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Freedom from the dock

Set against a gritty industrial backdrop of wharves, warehouses and Middlesbrough's famous transporter bridge, Middlehaven dock is being shaped into a new urban quarter. Andrew Mylius reports.

The sleek white ship moored in Middlehaven dock contrasts surreally with squat, mucky dump barges and a pair of dredgers toiling close by. Its elegant lines are accentuated by jagged piled walls and a veritable sea of wet, black silt, laid out over the surrounding land.

After dark the scene becomes stranger still. Women and men dressed to party traipse across the ravaged site and step aboard.

The liner is Middlesbrough's newest nightspot.

Christened the Tuxedo Royale, the floating club will, by next year, look less out of place.

Middlehaven dock is undergoing a transformation from derelict post-industrial wasteland to modern, mixed use, urban district. By the end of 2002, site owner English Partnerships, codeveloper Middlesbrough Council and regional development agency One North East envisage the dock will boast commerce, retail and housing. The Tuxedo Royale is a promise of things to come.

Reclamation of Middlehaven dock started in April 2000 and is due for completion in October this year. Work got under way with the demolition of abandoned steel and concrete framed industrial buildings. Main contractor Harbour & General then got stuck into the real meat of the project - recovering and remediating some 180,000m 3ofsilts contaminated with heavy metals and hydrocarbons, and remodelling the dock itself.

Middlehaven was opened in 1842 as a coal port serving the Stockton-Darlington railway.

Over the next century it was redeveloped five times to handle petrochemicals and materials for the steel industry. But growth in the size of ships and volume of shipping meant that by the early 1980s, Middlehaven was defunct.

The dock and surrounding area was left with widespread contamination. 'There was general contamination across the site, with local hydrocarbon hot spots, ' recalls Andrew Coapes, senior project manager with consultant White Young Green, English Partherships' agent. All materials on the site require some degree of treatment.

Reshaping Middlehaven calls for a huge amount of earth movement. Under a £12M lump sum design and build contract Harbour & General, with subcontractor CA Blackwell, is closing and filling three finger docks off the main basin. A sheet piled wall has been installed across the mouth of one finger at the north west of the dock already.

The contractor is now driving 22m sheet piles for a 220m long wall enclosing the middle finger.

A 220m long sheet piled wall right across the southern side of the main dock will incorporate the third finger into a larger reclaimed area. In all, 2ha of new land is being added to the existing 20ha. Middlehaven will be left with 7ha of water.

Harbour & General brought on board consultant Babtie to devise a reclamation and remediation strategy. 'The main advantage of a lump sum design and construct contract was that we were able to work together on methods for handling materials - there are a lot of movements and not very much space, ' comments Coapes. 'And from the client's point of view the benefit is certainty of outcome sum with limited risk.'

The huge volume of material required to achieve the reclamation placed a premium on minimising waste. To claw back as much material as possible from the local environment, and in a bid to improve dock water quality, Harbour & General's scope of works was broadened last year to include dredging 120,000m 3ofsilts from the dock floor.

A barge mounted 40t dipper dredger and a 45t long reach excavator operating from the quay wall are being used to carry out the work. Water level in the dock basin has been reduced by 4m to aid dredging, but the work is still being carried out through 6m of water. 'Wall stability wouldn't allow complete emptying of the dock, ' explains Harbour & General site manager Colin Embleton.

In addition to the dredged silts, the contractor has had to excavate 60,000m 3of old fill from the north finger dock, which was first closed off five years ago. Both dredged and excavated materials are contaminated. Seven hydrocarbon hot spots have also been excavated.

Despite locally high pollution concentrations, Embleton notes proudly, only 10 truckloads of material illegally fly-tipped on the site have required off site disposal.

Hydrocarbons are being removed through bioremediation and volatilisation. Dredged and excavated materials are laid out in 2m-3m high windrows across the site and periodically turned. This 'farming' allows naturally occurring microorganisms to break down organic pollutants, while volatile compounds escape to the air, explains Embleton. Remediation is being carried out to draft Contaminated Land Exposure Assessment standards under the watchful eye of the Environment Agency. Batches of material are tested for residual hydrocarbon contamination - the fastest rate of remediation achieved so far is six weeks. The slowest is six months and still going.

Heavy metals are not being removed but stabilised. 'Although concentrations of contaminants may be high, they need not necessarily be harmful to human beings if they are left where they are, ' notes Babtie technical director Colin Harding. To lock them into the soil matrix, 4% by weight of Portland cement is mixed with the silts in a specially developed mobile mixing unit.

Tight controls have been placed on the amount of contamination that leaches from the treated soils into the completed dock, which will be used for water sports. Tests carried out so far show that only a tenth of allowable leaching is taking place.

Reusing the silts for construction poses other challenges, though. 'Silt is a particularly difficult engineering material, ' comments Embleton. It is easiest to handle and 'gives optimum performance as a construction material with a moisture content of 28%'. Blending with 10mm-40mm granular material improves performance further still. Harbour & General has mixed over 100 trial batches - currently a ratio of 76% silt to 20% granular material and 4% of cement shows best results.

Much of the site will ultimately be open, recreational space.

However, across much of the site, the silt fill will have poor load bearing properties and Babtie and Harbour & General will prepare guidance on piling for the construction of buildings.

Contained within the fill zone, though, is a structural embankment designed to carry the development's principal access road.

The 350m long embankment is designed to highway specifications. Avoiding settlement is all the more crucial because it is fully serviced with electricity, communications cables, water, and a 200mm diameter high pressure oxygen main feeding local chemical and steel works. 'It is absolutely imperative there is no settlement in the structure, ' states Coapes. A 2.5m overburden has been constructed to ensure good compaction.

Harbour & General is importing granular fill from Tees port to make up a shortage of material.

Much of the coarse material being used, though, has been produced on site by crushing and grading existing concrete foundations. 'Some we have records for and others we are discovering as we go along, ' says Embleton. Like other materials in the reclamation, these are tested for contamination.

Across Middlehaven, the site will be cleared of foundations to a depth of 1m. Where housing will be built, in the north west quarter, a 300mm capillary break layer will be provided to rule out any possibility of residual contamination coming to the surface.

Earth moving and remediation is not the total sum of the Middlesbrough dock reclamation. Alongside the prodigious muck shift the contractor is repairing the existing dock wall to 2m below final water level.

Harbour & General is also facing the sheet piled wills with dressed stone salvaged from infilled areas of the dock.

Last month, with dock water levels stable behind a temporary bund, the original 100 year old dock gates were removed. They will be replaced with new mitre gates fabricated by Belfast shipbuilder Harland & Wolf.

A timber jetty protruding into the River Tees next to the dock mouth will also be refurbished, allowing Middlehaven's new residents to look out across the murky waters.

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