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Columns control dock silt

PENNINE VIBROPILING is using a traditional wet vibro technique to combat difficult ground in the heart of Liverpool.

The geotechnical contractor proposed using the method, introduced to the UK over 30 years ago, as an alternative to excavating 3.5m of silt and clay from the derelict Prince's Dock and backfilling with rockfill. Property developer David McLean Developments is building luxury offices and a hotel on the site as part of the city's regeneration of its dockland areas.

Previous attempts to remove this material had failed and so the area had been backfilled with a mixture of sand, demolition rubble and clay. And because of the water table is only 1m to 2m below the top of the filled dock, the fill could not be engineered in place. And as it was only placed in the last decade, it would not have responded to vibratory compaction techniques due to continued self-weight settlement.

Pennine proposed using a high powered vibroflot combined with water flushing to construct 1,400 stone columns, with a minimum diameter of 800m to depths of between 7.5m and 9.5m, penetrating the underlying sandstone bedrock. The combination of large diameter columns and the water flush tending to remove some of the silt in suspension ensures tight settlement control of the fill and the silt.

To confirm the long term settlement performance of the treated ground, in its present state and also following a surcharge of 37kN/m2, consultant Ove Arup & Partners carried out complex creep analysis and Pennine modelled a wide range of soil and groundwater environments.

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