A HEADY brew of organic and inorganic contaminants is posing a problem for geotechnical engineers at the St Oswald's Park shopping and leisure development in Gloucester (EF Winter 04).
As well being close to the flood plain of the River Severn (which flows about half mile away), a major stumbling block to main contractor Balfour Beatty was that the site was been used extensively for tipping domestic refuse up to the early 1960s.
Subsequently redeveloped as a cattle market, hardstanding and roads had suffered severe subsidence over a number of years due to degradation of the organic refuse and general settlement.
The question was how to improve the ground under car parking, roads and service areas without spreading the pollutants at the 20ha site. This ruled out traditional vibro piling techniques such as stone columns which can create drainage paths for the polluted fill.
Instead, subcontractor Pennine Group and is using rapid impact compaction on the £160,000 contract. Treatment will provide ground in the upper fills with an overall bearing pressure of up to 30kN/m 2.According to Pennine geotechnical consultant Graham Ellery, the site is underlain by 3m to 4m of made ground, with fill materials comprising a grey/black slightly clayey sand of ash and a gravel of brick concrete sandstone with paper, metal, wire, plastic, wood and glass associated with the domestic refuse.
This overlays soft silty clays, sometimes with gravel, extending to depths up to 16m, below which are medium dense gravels resting on stiff Lias clay. Groundwater is found within the fill material and at the base of the fill material.
Ellery explains: 'We needed to be able to compact the voids within the fill material to its full depth, with a system that ensured no contaminated soils were brought to the surface.
'Further degradation of the refuse and ongoing settlement was a major worry, as it could lead to movements within the upper made ground below car parking, roads and service areas.' The site is being upfilled with 300mm to 500mm of crushed concrete and brick. Once the first pass of rapid impact compaction has been completed, imprints are backfilled with a tracked excavator and a second pass is carried out.
The area is then backfilled once more, before a final pass with a vibrating roller.
Compaction should allow the site team to build the car park surfacing without undue settlement. Drainage can be placed across the site without fear of excessive movement causing breakages and leakage.
Work is being checked and monitored using pre- and post-cone penetration tests, along with plate load testing and large scale zone load testing.
The eight week project to treat around 34,000m 2of land in the first phase of work was due to be completed before Christmas. Pennine may return to site in the New Year to carry out further compaction.