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Dynamic combination


RECENT TRIALS by foundation contractor Roger Bullivant have shown a combination of vibro-stone columns and dynamic compaction to be a viable alternative to piled foundations for a range of ground conditions.

The firm carried out trials at two sites in Belvedere in east London and Lytham St Anne's in Lancashire. The company's RB Hardrock Raft system consists of vibro-stone columns, the tops of which are dynamically compacted with Bullivant's NRG single point tamper.

The resulting depressions, which link contiguously across the treated area, are infilled with crushed stone. The treated area is then compacted further with a multi-point tamper and roller.

Bullivant chief geotechnical engineer Steve Rogers says the result is a soil raft of high quality ground arching between the stone columns to transfer stresses down through them.

'Vibro replacement and dynamic compaction techniques are rarely combined, but the Belvedere trial has shown this package of ground improvement methods reduces settlement significantly, ' he said. 'The recorded settlement of the hardrock raft is one third of the figure calculated for an equivalent surface raft without any ground treatment.

'The trial indicates a wide range of soil types can be improved using these techniques, including weak alluvial and some organic soils. Ground at the site consisted of layers of soft alluvial clay, soft peat and loose sands, ' Rogers said.

Bullivant built the Belvedere trial Hardrock Raft last year and recently completed assessment of settlement. Vibro-stone columns were installed at 2.2m centres across a 10m grid to a depth of 4.5m, topped by dynamic compaction.

The area was rolled and loaded with a 10m by 10m soil mound to apply a stress of 40kN/m 2tothe soil raft for one year while settlement was recorded.

'We have been developing this idea for a number of years, ' said Rogers.'These trials have backed up our belief that a combination of ground improvement techniques can produce substantial reduction in settlement under a given load.

'Settlement of a similar structure under test loading at Lytham St Anne's, again in a combination of weak organic and alluvial soil, is still being assessed. But results so far are very encouraging with recorded figures far less than expected.'

Development of Bullivant's RB Hardrock Raft system has been helped by new items of vibrostone column and dynamic compaction plant built in-house. These include a vertically vibrating poker and the guided dynamic compactor.

The poker rig differs from conventional equipment by using hydraulic power to drive the mandrel into the ground rather than relying solely on its self-weight. This additional push force combined with vertical rather than horizontal vibration allows the stone to be pushed into the side of the shaft and produces a greater bearing capacity in the finished column.

The compactor enables areas of stone to more exact tolerances, said Rogers.'The combined effect of the vibro columns and dynamic compaction is a cost-effective alternative to piling, particularly in very soft ground.'

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