The Kilnwood Vale site consists of about 132 hectares of land, around 42 hectares of which is located over a former landfill which resulted in fill of around 18m being deposited. Planning permission has been obtained for 2,500 dwellings, new access points, a neighbourhood centre, retail space, a public house, an elderly care facility, a nursery and a primary school. We have also overseen the design of two road/railway bridges and are progressing the Reserved Matters submissions for the next phases.

CampbellReith has provided the remedial design, geotechnical, civils and earthworks expertise  to de-risk the landfill area and transform this into a development ready platform. In addition, the Practice has provided the design of roads, drainage and earthworks at both the masterplan and detailed design level. The site is being developed over a number of phases.

The fill soils comprised a number of different types of materials which are predominately cohesive in nature but with variable amounts of organic and deleterious mater. The treatment of these was necessary primarily to both control geotechnical settlement issues (particularly consolidation, creep and collapse compression) and consider a significant ground gas risk which rendered much of the landfill portion a NHBC red categorisation using conventional assessment (methane concentrations were regularly above 90% v/v). This meant that without re-assessment, standard gas protection measures beneath the houses would not be sufficient and land based protection measures, such as a gravel blanket above the residual landfill material, would be required.

A review of the conceptual site model along with investigation data indicated that the cause of the elevated methane concentrations and high flow rates was due partly to the construction of the landfill rather than the high organic load of the Made Ground.  Soils with an organic load had been historically covered by cohesive clays and the ground gases generated were trapped. It was therefore proposed that gas generation was at a low rate and gas was trapped in the soil pore spaces, which was released when the monitoring wells were constructed.

CampbellReith modified the remediation strategy in order to utilise forensic soil descriptions  (based upon CL:AIRE RB17) in order to allow the soils excavated from former landfill to be screened and classified for reuse based on organic load.  Upon completion  of works, surface gas emission scans, flux box monitoring and traditional borehole monitoring  is being implemented which has allowed removal of the need for land based protection measures for the proposed housing. The approach has also allowed the site to be redeveloped without the need for extensive post construction gas monitoring.

CampbellReith has produced an Earthworks Specification for each of the phases to control the works and to ensure that the site is suitable for the proposed residential end use. We maintain a daily onsite supervisory role to provide validation services for the client.