GROUND GAS RISK ASSESSMENT
How does gas get into the ground?
Property owners are often surprised to learn that in some areas, hazardous gases can find their way into the site and accumulate in basements or other subsurface features and roofs.
Gases appear in the soil and air from numerous sources. They can then get into the buildings through cracks, gaps, cavity walls and other entrances, and then accumulate under floors, in the roof, in the cellar and in drains.
- Methane is a common gas with potential harmful effects on health, property and vegetation. It can be dissolved and appear in ground water, and is flammable. Two common sources are nearby coal mines and worked underground coal seams, and landfill sites, but methane may also be present on brownfield sites and former agricultural land
- Carbon dioxide is also commonly found near organic decomposition such as landfill sites and is a potential hazard when sufficiently concentrated in confined spaces as it replaces oxygen within the respiratory tract in humans and can lead to death in minutes if a person is not removed from the hazardous atmosphere.
- Radon is a very common gas at low concentrations and is a product of the decomposition of radium. Tests may last several weeks and aim to find out concentration of radon in the soil, and where necessary in nearby water courses. It can dissolve in water and is sometimes found in harmful concentrations in ground water and spring water, as well as migrating through bedrooms
Landfills are surprisingly common in the UK. Most Phase 1 Desk Studies will reveal the presence of a landfill within the local area of a development site. DBS has vast experience in identifying, managing and mitigating risks from landfills and landfill gas. It is essential that gas monitoring regimes are interpreted correctly to establish actual risk to new development that does not lead to over elaborate mitigation schemes. Landfills undergo various stage of waste decomposition and gas production and it is imperative that any consultants managing risks from landfill understand how this works, and how to critically analyse results for anomalies and trends in gas data to identify what is actually happening in the ground.
Worked coal seams present ground gas risks from methane that is produced through a natural mechanism due to the geochemical processes that form coal underground. It also has carbon dioxide present, along with trace gases such as carbon monoxide. It can migrate to surface from worked seams and impact new development, particularly where ground disturbance can affect migration pathways.
DBS design and implement monitoring regimes for ground gas, and we can undertake ground gas risk assessments based on the findings. We use current guidance in all of our ground gas risk assessments including BS 8485:2015 Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings, CIRIA C665 / NHBC Guidance Assessing risks posed by hazardous ground gases to buildings and BS 8576:2013 Guidance on investigations for ground gas. Permanent gases and Volatile Organic Compounds (VOCs).