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Tree huggers

A small housing development on a brownfield site in Berkshire is making the most of a foundation system designed
for difficult ground conditions. NCE gets to the root of how to protect trees on an environmentally sensitive site.

The increased use of brownfield sites for new developments has made the job of foundation designers more complex as they attempt to work around existing obstructions or poor ground. Foundation specialist Abbey Pynford believes it has found the solution in its Housedeck system, which can be redesigned on the spot to take account of unexpected obstructions.

The latest application for the system is at a secluded, leafy development in Ascot, Berkshire, where developer Octagon is building three new luxury homes on a site that previously accommodated a single dwelling surrounded by trees.

Wary of damaging roots by building traditional ground beam-type foundations, the developer chose Housedeck, as Abbey Pynford's proposals manager Jeremy Jubb explains: "There are a lot of trees around the boundary, and they've all got tree protection orders on them. Conventional foundations or machine piling could go straight through the roots."

The company's system involves casting in-situ piles that are structurally integrated with the building's concrete floor slab. At the design stage, the optimum pile locations are calculated for the loading on the slab, and these are marked out on site, usually between four and five piles per square metre.

Pile design is not dictated purely by the structural loads, but also by the size of the rig that can realistically be used on the site. This then dictates the maximum pile size and length that can be constructed.

For most jobs where trees are likely to be an issue, a small or medium-sized rig will be used. The Ascot job was initially designed with 36 piles, each 300mm in diameter, and constructed to depths between 6m and 10m.

If obstructions, such as tree roots are anticipated, hand augering is undertaken at the pile locations to identify whether there will be a problem. At the Octagon site hand augering is being carried out to a depth of 3m to be sure that any tree roots are picked up. If the ground is clear at this depth, a piling rig finishes off the job and the in-situ reinforced concrete pile is constructed in the traditional fashion.

So far, the Abbey Pynford team has not discovered any tree roots, but if one should be located during the hand augering, the company's technical coordinator, Devji Bhuva, is on hand to identify a new pile position. "The reason we hand auger is to make sure the pile locations we've designed don't clash with any tree roots," he explains. "But if they do, we can redesign it on the spot."

If a new pile location is required, Bhuva calculates the best position and runs a computer model that recalculates the loads throughout the slab using finite element analysis. "If it happens that we can't get a pile in, we may have to change the thickness of the slab or put in more reinforcement locally," he explains. "Or we may have to replace one pile with two piles."

The Housedeck system does not require pile caps, so any redesign required can be taken up purely by adding extra piles or by changes to the slab, which can be easily accommodated when slab construction is under way. As a result, the system gives the developer far more flexibility than other foundation methods, which cannot usually be changed if obstructions are found.Abbey Pynford has designed various versions of Housedeck, suitable for differing ground conditions. All are designed such that, even if the slab sits on the ground, the entire load in the slab goes into the piles.

If the ground is likely to heave (as is likely with London clay), or if gases are present in the ground, the slab will be built above a void. But if there is no need for a void, the slab is built directly on top of a concrete blinding layer.

In situations like the Ascot job, where tree roots are an issue, the blinding will be replaced by a shingle layer on top of a geotextile membrane, which allows water and oxygen to permeate through to the roots. The 225mm thick slab sits directly on top of the shingle.

After winning the contract for one plot on the site, Abbey Pynford persuaded Octagon to use Housedeck for the remaining two houses, even though the ground is very good, so piling would not have been the obvious choice from an engineering perspective.

"People think that piling is expensive, and it's a specialist item," says Bhuva. But, according to Jubb "the actual cost is about the same, and we're saving Octagon six weeks in construction time".

"We're getting more enquiries for similar situations," he continues. "Developers realise that if we hit an obstruction – whatever it is – we can redesign the slab to suit. It saves messing around and breaking out the obstruction."

Developers realise that if we hit an obstruction - whatever it is - we can redesign the slab to suit.

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