Ground Gas Assessments
Ruddlesden geotechnical has considerable expertise in undertaking ground gas assessments, in accordance with BS 8485: 2015 'Code of Practice for the Design of Protective Measures for Methane and Carbon Dioxide Ground Gases for New Buildings' and NHBC Report 'Guidance on Evaluation of Development Proposals on Sites where Methane and Carbon Dioxide are Present'.
The breakdown of organic material in made ground can produce ground gas, though it may also be produced by other, natural, sources (e.g. coal, peat). The principal components of ground gas are methane (potentially explosive) and carbon dioxide (potential asphyxiant).
If desk study information and/ or initial intrusive investigation indicates there to be a potential risk from ground gas, a detailed assessment of ground gas regime should be carried out using the recorded levels of methane and carbon dioxide combined with gas flow rates, to produce gas screening values (GSVs), so that the site's BS 8485 Characteristic Situation (CS) or NHBC Traffic Light classification can be ascertained.
Depending on the site's CS/ classification and the proposed development, gas protection measures are recommended, which might include a combination of floor slab construction, gas resistant membrane and/ or subfloor ventilation.
Ground Gas Assessment for Large Residential Development Site
After completing a Phase 1 contamination assessment, Ruddlesden were instructed to undertake a ground gas risk assessment for a proposed residential development near Rugby.
The southern and western boundaries of the site were bordered by an active landfill site and historical landfilling activities had taken place within the southwest of the site.
Twelve ground gas monitoring wells were installed to varying depths within the confines of the proposed residential development site.
The wells were monitored on twelve occasions following installation with a landfill gas analyser, for oxygen, methane, carbon dioxide and gas flow. Atmospheric pressure and other environmental conditions were also recorded.
The results of the monitoring were used to produce a gas screening value (GSV), which was used in the assessment to ascertain whether any protective measures were required for new dwellings.
The results indicated that a suitably installed gas proof membrane and subfloor ventilation would provide sufficient protection against the levels of ground gas present.