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CLEANING UP THE PAST

GEOENVIRONMENTAL ENGINEERING

As English Partnerships continues to vacuum up old coking works sites as part of the National Coalfields Programme, Damon Schünmann visits a site in the north east where remediation will soon begin to clean the ground.

One of the issues at the site of the former Lambton coking works, just south of Sunderland, is what to put where in the limited space available. It may seem strange to talk about restricted access work on a 64ha project. But when large quantities of contaminated ground must be shifted for treatment and certifi ed material laid in its place for a 350-strong housing development, a child's sliding tile puzzle picture immediately springs to mind.

'Although it looks like there is a lot of space to work with, in fact one of the issues for the site is the amount of space available, ' says Faber Maunsell principal hydrogeologist, Jonathan Welch.

'But we are trying to do as much here as possible at once, ' adds the company's principal engineer, Matt Agar. This means shifting some of the hydrocarbon contaminated material from the development area to a holding zone while clean clay is brought in from a borrow pit to replace it. The contaminated material will end up in the borrow pit in a giant merry-go-round of materials transfer.

The work is part of the National Coalfi elds Programme that is dealing with 101 sites and is due to complete its work in 2012.

As well as making the site suitable for a residential development platform, the early contractor involvement project, worth up to £19M, will clean up the site ready for country park and road corridors.

Faber Maunsell is the designer for client English Partnerships, with Carillion as the principal contractor.

Agar explains that the site area has had several industrial uses including coalmining, stockpiling coal from the Durham Coalfi eld, brickworks and clay pits, railways and yards, coke works and ovens and has had gasometers, pipes and storage tanks on it.

Large amounts of colliery and coke works waste materials have been deposited on the site contributing greatly to its current land form.

Some of that industrial past is being protected. 'There is a 17th century wooden wagon way associated with the colliery that has been buried to protect it from the environment, ' says English Partnerships communications manager Tom Hustler.

Agar says made ground across the site has resulted in a land raise over the natural ground surface level.

The area is extensively overlain by colliery spoil including shale, coal and coke deposits.

The made ground depth varies from 1m to 17m and this is underlain by up to 10m thick drift deposits of glacial tills, sands and gravels, alluvium and laminated clays.

Parts of it - mainly the coke works area - are overlain by materials including ash, blue billy and tar deposits. There are land drains and other below ground structures within the made ground, some of which contain liquid coke works waste.

'Our best guess is there is about 100,000m3 of material that ranges in levels of contamination, ' says Agar. 'We need a range of solutions for different areas of the site.' These options are still being reviewed and remediation has not yet begun in earnest.

But Welch says volumes are hard to quantify. 'We don't know how much contamination is here as the de nition changes for the different land uses and treatments proposed.'

It can be hard to predict quantities at a site that has seen so much mixed industrial activity. Welch adds: 'There are light hydrocarbons that concentrate at the groundwater table and are mobile. The material affected by them is more than expected and there are concerns over these volumes. But the good thing is it's easy to treat with bioremediation, which is one of the cheaper options, and is sustainable as it destroys the contamination. There is no chance it could be left and become re-mobilised in the future.'

Although the team is still deciding on appropriate remediation techniques, one idea was to use the natural geology as part of the process. 'There is a buried valley in glacial deposits and the geology is good as it stops the movement of contaminants. This buried geomorphology directs its ow towards the Herrington Burn, ' says Welch.

'We have considered thermal desorption, which will heat the soil and drive off hydrocarbons to be burnt, ' he says. 'Its advantage is you're left with a clean soil, but it uses a huge amount of fuel, which may not be feasible and requires land use. There is also a public perception issue as it looks like an incinerator. In addition, we may not have time within the programme and there are licensing issues as well.'

Between the cost of bioremediation and thermal desorption lies another option. This is stabilisation (locking in the contamination) but this would not destroy it and there's a chance of future remobilisation.

Another possibility is soil washing to separate the coarse fraction from the ner where the pollution will be more concentrated. This can then be disposed of off site or stabilised.

Metals normally associated with coking works such as arsenic are also present. Less of a problem are the high concentrations of iron found in the site. Welch says: 'It's not toxic but we can taste it in the water and it can stain washing. People are not going to use this water here but there are standards we must compare it with.'

More of a worry is the loose coal in the colliery spoil which could become naturally combustible if mixed with air and water. 'If we built housing on the land as it is now it would not be appropriate for residents, ' says Welch. 'It might become a problem in the future even if it isn't now.' The solution for housing is to remove the colliery spoil and any waste for the housing site and create a clean building platform of natural clay.

Another issue is mine shafts.

Apart from subsidence concerns, these could provide a gas hazard.

So far only one has been found.

Agar says: 'We might build a road over them but not a house. We will in l with grout and cap them if we nd more. In the areas where there'll be buildings we'll take the ground down to natural levels to look for them.'

This means the ground can be checked at the same time for the potentially self combustible loosely compacted coal deposits.

The clay being used from the site's borrow pit, as well as the underlying natural clay, is not a source of contaminants. When the made ground in the area of the housing platform is stripped down to the clay and additional material from the borrow pit has been laid here, there will be a clean foundation ready for the residential properties being built.

As if mineral contamination is not enough to contend with, there are also living incursions to be sorted out. 'Another problem is Japanese knotweed dotted about the site and we have had to take out 40,000m 3 of material, ' says Agar. 'But we have treated it three times and we will move it to the bottom of the borrow pit. If it survives after that it deserves to live, ' he laughs.

Not all of the site's industrial past poses problems. When handed over for redevelopment, one of the rst tasks was to demolish the buildings and contractors will reuse this material for drains and as subbase for roads and footpaths.

Because of a topsoil shortfall, the plan is to manufacture material by mixing colliery shale - not in short supply - with paper mill crumble and green waste compost.

The reuse of three materials to create a growing medium is perhaps appropriate for the regeneration of the land that will be getting a fresh, and hopefully at long last, a sustainable face.

Project phases

Faber Maunsell says Sunderland City Council has been flexible in allowing the planning of the residential development platform before the detailed planning of all the remediation has been completed.

Phase one: site investigation.

Phase two: enabling works.

Phase three (current): development platform and borrow pit material exchange.

Phase four: treatment of contaminants, building a link road, contouring remainder of the country park.

The railway

Protecting a disused railway to the west of the site is of geotechnical concern on this geoenvironmental project. It must not be destabilised by extraction work in the clay borrow pit. Engineers have had to carefully consider the pit's size and depth so that it does not affect the nearby line's embankment as there is always the possibility that it might be used again at some future date.

The scope of works

Removal of existing foundations and structures from below ground.

Removal of potentially contaminated made ground and building a development platform for 350 homes using clean site won clay from a borrow pit.

Filling of the borrow pit with suitable existing site won made ground.

Construction of a structural road embankment.

Re-profiling of the remaining ground for use as a country park and public open space.

Construction of the associated footpaths and cycle ways.

Construction of the east access road and the culvert crossing to the Herrington Burn.

Re-alignment of the existing Herrington Burn and the formation of new waterways, wetlands and ponds.

Processing of contaminated or unsuitable soils arising from earthworks.

Creation of suitable site won materials by blending imported materials to form a soil material.

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