Ground gas protection measures were recommended to be incorporated within the design of the development. A ground gas resistant membrane was understood to have been installed during construction phase, however the developer could not provide evidence to the integrity of the installed membrane.
The development had reached an advanced stage and so verifying the integrity of the installed membrane retrospectively through inspection was not possible. Therefore, GGS was approached by a Geo-Environmental Consultant, working on behalf of the housebuilder, to undertake a period of continuous sub-floor void monitoring.
A GGS specialist attended site to undertake a detailed site walkover and high-resolution ‘gas sweep survey’. During the survey, background concentrations of methane and carbon dioxide were recorded within all internal spaces to parts per million (ppm) level. Close attention was made to areas likely to provide a potential preferential pathway to ground gases (such as service entry points) and where ground gases could potentially accumulate (such as cupboard spaces). All site observations pertinent to ground gas were also recorded.
The walkover and survey results informed deployment locations of high-resolution continuous monitoring instruments. GGS secured the instruments to the outer wall of the selected properties and inserted a length of semi-rigid sample tubing inserted through an external air brick into the sub floor void. All instruments recorded atmospheric pressure, temperature and gas concentrations (methane, carbon dioxide and oxygen) within the sub floor void for a 4-week monitoring period.
GGS presented the detailed site walkover, gas sweep survey and continuous monitoring data within a concise DataPack® report, with comparison against relevant guidance and technical commentary of results. GGS assisted the consultant with demonstrating that the installed protection measures were functional, enabling the developer to proceed without further significant costs and project delays.
This complex project involved demolishing house extensions, construction of a gas-barrier, rebuilding the extensions, and retesting for gas.
GGS produced a ground gas management plan for a wastewater treatment works to effectively manage the risk from and migration of ground gases.