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How to bear it all

Spotlight

Foundations specialist Roger Bullivant has completed the remediation of an area of weak, cohesive ground containing colliery spoil in Kettering. The land will now be used to develop five large industrial warehouses.

Three separate dynamic compaction techniques have been used to firm up made ground on the 40,000m 2 site.

The first UK use of Bullivant's Mega Probe cone enabled the creation of rock pillars across the site to carry foundation loads down to firm underlying strata. The Mega Probe's pointed steel cone is filled with lead and weighs 5t. It was repeatedly dropped from 10m by a 9000 series NRG rig. Several drops opened up a void in the ground some 3m deep which was filled with clean stone. Further dropping of the cone forced stone into the surrounding ground.

'Displaced stone also helps to improve the bearing capacity of the made ground, ' says Bullivant's Midlands ground improvement manager Andy Street.

A second dynamic compaction weight called a Multi-tamper was used to firm up shallow fill material of no more than 500mm depth on one third of the site and to compact a 3m layer of fill in a second area. The base of the weight features a 2m 2 plate containing nine protruding points. This was dropped from a height of up to 10m. The Multi-tamper was also used near recently formed rock pillars to tighten up ground and eliminate voids.

The final third of the site contained up to 1.5m of fill and was compacted by dropping a Flowerpot cone from a height of 10m. This features a 2m square plate with a flat, flowerpot-shaped cone.

'Use of the Mega Probe and the other two dynamic compaction cones worked as expected, but the initial design of the work was fairly complex, ' says Street. 'We had to carry out a detailed ground investigation that involved digging 25 trial pits to determine the depth of competent material across the site.' Ground improvement took five weeks and was completed in late June.

www. roger-bullivant. co. uk

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