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A level playing field

Use of an innovative piling platform has helped to fast-track foundations work on a new sewage works in Staffordshire, as well as meet the site’s access restrictions. Claire Symes reports.

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Having only one road providing access to a site is not normally a major problem, but when every truckload coming in or out has to cross a busy rail line, the logistics become more complicated.

It was just such an issue that proved a major constraint for MWH when it came to planning the foundations work for the new Clay Mills sewage treatment works in Staffordshire.

The solution, for what will become Severn Trent Water’s (STW) flagship facility when it opens in late 2014, was the first UK use of a new approach to piling platforms.

According to MWH, the combination of the platform and Bullivant’s proposal to use pre-cast piles helped avoid import of 6,000t of aggregates to the site and prevented delays to the piling programme from ready mixed concrete deliveries being held up at the level crossing.

Valued at £39M, construction work at Clay Mill’s is currently STW’s largest project and will deliver a cutting edge sewage treatment works that will provide the additional capacity to meet the demand of the growing local population.

“A piled solution was essential as large areas of the existing site had been filled or built up with spoil from previous excavations.”

The work is being led by MWH, but also involves other suppliers in STW’s framework agreement, including Costain, MMB and NM Construction. While the scale of the scheme and the access were both challenging, there was another complication - the work had to be carried out while the existing facility remained in use.

The currently used treatment facility, constructed next to the original Victorian treatment works, was built more than 40 years ago - and was one of the first major civil engineering projects Roger Bullivant worked on as an engineer before setting up his foundations business.

Bullivant’s contract with MWH was to install almost 1,400 precast concrete piles for the six new final settlement tanks (FST) - which will be 50m in diameter and 20m in height - the 6.5m high concrete structure of the 70m by 70m activated sludge plant (ASP), and the foundation slab for the 20m by 7m inter stage pump station.

Ground conditions at the site were varied with some gravel overlying a mudstone but, in some areas, much of the gravel had been extracted. “A piled solution was essential as large areas of the existing site had been filled or built up with spoil from previous excavations,” says Roger Bullivant contracts manager Will Payne.

“Use of driven precast piles removed the issue of getting concrete supplied onto site and also eliminated the problem of having to remove spoil from the site too.”

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The foundation design used a mix of 250mm by 250mm 500kN capacity, 300mm by 300mm 750kN capacity and 350mm by 350mm 900kN capacity square section piles driven to depths of 8m to up to 14m, depending on the location on the site.

Three preliminary test piles - one of each size - were installed at the site late last year ahead of the main works with each at a different location. The piles were tested to 1.5 times their safe working load and settlements of less than 6mm were recorded.

With the design proven, Bullivant moved onto site in February to start work. Piles for the tanks were installed in two phases. The first involved installing 26 300mm by 300mm piles to 8m to support the central core of the tank.

The second phase of work for the FST structures was carried out after MWH had constructed the earthworks for the conical shaped tank bases, which slope down to the central core that Bullivant had already installed the piles for.

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The second phase involved installation of 140 piles to a depth of 8m. The outer wall of the circular tanks was supported by 300mm section, 750kN capacity piles. The piling work for the tanks was fairly conventional but work for the ASP structure, which called for 640 350mm by 350mm 900kN piles to a depth of 7m, is where the innovative piling platform proved its worth.

“The piling mat for the FSTs was a conventional 6F2 material but for the ASP we trialled a new product called Roadcem,” says Roger Bullivant operations manager Alan Oakton.

“The system is a form of ground improvement but unlike normal limebased soil strengthening products, which fracture and break up when you pile through them, the Roadcem material makes the ground flexible so the pile can punch through but it doesn’t break up.”

The other added benefit for this scheme was that it avoided considerable volume of material being brought in over the railway line and taken back out of the site at the end of the work too.

“This was the first UK use of the product for a piling platform. It certainly helped to improve productivity.”

Oakton added that the system also meant that its piling rigs were able to work at ground level rather than raised up on a piling platform, which made the work safer.

The Roadcem was applied to a 150m by 150m area of the existing ground using a Wirtgen 2500 rotavator by contractor Hawk Group to treat the ground to a depth of 250mm. CBR testing was carried out to prove that the technique had improved the strength of the ground sufficiently for Bullivant’s 5000 series rig to operate safely.

“The material was expected to reach full strength within 28 days but was strong enough for us to work from within 12 days,” says Oakton. Bullivant estimates that £15,000 was saved on importing 6,000t of aggregates, before the savings of eliminating the need for geotextiles within the platform are added in.

It was also quicker - the Roadcem was applied in five days compared to Oakton’s estimate of two and a half weeks to construct a conventional piling platform. “The ground just needs to be rotovated again to return it to its normal state,” he says. “This was the first UK use of the product for a piling platform and it is something that Bullivant hopes to roll out to other projects. It certainly helped to improve productivity.”

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It is clear that STW is also pleased with how the overall foundations element of the scheme has gone - the water company presented Bullivant with its “Supplier of the Quarter” award for successful delivery of the piling work.

While the foundations work is now complete, the Roadcemformed piling platform is benefiting the follow-on trades in the form of level working platform for the construction of the ASP structure.

Elsewhere on MWH is working on building the six FSTs using precast concrete segments supplied by Ireland-based Carlow Precast and work is currently on target for the 2014 completion date.

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