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Curtailing cracks

An impressive track record of successes include not only domestic properties, but also historical and ecclesiastical structures.

However a humble bungalow provides one of the method's most convincing case studies. In 1990, BRE started monitoring a distressed bungalow at Frinton in Essex, as part of its research on shrinkable clays.

The property, built on high plasticity London Clay, and positioned beside a mature oak woodland, was evidently suffering both seasonal induced movements and localised subsidence.

BRE recorded differential vertical movements exceeding 100mm and lateral movements of 66mm, resulting in severe cracking.

In spring 1993 the local authority, which owned the woodland, felled the nearest oak tree and pollarded others nearby in an attempt to reduce the water demand. The bungalow now started to heave, more than recovering the settlement undergone since monitoring started.

A year later Hoopsafe was installed, although significantly the masonry was not repaired as BRE was continuing its monitoring.

Results from this have shown that, as expected, vertical movement has continued with seasonal ups and downs of up to 26mm superimposed on the general upward heave caused by the removal of the

tree. While post-tensioning had a beneficial effect, without repair of the masonry, there was a continuing, albeit reduced, change in crack width.

However following masonry repairs in October 1995, no further cracking has appeared des- pite on-going total found- ation movements.

This says Handley shows that 'Hoopsafe has achieved its aim of allowing the structure to

tolerate quite large foundation movements without distress.'

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