The site comprised a landfill with inert waste, which was active between 1975 and 1986. The made ground within the landfill contained extraneous waste items, such as brick, construction debris, wood, glass, plastic, ceramic and ash, which was overlain by an overburden capping layer of silt.

The key elements of our works included:

• Site-wide stripping of topsoil and stockpiling for later use during landscaping.
• Cut and fill of materials to meet the development topography.
• Stock piling of clean excavated soils for re-use within the capping layer. Materials management was assisted through the use of comprehensive drone surveys.
• Stabilisation of soils for proposed building plots and roads.
• Installation of a geomembrane marker layer beneath the capping layer.
• Installation of a subsoil/topsoil capping layer.
• Construction of piling platforms.
• Geotechnical testing of stabilised soils and piling platforms to confirm strengths.
• Processing and disposal of deleterious excavated materials that were unsuitable for re use.
• Geochemical testing of all soils used within the capping and sub-capping layers.
• Treatment of surface waters prior to discharge into the sewer to remove suspended silt.
• Excavation of voids for attenuation tank installation.
• Installation of two attenuation tanks.

The remediation and earthworks were carried out in accordance with Cognition’s remediation strategy and proved to be highly sustainable delivering significant savings to the client. A total of 20,735m3 of soil material was excavated and treated using segregation, enabling re-use on site and reducing import and disposal costs. Voids were excavated to allow for the installation of attenuation tanks and the use of surface water treatment throughout the works minimised the site’s output of suspended silts into the local sewer network. Soil stabilisation not only delivered firm construction plots and piling platforms but also formed a suitable subgrade for road construction for the new development. Cognition demonstrated exceptional materials management and robust geochemical and environmental testing maximising the reuse of materials within the capping layer.

The site then went on to be redeveloped into Heathside Walton-on-Thames Secondary School, which opened its doors to students in September 2022!