Publication C665 Assessing risks posed by hazardous ground gases to buildings Errata Readers are advised of the following corrections to the document: Figure 1.1, page 5 Figure revision (see attached page) Table 8.5, page 88 Table revision (see attached page) Table 8.6, page 90 Table revision (see attached page) te text, page 91 Text revision (see attached page) Box 8.2, page 91 Box revision (see attached page) Section 8.3.2, page 92 Text revision (see attached page) Table 8.7, page 92 Table revision (see attached page) Box 8.4, page 94 Box revision (see attached page) We apologise for any inconvenience this may have caused.
Site characterisation Assessment of risk Determination/validation of remediation Define the context an set the objectives Review data Identify remedial objectives to mitigate unacceptable risks Carry out phase one desk study Develop initial conceptual site model and undertake preliminary risk assessment Is data reliable? (eg appropriate response zones, variable/ unrepresentative groundwater levels) Identify feasible remedial options Undertake additional intrusive investigations Has development of model included site specific factors that may influence gas/ vapour regime? Has monitoring been carried out under varying conditions likely to influence the gas vapour regime? Undertake additional intrusive investigations and/or monitoring and sampling Has sufficient data been obtained to allow the selection/design of appropriate remedial solutions? Have these factors identified the potential presence of gases/vapour Results sufficiently consistent/reliable Detailed evaluation of remedial options Develop a remedial strategy Are there potential unacceptable risks? Identify further actions to clarify potential unacceptable risks Source(s) of gas(es)/ vapour(s) identified? Extent of source(s) established? Design, implementation and verification of remedial measures Is post installation/ construction monitoring required? Undertake post installation/ construction monitoring Review amend remedial strategy Establish objectives of any further investigations Carry out further investigation (desk based/intrusive/monitoring Refine conceptual site model Consider odour and toxicity and incorporate into risk assessment as appropriate Undertake appropriate risk assessment modelling. Define gas regime Is the monitoring data acceptable? Completion/ validation report Green Amber Red further action required Does risk assessment demonstrate corrective action required? further action required further action required Figure 1.1 The process of managing risks related to hazardous ground gases 5
Table 8.5 Modified Wilson and Card classification Characteristic situation (CIRIA R149) Comparable classification in DETR et al (1999) Risk classification Gas screening value (GSV) (CH 4 or CO 2 ) (l/hr) 1 Threshold Additional factors Typical source of generation 1 A Very low risk <0.07 Typically methane <1 % and/or carbon dioxide <5 %. Otherwise consider increase to Situation 2 Natural soils with low organic content Typical made ground 2 B Low risk <0.7 Borehole air flow rate not to exceed 70l/hr. Otherwise consider increase to characteristic Situation 3 Natural soil, high peat/organic content. Typical made ground 3 C Moderate risk <3.5 Old landfill, inert waste, mineworking flooded 4 D Moderate to high risk <15 Quantitative risk assessment required to evaluate scope of protective measures. Mineworking susceptible to flooding, completed landfill (WMP 26B criteria) 5 E High risk <70 Mineworking unflooded inactive with shallow workings near surface 6 F Very high risk >70 Recent landfill site tes on the use of Table 8.5 1 Gas screening value: (Litres of gas/hour) is calculated by multiplying the maximum gas concentration (%) by the maximum measured borehole flow rate (l/hr) see Glossary. 2 Site characterisation should be based on gas monitoring of concentrations and borehole flow rates for the minimum periods defined in Table 5.5. 3 Source of gas and generation potential/performance should be identified. 4 Soil gas investigation should be in accordance with guidance provided in Chapters 4 to 6. 5 If there is no detectable flow, use the limit of detection of the instrument. 6 The boundaries between the Partners in Technology classifications do not fit exactly with the boundaries for the CIRIA classification. Box 8.1 Situation A examples Example 1 Site to be developed for commercial/industrial units with some residential flats and the soil gas investigation has identified a maximum carbon dioxide concentration of 3.0 per cent with a worst-case flow rate of 2.0 l/hr. The gas screening value (GSV) can be calculated as: 0.03 2.0 = 0.06 l/hr So the site will be characterised as characteristic situation 1. Example 2 Site to be developed for commercial/industrial units with some residential flats and the soil gas investigation has identified a maximum methane concentration of 4.2 per cent with a worst-case flow rate of 5.0 l/hr. The gas screening value (GSV) can be calculated as: 0.042 5.0 = 0.21 l/hr So the site will be characterised as characteristic situation 2. 88
