Dunton Environmental Ltd / Persimmon Homes
The scheme comprises comprehensive remediation and re-engineering of a former landfill to leave the site suitable for residential development. CampbellReith provided detailed assessment and remedial design for geotechnical, ground gas risk and contamination issues, and subsequently designed and validated the remedial earthworks.
The site’s historic use as a landfilled former brick pit resulted in depths of up to 25m of highly variable landfill materials, including putrescible wastes, combined with a steep ‘high wall’ interface with the natural geology. Methane concentrations had been regularly detected up to 99% and the site is further complicated by practical issues such as a partially culverted stream running inside the boundary and housing immediately adjacent to the site.
The historical remedial strategy, by others, was incorporated into the previous planning permission and comprised a horizontal gas venting layer across the site, a vertical barrier around the site boundary and the exclusion of piled foundations. The strategy severely compromised the viability of the development and CampbellReith were employed to re-assess the site, which was done via additional, targeted investigations and detailed quantitative risk assessment.
An assessment of the gas emission potential of the landfill soils was carried out using forensic analysis of the organic component of the landfill combined with quantitative gas modelling. This demonstrated that the gas venting layer and vertical barrier were not required and an earthworks specification was produced to establish the parameters for soils reuse thereby enabling the gas emission potential of the soils to be controlled to facilitate ‘traditional’ passive gas protection measures for structures. This approach was subsequently approved by the regulatory authorities and NHBC.
The geotechnical appraisal led to the design of an engineered earthworks ‘load transfer’ platform, combined with raft foundations for structures to provide a cost effective solution.
CampbellReith subsequently produced an Earthworks Specification for the remedial earthworks and are currently validating the works on behalf of the contractor, Dunton. We have since been asked to produce the Earthworks Specification and provide validation for the main construction works.
The scheme reflects the substantial added value that can be achieved via a process of detailed investigation, risk assessment and bespoke engineering design. The solution unlocked a scheme that had previously been deemed cost prohibitive and considered unsuitable for development.