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Middlesbrough makes the most of its muck

CONTRACTS

REGENERATION OF a disused dock in Middlehaven, Middlesbrough, involved improvement, treatment and reuse of 200,000m 3of semi-liquid and contaminated silts dredged from the dock floor.

Middlehaven was once the heart of the expanding industrial town in north east England, but the dock and the older part of Middlesbrough are now run-down and isolated from the modern town.

Regeneration agency English Partnerships is carrying out a project to revitalise the area. The first phase involved construction of a 1.5ha engineered platform within the 8ha dock to create an attractive development site. The design and construct project was let to Harbour & General with the Babtie Group as designer.

The sustainable earthworks and remediation strategy aimed to minimise the amount of material removed from site and reduce imported construction fill. This meant improving material that would normally have been unsuitable using bioremediation, aeration, crushing, blending and cement stabilisation.

The surface layer of fill was originally placed to raise the dock above the former tidal flats. This had widespread low-level heavy metal contamination and occasional hot spots of organic contamination. This was underlain by soft laminated alluvial clays, alluvial sands and glacial till. Soil and boulders had been tipped on site, a number of buildings had been demolished and numerous foundations dug out.

The dock basin floor was covered by a layer of silt up to 4m thick. This had widespread lowlevel heavy metal and organic contamination and poor physical properties. The silt from one of the old 'finger' docks was even more contaminated.

Both materials were to be used as fill behind the new dock wall and so treatment was needed.

About 200,000m 3of silt was dredged from the dock. The semiliquid material was placed in temporary lagoons and improved using cement stabilisation and blending with an imported 6A type fill - a local incinerator bottom ash. The amount of cement and blend proportions were determined by site trials.

Moisture content was typically lowered to about 20% and leachable contaminant levels were reduced to below targets.

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