CampbellReith has been actively involved in the development of several former landfills. Our expertise enabled the sites to be unlocked since each were in danger of stalling at the design stage due to what were seen as onerous land quality issues, particularly those associated with ground gas.

Over the years, CampbellReith has been involved in the development of a number of landfill sites but more recently, we have developed our expertise, particularly with respect to ground gas assessment and settlement analysis. Consequently, we are now working on three such sites that were previously struggling to be developed due to what were seen as cost prohibitive design issues.

The sites, all of which are proposed for residential development, include:

• Kilnwood Vale. A 132ha site near Crawley, West Sussex, of which 42 ha is located over a former landfill with fill up to 18m depth. Methane concentrations regularly detected above 90% v/v;
• Holly Lane. A 6ha site in Erdington, Birmingham. It is a former quarry for a brick works, backfilled in the early 1970s with a variety of wastes including deleterious materials such as wood, metal, ash and localised hydrocarbon contamination. The landfill covers most of the site and has steep walls close to the site boundaries with fill extending to 25m depth. Methane concentrations regularly detected above 90% v/v; and,
• Unnamed 20 ha site in Berkshire (the project is at a sensitive development stage and cannot be named). The site was subject to landfilling across the western portion. It borders a landfill to the north, with residential estates to the west and south. Methane concentrations detected up to 30% v/v.

The remedial recommendations by others comprised a mixture of gas collection / venting systems beneath all properties, or whole scale removal of the organic soils from site. One site also required a vertical barrier around two thirds of the site perimeter to prevent off site migration and each strategy included the use of standard gas protection measures.

As well as adding significant costs to the budgets, these measures also significantly compromised the geotechnical foundations options, thereby either increasing costs still further or making the project/s unviable. For example, the presence of the horizontal gas collection systems beneath properties meant that piled foundations could not be installed.

CampbellReith’s solution for each site was to undertake detailed ground gas monitoring and assessment. This enabled calculation of the gas emission potential of the soils, which, in each case, confirmed that the gas risk was far lower than originally identified, enabling remedial measures to be downgraded to standard gas protection measures (except for site [3] where no protection is required at all. In addition, these measures meant that more suitable geotechnical solutions could be employed.

The standard approach to gas assessment in industry relies on comparison of borehole results to threshold values. This can often lead to very over conservative and costly remedial design. By undertaking detailed assessment, supplemented by validation techniques throughout the construction works, the remedial measures can be reduced to a required minimum, potentially saving significant sums of money.