Ground Gas Monitoring & Risk Assessment

Introduction to Ground Gas

 

Ground gases are those generated as a result of sub-surface processes and may include methane, carbon dioxide, hydrogen sulphide, carbon monoxide and radon. The sources of these gases may be natural, such as geological processes like weathering and erosion, or anthropogenic, such as mining and waste disposal. Understanding the risks posed by ground gases is essential for the design of a safe development.  Past incidents, including explosions and the relocation of entire villages, have demonstrated the potentially catastrophic implications of a poor understanding of site specific ground gas risks.  Effective ground gas monitoring and risk assessment are essential for identifying potential risks, designing appropriate mitigation measures and ensuring compliance with relevant legislation and guidance.​​​​​

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When to Undertake Ground Gas Monitoring

 

​​​The first stage of a gas risk assessment is determining whether a potential risk from ground gases exists and, therefore, whether any gas monitoring is justified. Expert advice is crucial at this stage to ensure accurate determination. This should be established by the Phase I Desk Study and/or the Phase II Site Investigation. Undertaking gas monitoring when no viable source and/or migration pathway has been identified can be costly, time-consuming and create issues with the misinterpretation of ‘normal’ ground gas concentrations, which may result in the installation of gas protection measures where none are necessary. Ground gases generally only present a risk where conditions are conducive to the generation of significant volumes of hazardous gases (principally methane and carbon dioxide), usually as a result of the (relatively) rapid degradation of organic matter, or where a pathway and flow mechanism exists between a significant gas ‘reservoir’ in the ground and a receptor.

 

The most common sources of ground gases are buried waste (i.e. landfills), significant thicknesses of Made Ground with a high organic content, natural soils with a high organic content (such as peat and alluvium) and old mine workings. Recent household waste landfills tend to present the greatest risk as they can contain a high proportion of easily degradable organic material, which continues to generate large volumes of ground gases. Older landfills may present less of a risk, as while they may contain high gas concentrations, they are less likely to be a continuing source of gas generation. Similarly, old mine workings and soils with a high organic content may be sources of high gas concentrations (and high volumes of gas), but are unlikely to be continuing to generate gases in significant quantities. ​​

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Site Investigation

 

​​​Where a potential ground gas risk is identified by a Phase 1 assessment, a Phase 2 intrusive site investigation should be undertaken which includes elements designed specifically to address the identified risks, by targeting likely source areas and migration pathways.  This will involve consideration of the anticipated ground conditions, groundwater levels, potential gas source areas and the form of the proposed (or existing) development, to construct a detailed conceptual site model. Monitoring standpipes should be designed with response zones targeting specific strata and to ensure the effects of groundwater are mitigated and / or understood as part of the assessment.  Gas monitoring may be undertaken as one-time 'spot monitoring' visits which measure gas concentrations, flow rates and pressures at the time of the visit, or it may be appropriate to install equipment within the standpipe that provides continues monitoring data.  The aim of the investigation should be to provide data that shows how gas concentrations and flow rates vary under different conditions (atmospheric, meteorological, groundwater/tidal), and identify the 'worst-case' which should be used as the basis of the risk assessment.

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Interpretation of Gas Monitoring Data

 

​​​​​The presence of elevated ground gas concentrations does not necessarily indicate a risk on its own. The greatest risks occur where a pressure gradient allows the flow of significant volumes of ground gases via a pathway and into buildings/enclosures where gas can accumulate. Pressure gradients may result from the ongoing generation of significant volumes of gases within a source area, a change in atmospheric pressure, a rise or fall in groundwater level, groundworks such as mine-treatment and piling, or a pressure differential within a building. Gases may migrate between the source and a building (receptor) via various pathways, including high permeability deposits, fractures or faults within the ground, old shafts and boreholes, and service corridors. Gathering multiple lines of evidence is key to ensure robust, evidence-based assessments.

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It is important to recognise that there are also processes that can result in the generation of ground gases other than methane and carbon dioxide. For example, underground fires and chemical reactions can be sources of gases such as hydrogen sulphide and carbon monoxide, chemical contamination may be a source of volatile organic compound vapours and methane, while radon can occur naturally in certain geological formations.

 

The Conceptual Site Model is again important in determining if a potential risk exists, considering the possible sources, pathways and receptors as described above. The aim of gas monitoring is generally to provide a reasonable characterisation of the gas regime and identify the likely ‘worst case’ conditions, which may, for example, correspond with periods of falling atmospheric pressure or rising groundwater levels.

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Migration Pathways

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Migration pathways refer to the routes by which ground gases can move through the soil and into buildings or confined spaces. These pathways can include pore spaces in sands or gravels, fractures, joints, bedding planes, and fault lines, as well as anthropogenic features such as boreholes, mineshafts, piles, sewers and service ducts. Understanding migration pathways is one of the key components for identifying potential ground gas risks and designing effective mitigation measures. Factors such as atmospheric pressure, groundwater flow, and soil permeability can influence the movement of ground gases, and the presence of preferential pathways can increase the risk of gas accumulation in buildings. 

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Risk Assessment and Mitigation

 

​​​A ground gas risk assessment requires the evaluation of the potential risks, including toxic and explosive concentrations, and determining if and what mitigation measures are required. The assessment takes into account factors such as the type and concentration of gases present, the permeability of the soil, the behaviour of gases under different conditions and the likelihood of migration and accumulation. Mitigation measures may include the installation of gas membranes, ventilation systems, gas detection equipment, as well as the designing a development around potential gas sources.  It is essential that any gas protection system is properly installed and verified by an experienced and independent consultant, as incorrectly installed systems may offer little or no protection.

 

Effective risk assessment and mitigation require a comprehensive understanding of ground gas behaviour.  Detailed guidance on the risk assessment process and the design of mitigation measures is contained within a wealth of industry guidance, such as BS8485:2015 and various reports by CIRIA Reports. Through suitable risk assessment and mitigation, developers can ensure the safe and successful completion of construction projects, that minimise the risks associated with ground gas and protect the health and safety of occupants.​​​

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G&J Gas Monitoring Services

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Where there is gas risk potential, G&J can provide monitoring and assessment services, including spot monitoring, where gas concentrations and flow rates are measured from standpipes at discrete times over a defined period (e.g. fortnightly for three months), or continuous monitoring whereby monitoring equipment is left in-situ to allow real-time recording of gas concentrations and flow rates. Continuous monitoring is often used on higher-risk sites with a known gas issue or where a shorter monitoring period is required.​  For advice related to ground gas risk management and monitoring, please contact us.

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