prescriptive approach to detailing protective systems and allow a wider choice in the use of different components. For example a ventilated underfloor void or a positive pressurisation system is one level of protection. The key issue surrounding gas membranes is their ability to survive the construction process intact (see Chapter 9) and also possibly resist differential settlements. Membranes should be selected based on their performance characteristics and ability to survive the construction phase. An unreinforced 1200 g membrane is unlikely to achieve this and the minimum thickness of gas resistant membrane proposed is unreinforced 2000 g for low-risk sites. The range of protective measures that are available, and their detailed design, is discussed in Chapter 9. Table 8.6 Typical scope of gas protective measures Characteristic situation (From Table 8.5) Residential building (not those which belong to Situation B) 1 Number of levels of protection Typical scope of protective measures Office/commercial/industrial development Number of levels of protection Typical scope of protective measures 1 ne special precautions ne special precautions 2 2 a. Reinforced concrete cast in situ floor slab (suspended, nonsuspended or raft) with at least 1200 g DPM and underfloor venting. b. Beam and block or pre-cast concrete and 2000 g DPM/ reinforced gas membrane and underfloor venting. 1 to 2 a) Reinforced concrete cast in situ floor slab (suspended, non-suspended or raft) with at least 1200 g DPM. b) Beam and block or pre cast concrete slab and minimum 2000 g DPM/reinforced gas membrane. c) Possibly underfloor venting or pressurisation in combination with a) and b) depending on use. 3 2 All types of floor slab as above. Proprietary gas resistant membrane and passively ventilated or positively pressurised underfloor sub-space. 4 3 All types of floor slab as above. Proprietary gas resistant membrane and passively ventilated underfloor subspace or positively pressurised underfloor sub-space, oversite capping or blinding and in-ground venting layer. 5 4 Reinforced concrete cast in situ floor slab (suspended, non-suspended or raft). Proprietary gas resistant membrane and ventilated or positively pressurised underfloor sub-space, oversite capping and in-ground venting layer and inground venting wells or barriers. 6 5 t suitable unless gas regime is reduced first and quantitative risk assessment carried out to assess design of protection measures in conjunction with foundation design. 1 to 2 All types of floor slab as above. Minimum 2000 g/reinforced gas proof membrane and passively ventilated underfloor sub-space or positively pressurised underfloor sub-space 2 to 3 All types of floor slab as above. Proprietary gas resistant membrane and passively ventilated or positively pressurised underfloor sub-space with monitoring facility. 3 to 4 Reinforced concrete cast in-situ floor slab (suspended, non-suspended or raft). Proprietary gas resistant membrane and passively ventilated or positively pressurised underfloor sub-space with monitoring facility. In ground venting wells or barriers. 4 to 5 Reinforced concrete cast in-situ floor slab (suspended, non-suspended or raft). Proprietary gas resistant membrane and actively ventilated or positively pressurised underfloor sub-space with monitoring facility, with monitoring. In ground venting wells and reduction of gas regime. te: 1 For low rise traditional housing with ventilated clear underfloor void (ie Situation B. See Table 8.7, Box 8.4 and Section 8.3.2). Typical scope of protective measures may be rationalised for specific developments on the basis of quantitative risk assessments. te the type of protection is given for illustration purposes only. Information on the detailing and construction of passive protection measures is given in BR414 (Johnson, 2001). Individual site-specific designs should provide the same number of separate protective methods for any given characteristic situation (see Card, 1996). 90
