
“Because of their length, there had to be a mid-span support to counter the sag due to self-weight, but there could only be one support on each prop to achieve the flying shore solution that would give Modebest the practicality and speed benefits it was looking for.”
Again, Modebest brought in Coyle Kennedy to design a simple support system that would counter any sag in the props without obstructing the excavation work. Its solution was a “goalpost” formed of plunge columns toeing in 8m below dig level with a steel section as the “crossbar”, from which the props were hung.
One stipulation that the contractor made to Groundforce was that every prop had to be designed to be lifted into position in a single lift. Each prop arrived in sections on three articulated lorries and was bolted together on the ground using mobile cranes, before being lifted into place using tandem lifts. The longest props weighed 40t.
These 65m long props are the shoring equipment firm’s 1.2m diameter MP250 hydraulic struts with
“Super Extensions”, which have a 250t axial capacity. The same specification was used for all the corner props, other than the shortest strut, which is a 610mm diameter MP250.
“We designed the layout for the worst case wall loadings, to allow the internal pits to be excavated in close proximity to the piled wall without having adverse effects,” explains Nagah.
Four of the props were fitted with the firm’s load cells to monitor in real time the actual load going through them.
For Groundforce, one of the biggest challenges was the logistics of designing and supplying the system in a very short time frame.
“We had only four weeks from initial enquiry to design and then delivery of 785 linear metres of steel tube,” explains Groundforce general manager, major projects Mark Whitmore.
The first props in its major projects depot, and calculated that there would be just 70mm of lateral sag over their whole length, giving confidence that they could be built on site and lifted in as Modebest required.
“They were very simple to put together, with only 16 bolts per joint,” says Brennan. “We were able to install two 65m props in one and a half days.”
The first props were installed in June 2016, with Modebest working progressively from one end to dig out the ground to slab level.
“The top 3m is fill which was removed prior to the piling works which were installed from minus
900mm, then you get an alluvial layer, and at around minus 1.5m you get into the gravel layer,” explains Brennan. “We started to hit clay at about minus 5.4m down, which was the bottom of our dig; so the majority is in sand and gravel.”
The contractor has made a start on the slipformed cores for the residential towers, with the first pour for the Maine tower due to take place once a massive transfer slab had been completed late last year. “The key is getting this transfer slab in, and then we’ll be pushing on with the post tensioned floors,” says Brennan