What is ground gas and why is it dangerous?
Ground gases are natural gases that are generated by various mechanisms and gather in the spaces around soil particles (pore spaces).
- Ground gases often gather in wetlands or soils with a high organic content, such as peat.
- They can occur as a result of land contamination, for example old mines, brownfield sites and landfill sites.
- Inert fills such as ash or foundry sands can also create potentially hazardous levels of ground gas.
- They can be a result of soil gas migrating from a nearby source.
PAGeoTechnical offers a number of measures to
mitigate the risk from ground gas.
There is a misconception that ‘ground gas’ refers to just carbon dioxide and methane. However, radon, carbon monoxide volatile organic compounds (VOCs) and nitrogen, as well as numerous other gases, can be classified as ground gasses. Landfill gases may also include ammonia and hydrogen sulphide, which are responsible for most of the odours associated with landfill sites.
In low volumes, these gases do not pose a significant risk. However, they can migrate through the subsoil and into buildings, where they can build up to hazardous levels. They may then cause harm to health or compromise safety. This could be via:
- Asphyxiation due to decreased oxygen levels.
- The toxicity of gasses at varying levels, such as carbon dioxide or monoxide.
- VOCs which can aggressively permeate through plastics and are flammable, toxic and have an unpleasant odour.
- Methane, which is combustable, can explode at concentrations from 5% to 17%.
- Other health risks, ranging from headaches, nausea and dizziness to cancer.
Building design and ground gas
A building’s design and use is a factor; greater risk from ground gases can be caused by:
- Smaller, enclosed spaces on the ground floor.
- Alterations to buildings in unmanaged properties.
- The proposed usage and a lack of internal ventilation.
Analysing the risk of ground gas
Methods of analysing the potential risk from ground gasses and site classification are outlined in CIRIA C665 and BS8485:2015+A1:2019. These advise that sites are classified on a scale from CS1-CS6, where CS1 sites require no protection measures and CS5-6 sites are generally considered unsuitable for housing.
Site classification should be performed by a competent individual using a combination of ground gas monitoring data and the conceptual site model. Both source and mechanisms driving gas flow should be considered, such as ground gas generation rates, air pressure due to weather, groundwater levels, tidal pumping and the soil makeup.