星空传媒

It was a site bounded on four sides by roads. The underground car park extends right to the edges of the plot beneath a three-storey office building. The rock head, mudstone, is 3m below the surface.

The solution is a novel one. Excavate a 5m trench round the perimeter, lower in sheet piles, pour in 1.5m of concrete to hold them in place. Excavate out and paint up the sheet piles to form the walls of the car park. Well, it's not quite that simple. There was some clever sealant stuff going on in the pile interlocks.

For design and build contractor Houseman and Falshaw, this meant savings of 拢80,000 or more on a 拢5m contract and a much quicker build-time. Not to mention a quiet life for the neighbours. "You can't put a price on the degree of disruption we saved the other people on the estate," says contracts director Clive Mitchell.

The contractor is building the three-storey office, dubbed 'The Lens' due to its curved glass facade, for Hornbeam Park Developments which owns much of the former ICI works site. Developer and contractor negotiated this contract, having worked well together on previous jobs on the park.

Since the developer owns the three minor roads surrounding the site, Houseman and Falshaw could have opted for a conventional reinforced concrete solution.

"The problem is cost and time," says Mitchell. "Because you are taking up all of this site, you need such a big working space to be able to construct the concrete wall."

This would have involved excavating a 3.5m deep trench around the edges of the 65m by 35m site. The trench would extend outside the site boundary to accommodate the toe of a concrete wall. Battering back at 45 degrees would eat into the surrounding roads.

A temporary cofferdam was a no-no since silent piling 鈥� or vibropiling 鈥� would not have penetrated the mudstone. And driven piling is far too noisy for a site with near neighbours, the developer's existing tenants.

Houseman and Falshaw's structural engineer Hill Cannon came across Hydrobarrier from Corus. Basically it's sheet piles. What makes them special is the work Corus did on sealant systems for the interlock. It makes them waterproof instantly [see below].

John Theos, Hydrobarrier's business manager, says the seal alone would provide a basement suitable for some car parks. This would be equivalent to BS 8102 grade 1 which allows for some leakage. There are four grades in all, grade 3 being habitable. For the Harrogate site, Corus's contractors will make it grade 3 by returning to shot blast the piles, weld the interlocks and paint.

This job only required a low-grade sealant since the regular water level is at just 0.5m from the bottom of the piles. So why not use bog-standard piles? Because with standard piles, some water would be bound to leak through which would effect the quality of the welding: "You will get holes in the weld," says Theos. "It's a crappy weld basically. You have to keep going over it. It doesn't look good and it's not an efficient way of working."

Before excavating the 5m trench for the piles, Houseman and Falshaw dug a number of trial trenches and left them to stand. It had to make sure the clay walls would not collapse at such a depth.

Meanwhile Corus was welding the piles together in fours, reducing installation and welding-on-site time and ensuring three out of four joins happened in factory conditions. This caused some fun at first on site, as Corus' contractor had to experiment with the way it was slinging the pile panels so that they hung vertically. The contractor installed girders over the trenches to control the line of the wall.

"Once we had ironed out the practicalities, things went well," says Mitchell.

No reservations
Despite Corus' assurances that the piles won't leak, Hill Cannon specified concrete as the backfill rather than granular material. "The engineers were concerned that if you get a build up of water, that could cause problems. So for a few thousand pounds, it was worth concreting," says Mitchell.

The contractor used granular material to temporarily fill the gap at the front of the piles. Ground anchors will hold the piles in place while the contractor excavates and constructs the basement. And once the ringbeam and crossbeams are linked, the anchors become redundant.

Mitchell reckons using Hydrobarrier saved in the region of 拢50,000 on temporary works and 拢30,000 on the difference in cost between piles and reinforced concrete.

Would he use it again? Mitchell points out that the success of this particular solution is dependant on the stiff ground conditions. But he says: "If we came across this problem, I'd have no hesitation. I think the system is right for the job."