When building or developing on or adjacent to brownfield sites, in areas of mine workings, or sites where degradable materials are present within the ground; there is a potential risk of the generation of ground gases and accumulation of these gases within buildings. Under adverse circumstances, methane and carbon dioxide gases can build up to hazardous concentrations within confined spaces giving rise to a potential risk of asphyxiation or explosion.
Gas monitoring is undertaken to determine the levels of methane, carbon dioxide, oxygen and other gases which can be generated by made ground or backfilled materials. The gas flow is dependent on the atmospheric pressure at the time the readings are taken.
If a potential risk from ground gas is established during the desk study; the design of the intrusive phase of the investigation will consider the installation of monitoring wells within selected boreholes. The monitoring wells or standpipes are constructed using slotted plastic pipe. The boreholes are backfilled with granular filter material surrounding the monitoring wells and a bentonite seal is installed at the surface.
The wells are fitted with a gas tap and protected at ground level with either a flush or raised metal cover. The wells are monitored for groundwater and ground gas levels on several occasions. The gas levels can be affected by prevailing weather conditions, and the monitoring should be carried during periods of low and falling barometric pressure. If a potential gas risk is identified, a minimum of three wells and at least three monitoring visits should be carried out. Although if gas is recorded in measurable quantities or flows then further visits may be required.
Gas monitoring involves attaching a specialist detector to the gas valve which then analyses the gases pumped out of the standpipe. The gas detector will monitor the levels of methane, carbon dioxide, oxygen, hydrogen sulphide and carbon monoxide. The groundwater levels are also recorded at each borehole location.
A photo-ionisation detector (PID) is used to obtain semi-qualitative readings from the monitoring wells or the recovered soil samples. The results are used to determine whether further laboratory analysis of the soil samples should be considered.