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Military debriefing

Soil washing is helping to make the historic Royal Arsenal at Woolwich fit for redevelopment.

At its peak during the First World War, the Royal Arsenal at Woolwich in south east London stretched almost 10km along the south bank of the River Thames and employed more than 80,000 people.

The site even boasted its own railway to ferry workers from the main entrance to the massive testing ranges on the marshes to the east.

Arms development and manufacture had been carried out on the site for nearly 300 years when main production stopped in 1967. By 1994, when government development agency English Partnerships acquired the remaining 31ha from the Ministry of Defence, most of the rest of the site had already been sold for redevelopment, including the town of Thamesmead built in the 1960s and 1970s.

With a view to redevelopment for mixed leisure, commercial and residential use, English Partnerships set about devising a remediation plan for the site, which includes most of the Arsenal's early buildings, many of them listed.

The MoD had begun a phased programme of investigations in 1993. Investigations continued during the remediation scheme devised by English Partnerships and its design team of Campbell Reith Hill and LlewelynDavies.

This second tranche of investigations focused on site contamination and was split in two sections. Listed buildings destined for refurbishment were examined for asbestos and toxic metals in paint, and dust, internal surfaces and silt and sediment in the drainage systems were tested for residues from explosive manufacture, radiation and anthrax. Asbestos and paint containing lead, zinc and copper were present in most of the buildings and had to be removed, but no anthrax or radiation were found.

The other part of the investigation aimed to determine site geology and to discover any potentially hazardous substances in the ground and groundwater, to ICRCL guidelines.

This involved 37 boreholes, 500 trial pits and some 1,000 chemical tests for leachate and soil gas, as well as geophysical surveys in six areas identified as possible locations of unexploded Second World War ordnance.

The whole area was found to be covered with up to 5m of mixed fill, containing 'significant concentrations' of lead, copper, nickel, zinc, chromium, polycyclic aromatic hydrocarbons, diesel range hydrocarbons and chlorinated solvents. These were also found in the groundwater, although it was decided that widespread remediation was not necessary, a view shared by the Environment Agency.

Based on these results, English Partnerships decided that remediation was needed where new roads, drainage and serv ices were to be bu i lt and in a number of the vacant development plots. These phase one works required excavation of more than 170,000m3with some 130,000m3ofengineered fill needed for final construction.

'Once the nature and scale of contamination became apparent, it was realised that lorry movements for dig and dump would be unacceptable, so we looked at other options, ' says Campbell Reith Hill associate Gregg Acheson.

English Partnerships was willing to consider alternative solutions to reduce the amount of material that had to be taken off site.

Because most of the contaminated material was granular, it lent itself to processing and recycling as engineering fill. Campbell Reith Hill suggested soil washing.

'VHE was involved in soil washing work at Basford Gasworks (Ground Engineering July 1998).We went to see the site with the client, who was impressed and so we went to tender with soil washing, 'says Acheson.

The £15M phase one contract was won by a joint venture of Mowlem Civil Engineering and VHE Construction, with Mowlem constructing new infrastructure and VHE focusing on remediation of the fill and underlying Thames Gravel using soil washing.

Remediation typically involves excavating between 0.75m and 1.5m of the contaminated ground and replacing it with 'compliant material'. About 25% of excavated material is rejected immediately as being unsuitable for the soil washing process, and the remainder is earmarked for recycling.

Excavation has been deeper in some areas, where buried canals and shafts are present.

Recycled material that marginally fails the compliance procedure can be used below 1.5m depth in these areas.

Work began in February 1999 with the commissioning of the soil washing plant.VHE Construction project manager Lyndon Absolon explains that while the firm used proprietary soil washing systems from the UK and the Netherlands at Basford, this time it decided to use the knowledge gained on the project to design and develop the plant inhouse, using a combination of equipment sourced from the UK.

Soil washing works on the principle that contamination forms only a surface coating on soil particles, meaning fine grained material is potentially more contaminated than coarser granular material. The process separates the finer material from the coarser sand and gravel fractions, which are cleaned and then blended for re-use. The fine material is concentrated into a contaminated sludge which is then disposed off site.

VHE Technology general manager Mike Summersgill says 80% to 85% or more of the input material at Woolwich has been recovered.

Cleaned material is about 70% gravel and 30% sand, with up to 1,000t being output per day.

The washed aggregate is stored in quarantine bays next to the plant where it undergoes compliance testing (both chemical and geotechnical) before being selected and blended for re-use elsewhere on site for engineering fill or capping for roads.

Any material failing to meet requirements can be reprocessed or disposed of, although Acheson says the generated fill is actually better than imported material as it is cleaner and already graded.

Originally, it was expected that the ratio of disposal to recycling of suitable excavated material would be around 60/40 but in fact the figures are nearer 40/60, Acheson says. This is of benefit not only to the client, with reduced lorry movements and a reduction in the amount of imported fill needed, but also to the joint venture contractor, whose contract includes an incentive scheme once recovered volumes from soil washing exceed 60% of the fill requirements.

Phase one is due to finish in October, by which time Campbell Reith Hill anticipates that nearly 90,000m

3of reclaimed material will have been used in construction.

Washing cycle

At Woolwich, excavated material is crushed and screened to produce material below 75mm for washing.The blended material moves beneath a powerful magnet to remove ferrous metal before passing through a high pressure wash and screen to separate and remove coarse material.

The coarse gravel then feeds into the attrition scrubber, where it is washed and clean coarse gravel reclaimed.Any wood, plastic, ash and clinker is floated off for disposal at this stage .

Finer material then passes through the primary desander, where cleaned fine (2mm-5mm) gravel is screened off and the clean sand fraction (0.15mm-2mm) recovered by hydrocyclone separation.The silt fraction, which contains some of the contamination, is then recovered by microfine hydrocyclone separation in a separation unit and disposed of off site.

The remaining clay/water slurry produced from the hydrocyclones is collected and flocculants added to thicken it into a sludge, which is then dewatered in a centrifuge.This material contains much of the contamination and is disposed of off site.All the process water is collected throughout the washing process and re-used.

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