When building basements and below ground structures, it is vital that the structure and its users are protected from hazardous ground gases.
The gases present on a site may be naturally occurring or a result of its historical uses. Naturally occurring gases include methane, carbon dioxide, nitrogen and radon and many of these can be present within soils (including natural soils) at elevated concentrations relative to normal atmospheric gases. Past uses of a site which may generate hazardous gases include landfill, industrial use, and activities where petrol, oil or solvents may have spilled. Gases occurring from these activities include methane, carbon dioxide, volatile organic compounds (VOCs) such as benzene and trace gas components such as hydrogen sulphide and carbon monoxide.
The above gases and vapours have a range of properties, some are flammable and explosive, some asphyxiate, others are highly toxic and are also known to cause cancer through low exposure over several decades. Some may also chemically interact with building materials including concrete and plastics, so care must be taken when specifying materials.
It is vital that the presence of ground gases and the risks associated with them are assessed and mitigated to protect the structure and its users. When building below ground the need to protect structures against water ingress is also a key requirement. Water ingress can cause damp and mould and associated health issues, as well as causing damage to structures, finishes and the building contents.
However, when addressing both structural waterproofing and ground gases, the complex number of scenarios and a lack of synergy between the relevant standards mean that determining a correct solution is not easily achieved. Current UK guidance for waterproofing is provided by BS 8102:2009 – Code of Practice for protection of below ground structures against water from the ground, and for ground gases, guidance is provided by BS 8485:2015+A1:2019 – Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings.
Dealing with ground gas itself is particularly challenging. Each gas behaves differently in the sub-surface and presents different risks to receptors (for example VOCs can degrade plastic liners and some gases present risks to human health at very low concentrations). As a result, there are a number of sources of guidance and gases are measured and assessed differently. BS 8485:2015+A1:2019 focuses on methane and carbon dioxide but the protection approach it sets out may be used to manage risks from other gases. Further guidance on VOCs is available in CIRIA guides C682, C716, and C748.
Guidance on radon is provided in BRE Report BR211, which provides details of radon affected areas. Chapter 6.12 of the Report highlights the increased radon risk for basement construction and the other occupied spaces below ground level. General guidance is also available for ground investigations for ground gas in BS 8576:2013 and on testing and verification of gas protection measures in CIRIA C735.
At the early stages of a project, a site evaluation and risk assessment should be carried out. Initially a Phase 1 Desk Study and Preliminary Risk Assessment should be undertaken. The findings at Phase 1 will determine whether additional investigative actions (e.g. Phase 2 investigations – intrusive, borehole, monitoring) are necessary. These investigations should result in the site being categorised for ground gas based on the associated risk (characteristic gas situation) which quantifies the level of risk and from which design options can be considered by the design team.
Dependant on the characteristic gas situation (CS), an associated ‘gas protection’ score must be achieved based on the type of building (Type A-D), as described in Table 4 of BS 8485:2015+A1:2019. This score is used to determine the required scope of the gas protection measures and should be achieved using a combination of two or more of the following elements within the design to provide appropriate system redundancy (available scores for each element are indicated in brackets):
- Ventilation (0.5 - 2.5 points);
- Structural barrier (2 - 2.5 points – refer to Table 5 of BS 8485+A1:2019); and
- Gas resistant membrane (2 points – refer to Table 7 of BS 8485+A1:2019).
Further information on the requirements for each element of protection can be found in BS 8485:2015+A1:2019.
Once a detailed design has been developed, this should be verified by the project team. Additionally, in accordance with CIRIA C735, an independent ‘Gas protection verification plan’ is required. A robust design and construction process should then be followed to install the gas protection measures and the requirements of ‘verification plan’ implemented. Once the construction has passed the independent verification, handover documentation should be prepared – this should highlight any measures required during the remainder of the construction process, for example to prevent damage, as well as during the maintenance and operation of the building.
Based on extracts from The Basement Information Centre’s ‘Basements: Ground Gases and Structural Waterproofing’. This publication aims to provide a best practice approach for below ground structures that addresses both gas proofing and waterproofing. It sets out a design and construction process, illustrated in the flowchart below, and provides expertise and advice as well as identifying some of the areas where designers need to be aware of potential conflicts between standards.