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SUBSIDENCE - Steel cased mini piles are underpinning bungalows on peaty ground in Scotland.Mike Walter reports.

Residents will soon be returning to the Greenhill Estate in Bonnybridge, near Stirling, as contractor Roger Bullivant has just completed main structural work for a large underpinning contract.

Properties on the 60-house estate are founded on ground containing peat and prolonged periods of dry weather in recent years had led to subsidence. Large cracks had appeared in the walls of many homes and floors had begun to drop.

Homeowners whose properties were affected had been offered compensation by the National House Building Council. Some chose to have their homes underpinned, but others had moved away. Around half the properties had been abandoned.

Bullivant was called in early last year to install heavy duty mini piles to depths of between 9m and 15m to transfer load from the houses down to a firmer sands and gravels.

The company decided to buy 31 empty bungalows and one house with a view to underpinning and refurbishing them for re-sale.

Senior staff from Bullivant hosted a public meeting to explain to concerned home owners on the estate what the underpinning work would involve and to assure them that no homes would need to be demolished. This meeting was followed by an open weekend for local residents and other interested parties to explain the underpinning process further.

The fi rm used a combination of 100mm and 150mm diameter steel cased mini piles, reinforced with concrete. One test pile was installed beneath each house before the underpinning began, to ensure that each building could receive the correct guarantees.

Building control warrants were secured in April 2005 to underpin seven properties and garages. Completion of this fi rst phase was followed by underpinning of a further 25 bungalows with structural work due to fi nish towards the end of December 2005.

The company is also renovating the properties and landscaping gardens, many of which had also subsided. Works are due to be completed by next month.

Bullivant director John Patch says: 'The bungalows are of timber frame construction supported by a reinforced concrete raft and a 600mm high brick support around the perimeter. Our underpinning operations involved drilling holes through the concrete raft and inserting mini piles through them. Steel tubes were driven to refusal and each was fi lled with concrete and a T16 reinforcing bar.' There are four different sizes of bungalow on the estate; from two bedroom semi detached properties to three bedroom detached. The smaller bungalows have had around 43 piles installed around their perimeter and at key positions inside the buildings. The larger properties have been underpinned in up to 70 places.

Mini piles were installed vertically using two bottom driven hammers;

a Grundomat pneumatic piling hammer which is driven using compressed air and a more conventional drop hammer for the larger piles.

Up to fi ve teams worked on five properties at once at the height of the underpinning works last autumn.

The Bullivant mini rigs were able to gain access through doorways and operate comfortably beneath room ceiling height.

'Piles were installed around the perimeter of each property to pick up the load from external walls.

Three lines of piles were installed internally at dwarf wall locations to provide additional support, ' says Patch.

Evidence suggested that the worst of the affected bungalows had settled by as much as 300mm and 13 of the homes were visibly tilting.

These properties required jacking up to their original position.

Timber frames were raised using hand pump jacks and new brickwork was installed in the void between frames and sleeper walls.

External brickwork was then reinstated.

Bullivant also installed piled foundations to support a culvert over a water course which was also found to have settled.

'I have not come across an underpinning contract quite like this before, ' Patch says. 'It was an enjoyable challenge to not only underpin but also to renovate and re-sell properties on the estate. It is good to see new residents moving in.'

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