Sensitive earthwork designs underpin a scheme to restore a colliery tip near Nottingham as a habitat for plants and wildlife. Mike Walter reports.
Closure of hundreds of collieries throughout the UK over the past 20 years has had a significant impact not only on local economies and those who worked the mines but also on the environment.
Calverton Colliery in Nottinghamshire is a case in point. Closed in 1999, its legacy is a spoil tip covering 190ha. Huge stockpiles of unwanted coal deposits dominate the skyline.
The tip is a barren, undulating environment, off limits to the public. Three giant lagoons built to drain water from fine coal deposits lay dormant towards the top, and the largely unvegetated stockpiles are uninviting for animals or birds.
Now the site is set for a major facelift. Nearly 50 years ago, planning permission was granted to tip unwanted material at the site, leased from the Forestry Commission. Part of the deal was that the colliery owner had to start remediation of the site once mining had ceased.
When the licensee, RJB Mining, stopped mining the colliery was handed back to the Coal Authority.
Since 1999 the authority has worked closely with local authorities to develop an environmentally enhanced restoration proposal.
Part of Calverton spoil tip has already undergone minor remediation with grass well established on lower regions of the site, but a more comprehensive restoration of the tip was deemed necessary.
Earlier this year the Coal Authority asked consulting engineer Scott Wilson to come up with a restoration design. Regeneration of the site will involve capping material in each lagoon, reprofiling the site and reclaiming material to support vegetation growth.
The hope is that the site's limited wildlife will be greatly increased and that tourists will visit the area. When work is finished the site will be handed back to the Forestry Commission.
Scott Wilson project manager John Armistead says the remediation design his team has developed involves reusing as much of the material on site as possible.
'The authority's strategy involves reclaiming material from the site and restoring open space without importing soils from elsewhere. We see little point in digging a hole a few miles away to win material to be used on this site, ' he says.
Inert colliery spoil will be piled on top of each of the three 9m deep lagoons to a depth of about 2m. This will effectively cap the fine material in each lagoon which is unsafe to walk on and is mildly contaminated with traces of sulphates and arsenic. As part of the capping process, surface water will be drained from the surface of the lagoons to enable them to dry to form a crust. This will provide a level platform from which reprofiling can begin.
Particular care must be taken when loading material on top of each lagoon, says Armistead: 'If lagoon deposits were to spill over the steeply sloping stockpiles, the environmental effect on adjacent land could be significant. '
The Environment Agency has been monitoring local brooks and other watercourses for contamination, but Armistead stresses that during the works the main requirement will be to protect watercourses from the effects of suspended solids in the surface water run-off from the site.
Before excavation and reprofiling of the tip can begin in earnest, the extent and condition of a number of redundant lagoons known to be buried beneath the stockpiles of spoil need to be identified. Cone penetration testing and boreholes were used to locate each lagoon and test for material strength.
Reprofiling material will help reduce the height of the steep mounds and allow material won from one area to be used in another. Gradients at the site vary from 1 in 15 to as steep as 1 in 5, and the contours will need to be made less dramatic to allow for a variety of end uses. Excavation of material in areas where grassland has become well established will be kept to a minimum.
Scott Wilson's restoration design suggests that coniferous and fast-growing softwood trees are planted on about a third of the site, with another third planted with hardwood trees and the rest returned to open space.
Provision of a growing medium on the top of the tip is an important part of restoration.
Techniques using treated sewage sludge or paper sludge, either to be mixed into or sprayed onto the soil, are being investigated to help with the growth of surface vegetation and trees.
Sandstone can provide another good source of nutrients for trees, as Scott Wilson assistant project manager Stuart Miller explains.
'Sandstone in this area has been found to be an excellent growing medium, ' he says. 'Once broken up it is friable and easily broken down by rain.
'Coniferous trees are very good at surviving in acidic soils and the sandstone will produce a loamy topsoil. The soil and sandstone will allow tree roots as much room as they need to grow. '
Calverton spoil tip is within Sherwood Forest and the immediate surroundings include agricultural land as well as mixed forestry. Regeneration will be carried out in harmony with the needs of the local environment.
Armistead says: 'The remediation work will be carried out over two summers because we want to avoid disturbing natural habitats and restoration is very dependent on the weather.
It is also necessary to plan earthworks to avoid disturbing bird life which has already colonised one part of the site, he adds.
Following reclamation, the Forestry Commission aims to construct footpaths and a car park to encourage visitors to the site. Further incentives may include hides for bird-watching and a visitor centre.
Approval of reclamation designs by the planning authority and the Forestry Commission is expected this autumn. The work will be put out to tender at the beginning of 2002.
'Scott Wilson has a good track record of reclaiming colliery sites to be used mainly for retail, industrial and residential uses, with only small areas for open space and recreational use, ' says Armistead.
'But this is the first time we have reclaimed a colliery spoil tip purely for environmental purposes. It is an exciting challenge. '
Mike Walter works for Barrett Byrd Associates.