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Ring beams arrest bungalow heave

VAN ELLE'S subsidence repair division is part way through installing its Hoopsafe system on a detached bungalow in Redditch in the West Midlands.

The bungalow is suffering from differential movement caused by mature oak trees 11m from the flank wall.

Foundations found at 2.5m down and site investigation revealed clays of medium to high shrink potential with desiccation anticipated at 2.2m below ground level.

With no heave precautions installed during construction, the foundations have created a root barrier resulting in lateral clay heave within the building's footprint.

Combined with the desiccation, seasonal movements over 10mm caused cracking in the structure.

Lateral movement has also taken place at damp proof course level.

The initial proposal was a heave protected external linear piled underpinning scheme. However, this was ruled out because of concerns over future differential movements as a consequence of partial underpinning.

Hoopsafe acts by inducing compression into a structure through a series of internal and external ring beams which are post-tensioned.

Acting as a girdle, it prevents lateral movement, and while not stopping vertical movement, the structure acts as one unit to negate differential movement.

The solution consists of a reinforced concrete beam 600mm deep by 450mm wide incorporating three, 15.2mm Dyform unbonded tendons, centrally placed within the beam.

Van Elle was due to complete the £35,000 contract by 30 December.

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