In all cases there should be minimum penetration of ground slabs by services and minimum number of confined spaces such as cupboards above the ground slab. Any confined spaces should be ventilated. Foundation design should minimise differential settlement particularly between structural elements and ground-bearing slabs. Commercial buildings with basement car parks, provided with ventilation in accordance with the Building Regulations, may not require gas protection for characteristic situations 3 and 4. Floor slabs should provide an acceptable formation on which to lay the gas membrane. If a block beam floor is used it should be well detailed so it has no voids in it that membranes have to span, and all holes for service penetrations should be filled. The minimum density of the blocks should be 600 kg/m³ and the top surface should have a 4:1 sand cement grout brushed into all joints before placing any membrane (this is also good practice to stabilise the floor and should be carried out regardless of the need for gas membranes). The gas-resistant membrane can also act as the damp-proof membrane. 2 Based on Building Regulations Approved Document C (Office of the Deputy Prime Minister, 2004a) which states that a membrane below the concrete could be formed with a sheet of polyethylene, which should be at least 300 mu thick (1200 g). Please note the alteration from 300 mm (as stated in the Approved Document C) to 300 mu. 300 mm is a typographical error that has been recognised and corrected for this publication. The levels of protection referred to in Table 8.6 provides a number of protective elements (ie collectively the whole system) that are each capable of protecting a building on their own. In case of failure or damage of one element the remaining element(s) continue to effectively protect the building. The whole system should be designed accurately for the site risk with appropriate interdependence between components and levels of redundancy as determined by a risk-based approach. The primary method of protection should be the creation of an envelope below the building. Passive ventilation is preferred as it requires less maintenance than any system with fans. This is especially so in freehold/low rise residential developments. Any active system (dilution or positive air) should be able to vent passively in the event of fan failure. The methods of protection are discussed in Chapter 9. Box 8.2 Owen and Paul risk assessment methodology A methodology was derived by Owen and Paul in 1998 as a part of the DETR Partners in Technology Guide for design research report (DETR et al, 1999). This method is similar to the Wilson and Card approach using both gas concentrations and borehole flow rates to define a gas regime for methane and carbon dioxide. However, by using this approach the calculated regime will be a factor of 10 below the gas screening value derived using the Wilson and Card (1999) approach. This difference is due to the fact that Owen and Paul use the Pecksen approach (1986) in addition to gas concentration and borehole flow rate (Owen et al, 1998). A number of practitioners have raised concerns regarding the assumptions behind the Pecksen methodology, in particular the assumption of a 10 m² zone of influence of a standpipe. There is no doubt that further research is required in this area of the subject. In the meantime, practitioners should make proper consideration of the uncertainties associated with this type of calculation in their risk assessments (that is by carrying out appropriate sensitivity analyses) and should also consider obtaining data on direct surface emission rates to provide greater confidence in any such calculation of gas regimes. 91
8.3.2 Situation B Low rise housing with a ventilated underfloor void (min 150mm) Step 4B Characterise site and determine the gas screening value The NHBC have developed a characterisation system that is similar to the Wilson and Card system, but is specific to low-rise housing development with a clear ventilated underfloor void (Witherington and Boyle, 2006) (see Table 8.7). This is a risk-based approach that is designed to allow an identification of gas protection for a low-rise housing development by comparing the measured gas emission rates to generic traffic lights scenarios. The traffic lights include typical maximum concentrations and are provided for initial screening purposes and risk-based gas screening values (GSVs) for consideration in situations where the typical maximum concentrations are exceeded. However, the assessor should carefully evaluate the soil gas regime before proceeding with a design where the typical maximum concentration is exceeded. It should be noted that the method used to develop the GSV thresholds is based on a number of assumptions regarding the proposed structures, and designers should ensure that these assumptions are appropriate to their site. If the proposed low-rise housing development differs significantly from the model low-rise housing development (for example, deeper sub-floor void, increased ventilation