Project background

Peel Holdings (Environmental) and Island Gas (IGas) commissioned RSK to carry out contamination, slope stability and settlement assessments at a closed landfill site in Carrington, Greater Manchester. The work would assist IGas, part of Nexen Exploration UK, in developing the site for coal-bed methane extraction.

The assessments determined the ground conditions and the nature of the waste in the landfill at the location of two of four proposed conductor pipes. Specifically, the extent of the asbestos waste known to be present was characterised and delineated so that a safe system of work could be developed for IGas. This would detail the control measures necessary during development work to protect the workers on-site, the users of adjacent land and the local environment. In addition, RSK reviewed and updated the slope stability assessment using the site-specific data obtained during the intrusive investigation.

Conceptual model

A conceptual site model was developed to identify the potential distribution of asbestos waste using historical records obtained from the local authority and the former site operators. Landfill construction details were also reviewed. This information was used to create an investigation plan, which the Environment Agency and the local authority subsequently approved.

Sonic drilling

Two 17-m-deep exploratory boreholes were drilled to prove the depth to natural soils and collect samples for asbestos and chemical analysis. In sonic drilling, the recovered soil cores are sealed in a plastic membrane when they are brought to the surface. This retains sample integrity for analysis and controls worker exposure to contaminants. Soil samples were collected along the length of the boreholes for determining the asbestos distribution in the waste profile and providing better understanding of the soil types for the geotechnical assessment. A groundwater monitoring well was installed into one of the boreholes; the second borehole was decommissioned.

Asbestos monitoring

RSK provided a mobile asbestos laboratory manned by a qualified surveyor during the works. While drilling, real-time air monitoring in the work area was undertaken in accordance with the Health and Safety Executive’s HSG248 publication Asbestos: The Analysts’ Guide for sampling, analysis and clearance procedures. In addition, RSK collected and analysed bulk samples to determine the nature of asbestos in them. Amosite and chrysotile asbestos were identified to depths of 7.1 m. RSK collected soil samples during drilling, then used low-flow sampling after installation of the groundwater monitoring well to collect groundwater samples. All the samples, including geotechnical samples for determining parameters to assist in the slope stability assessment, were analysed at RSK’s in-house UKAS-accredited laboratory.

Safe system of work

On the basis of the laboratory analyses and field observations, a safe system of work document was produced for implementation during all future drilling operations on the site. Adherence to the procedures will ensure the safety of the employees involved in future drilling works, the users of the adjacent land and the local environment. The document covers the classification of soil for off-site disposal; a generic assessment of risk to human health from soil and groundwater ingestion and vapour inhalation; the mitigation measures required for direct excavations; the personal protective equipment necessary for all intrusive works; water quality data for any de-watering operations; and plans to show the exclusion zones for heavy equipment near the landfill’s edge.