Geological Risk Assessment
Ground Gas Risk Assessments
Ground gas risk assessments are a major part of site investigations due to large areas of land being affected by hazardous ground gases. Many sites are either underlain directly by made ground of significant thickness with the potential to generate large quantities of ground gases, or are within close proximity to landfill sites and other sites associated with waste disposal in particular historical landfill sites that pre date current legislation of the requirement to be lined properly to prevent contaminant and ground gas migration. In addition, mine gases can also present a risk to human health and to buildings.
The risks from ground gases are asphyxiation suffocation and explosion. Explosion can occur when concentrations of methane build up and become trapped such as beneath a ground floor slab at a development, when methane is between 5% and 15% within the atmosphere it becomes flammable. these limits are know as the Lower Explosive Limit (LEL) and the Upper explosive Limit (UEL).
A good ground gas risk assessment would involve gas monitoring of boreholes installed with gas monitoring standpipes at a spacing and frequency over a period of time all recommended within CIRIA 665 current guidance. The monitoring should also be carried out over varying atmospheric pressures, in particular at falling pressure. This enables a worst case scenario to be considered.
GeoCon use the latest gas analyser on the market to provide an accurate indication of the levels of ground gases at a site and in particular gas flow rates. Typically we monitor for Carbon Dioxide (CO2), Methane (CH4), Carbon Monoxide (CO), Hydrogen Sulphide (H2S) and of course Oxygen (O).
In addition to the above some sites may require monitoring for hydrocarbon vapours. This is done by using either a Photo-ionization Detector (PID) or a Flame Ionisation Detector (FID)