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Terms of interment

FOUNDATIONS: Construction of a massive cut-off wall at one of the UK's largest foot and mouth burial sites was finished just before Christmas. Max Soudain reports.

Great Orton, a disused hilltop airfield 16km west of Carlisle in Cumbria, was one of the major burial sites during the UK's foot and mouth epidemic last year.

Nearly half a million animal carcasses, mainly sheep, are interred here, killed as part of the county's contiguous or 3km cull. This meant herds and flocks within 3km of infected farms were slaughtered to prevent spread of the disease.

The last burial was in May 2001, but work has only just finished on a containment system to prevent leachate from burial pits reaching groundwater outside the site. The system comprises a deep drain and a massive cut-off wall around the site perimeter.

Geology was one of the main reasons for selection of Great Orton as a suitable disposal site. It was chosen after assessment by the Ministry of Agriculture Fisheries & Food - now the Department of Environment, Food and Rural Affairs which owns the site - and the Environment Agency.

A desk survey by the British Geological Survey revealed that the site is underlain by up to 5m of remoulded and natural boulder clay and argillaceous Lias Mudstone. Both are low permeability materials and the EA's preliminary environmental risk assessment showed they were not vulnerable to pollution.

The EA stipulated that burial pits had to be at least 250m from any wells, boreholes and springs and watercourses, and that all field drains within 50m of the site were isolated. Burial pits had to be no deeper than 4m and have at least 1m of cover over the carcasses. According to its study there were no licensed groundwater abstractions in the immediate vicinity, although some local farmers are known to have their own wells.

All this indicated that the site was in an area with low risk to water resources.

Slaughtering, carried out by the British Army, began in March 2001. Carcasses were buried in 26 trenches dug at 100m centres running at 90infinity north and south from the runway, stretching to within 50m of the site perimeter.

Typically, trenches have an under-drainage gravel layer containing three 150mm diameter pipes linked to manholes for leachate collection and treatment. Then comes a 2m thick layer of carcasses, followed by a 200mm 'odour buffer' layer of burnt lime acting against rising gases and as a capillary break to prevent body fluids rising to the surface.

After each day's quota, a 1m covering layer of clay was placed until leachate was drawn off. A month after disposal finished, boulder clay capping was laid on the trenches to aid surface run-off and to counter settlement.The entire area was then covered in topsoil and grass seeded.

Technical assistance and site supervision was provided by Water Management Consultants, which began work in March 2001.

WMC senior site engineer Eric Froggatt says it was critical to set up a containment system and get it running to reduce the risk of contamination reaching the local surface water and groundwater surrounding the site.

Site investigations carried out between early April and mid-May aimed to determine the bedrock profile, the geotechnical properties of the bedrock and soils, the piezometric levels and the direction of groundwater flow. Three rings of boreholes were put down, one inside the wall, one just outside and one at a distance from the site.

Standpipes were also installed for long-term monitoring of gas and groundwater.

While the clay and mudstone are low permeability materials, investigations revealed there were potential zones of relatively high permeability that would form preferential flow paths. Regional groundwater flow is to the south west and mainly shallow and lateral. Recharge within the burial area is from rainfall, with penetration restricted by the boulder clay.

Most lateral flow was expected through the fractured and fissured weathered mudstone layers at the top of the bedrock sequence and containment was intended to intercept this.

Another concern was the land drains that had been installed when the airport was built by flattening the hilltop. Any containment system had to also allow for these potential flow paths.

After considering a number of options, it was decided to use a combination of a deep drain running around the burial area, just inside a perimeter cut-off wall made up of a jet grout curtain and a conventional slurry wall.

The deep drain was laid through the weathered and competent bedrock to intercept any lateral flow and the cut-off wall installed to a maximum of 12m into competent bedrock to provide a barrier to flow at depth.

'Pumping from the deep drain causes artificial drawdown within the burial area to about 4.5m depth, ' explains Froggatt. Groundwater levels outside the wall stand at about 2m depth.'This means that any flow of groundwater [although small] is into the site rather than out of it, ' he says.

Five abstraction points were installed to draw off groundwater before sending it to the on-site treatment plant. Leachate is also drawn off from the trenches and treated; the fluids are pH adjusted to remove the foot and mouth disease virus before being tankered off-site for disposal.

While contamination was high soon after burial, levels have now dropped, Froggatt says.

Rapid removal of leachate allowed faster settlement of the trenches, he says, reducing long term flow rates and the risk of leachate penetrating into the surrounding groundwater.

Trials of the proposed jet grout curtain and slurry wall cut-off barrier were carried out by geotechnical contractor Keller Ground Engineering in mid-June 2001, with construction proper beginning in mid-August in the south west corner of the site.

The trial was intended to finalise design and construction methods including the jet grouting, explains Keller Ground Engineering project manager Paul Marsden. A 10m section was built and then exposed for inspection.

The 600mm thick wall is 2.23km long and was built in stages, with Keller working in two directions. First, 300mm diameter boreholes were drilled into the mudstone at 600mm centres to a maximum depth of 12m.

This is well past the weathered zone, says Marsden.'By 6.4m it is good, solid rock, ' he says. In all, 3,700 boreholes were drilled for the wall.

Alternate boreholes were jet grouted up from 12m to about 5m below ground level. A double jet system of air and grout was used, with high grouting pressures of 300bar, says Marsden. Although not originally intended, after the trial rotation of the jet grout nozzle was considered necessary to allow treatment of a wider area. Grout was tremied into the other boreholes, filling them and any open fissures, ensuring a complete cut-off.

The 10,000m 3slurry wall was then built by excavating to a typical depth of 6.4m, about 1.5m below the top of the mudstone and about 1.5m into the jet grouted zone. Excavators fitted with specially developed buckets then ripped through the rock. The trench was filled with spoil from the jet grout operations and topped up with conventional cement/bentonite slurry.

Froggatt explains that a standard slurry mix was used 'because we wanted to use materials and techniques that we had confidence in' Marsden adds: 'By using a slurry wall grout mix for jet grouting we could reuse the material in the wall, reducing costs and minimising spoil.'

Grout was supplied by two main batching plants, linked to the three high pressure pumps.The pumps were placed near the injection points and moved around the site as work progressed to maximise pumping distances while minimising the length of high pressure line. At the peak of work, Keller was using four pre-bore rigs, three jet grout rigs and two excavators.

Construction took only 15 weeks with Keller completing the 27,000m 2wall at the beginning of December 2001.

Drainage contractor Story was due to finish drain installation last month. Froggatt says a decision will be made this spring as to whether or not a gas collection layer will be needed, and final trench capping design will be made.

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