or larger building footprints), sufficient information should be presented so that the assessor can derive site-specific GSVs. This information is also contained within Appendix A7. The calculations should be carried out for both methane and carbon dioxide, and the worst case adopted in order to establish the appropriate protection measures. It is also important to note that the GSVs are derived from one air change per day in the subfloor void providing a simple assessment. As previously stated, if the designer can adequately demonstrate that vent rates are greater (for example, when calculated using BS 5925) then higher site-specific GSVs may be calculated. However, any such alternative assessment should include a sensitivity analysis to take into account the effects of occupiers blocking air vents, for example by construction of patios. As described in Section 8.3.1 it is important to recognise that the GSV is a guideline value and not an absolute threshold. That is, the GSV quoted in Table 8.7 can be exceeded in certain circumstances (see Footnote 5 to Table 8.7). Examples of the determination of the site characterisation (green, amber or red) under Situation B are given in Box 8.3. Table 8.7 NHBC Traffic light system for 150 mm void Traffic light Methane 1 Carbon dioxide 1 Typical maximum concentration 5 (% v/v) Gas screening value (GSV) 2,4,6 (litres per hour) Typical maximum concentration 5 (% v/v) Gas screening value (GSV) 2,3,4,5 (litres per hour) Green Amber 1 Amber 2 Red { { { 1 0.13 5 0.78 5 0.63 10 1.60 20 1.56 30 3.13 92
Box 8.3 Situation B examples (contd) Example 5 Site is to be developed for low-rise housing and the ground investigation has identified a maximum methane concentration of 69.3 per cent methane and a worst-case flow rate of 1.7 l/hr. The GSV will be calculated as: 0.693 1.7 = 1.178 l/hr The GSV puts the site in amber 2. However, the gas concentration is very high, at nearly three and a half times the typical maximum value for red. So consideration should be given to whether the site should be characterised as red. To still progress with a new build at the site, the assessor should be extremely confident that a very thorough site investigation has been carried out and that the ground gas regime, in particular the flow rates, has been appropriately characterised over a suitable length of time and at the worst-case conditions, and that all data is robust and beyond scrutiny. Further, consideration into all possible methane generation and migration potentials should have been fully characterised within a sound conceptual site model, which should take into account how the ground gas regime (especially flow rates) may be impacted by partial sealing of the site with the buildings and roads of the specific development. Step 5B Define scope of protection Based upon the traffic light classification that is calculated for the site for low-rise housing development only, the scope of protection can be defined using Box 8.4. Box 8.4 Gas protection measures for low-rise housing development based upon allocated NHBC Traffic light (Boyle and Witherington, 2007) Traffic light classification Green Amber 1 Amber 2 Red Protection measures required Negligible gas regime identified and gas protection measures are not considered necessary. Low to intermediate gas regime identified, which requires low-level gas protection measures, comprising a membrane and ventilated sub-floor void to create a permeability contrast to limit the ingress of gas into buildings. Gas protection measures should be as prescribed in BRE Report 414 (Johnson, 2001). Ventilation of the sub-floor void should facilitate a minimum of one complete volume change per 24 hours. Intermediate to high gas regime identified, which requires high-level gas protection measures, comprising a membrane and ventilated sub-floor void to create a permeability contrast to prevent the ingress of gas into buildings. Gas protection measures should be as prescribed in BRE Report 414 (Johnson, 2001). Membranes should always be fitted by a specialist contractor. As with amber 1, ventilation of the sub-floor void should facilitate a minimum of one complete volume change per 24 hours. Certification that these passive protection measures have been installed correctly should be provided. High gas regime identified. It is considered that standard residential housing would not normally be acceptable without a further gas risk assessment and/or possible remedial mitigation measures to reduce and/or remove the source of gas. In certain circumstances, active protection methods could be applied, but only when there is a legal agreement assuring the management and maintenance of the system for the life of the property. 94