Chapter Eight: Air Quality And Gas Methane Membrane
Installation of the gas membrane
8.1 In carrying out our review, it quickly became clear to us that we needed to separate out our consideration of the installation of the gas protective methane membrane and its consequences from other parts of our Report. This was (a) because there were very specific public concerns associated with the membrane, its purpose and its safety, and (b) because this was the only ongoing precautionary measure undertaken under the planning consent to address concerns.
8.2 The concerns raised with us related to lack of clarity on its purpose, the ongoing integrity of the membrane, risks of methane exposure, any impacts from the settlement of the building on the methane and worries about a sensor having been triggered (and what that meant for confidence and safety).
Purpose of the installation of the gas membrane
8.3 We set out in Chapter 4 the history of site investigation and should record that a gases risk assessment was as integral a part of that as every other risk assessment aspect. We refer to the relevant sections of Chapter 4 for fuller more general background to that work.
8.4 There were a number of reasons why a gas membrane was installed at the school as part of the conditions for it being built.
8.5 The following extracts from reports produced at that time explain this and set out the consideration given to this issue.
Gases Risk Assessment
8.6 The Ramboll Report of January 2010 described the ground conditions as follows:
"the geology underlying the site is understood to comprise Made Ground including topsoil overlying probable landfill material up to a maximum depth of 8.45mbgl. No significant capping layer is present above the probable landfill material. Underlying the Made Ground are superficial deposits of peat, glaciolacustrine clay, silt and sand and glacial till to a maximum depth of 23.2mbgl. These superficial deposits overlie the solid geology of the Middle Coal Measures, reported at depths between 7.40mbgl and 23.20mbgl and comprise sandstone and mudstone with occasional bands of coal. The site is underlain by several coal seams, some of which have been historically mined by shallow workings. In addition, mine shafts have been identified in and St. Ambrose Ground Contamination Risk Assessment Report"
"4No. ground gas monitoring visits were undertaken as part of the initial investigation (URS, 2006) between August and October 2006. Maximum concentrations of 12.6% carbon dioxide, 4.0% methane and a maximum flow rate of 2.5l/hr was recorded during these monitoring visits.
3.4.2 An additional 4No. rounds of monitoring were undertaken following the URS investigation (2008) during August 2008 and maximum concentrations of 30.2% carbon dioxide, 68.7% methane and a negligible flow rate were recorded during these monitoring visits. However, these results were deemed invalid due to the negligible flow rates reported.
3.4.3 An additional 12No. rounds of monitoring were carried out between November 2008 and April 2009. Maximum concentrations of 32.2% carbon dioxide, 75.4% methane and a maximum flow rate of 10.9l/hr were recorded during these monitoring visits.
3.4.4 Following the Geotechnics Ltd. Investigation in October 2009 a number of additional boreholes were drilled and installed. 4No. ground gas monitoring rounds were completed from these boreholes between October to November 2009. Maximum concentrations of 43.3% carbon dioxide, 69.0% methane and a maximum flow rate of 16.2l/hr were recorded during these monitoring visits.
3.4.5 Based on current guidance (CIRIA C665; BS8485), the worst case results from all available data would classify the site as Characteristic Situation 4."
8.7 Reference is made to paragraph 28 of the Report we commissioned (see below) as to the meaning of Characteristic Situation 4. Such a situation represents a ground gas regime as "moderate to high risk" and is typified by a gas regime from mine workings and closed landfills of typically greater than 25 years of age. This is a reasonable approach, though as below we note that a lower assessment would have been sufficient.
8.8 At the construction phase, it can be seen from the Balfour Beatty Framework Agreement Gateway 3 submission, as follows-
"1.3.13 Ground Gas Protection
"The following recommendations are made in order to reduce risks to human health from identified ground gas impacts assuming Characteristic Situation 4:
1. Reinforced concrete cast in situ floor slab (suspended, non-suspended or raft) or beam and block or pre-cast concrete slab;
2. All joints and penetrations sealed; and
3. Proprietary gas resistant membrane and passively ventilated underfloor subspace or positively pressurised underfloor sub-space, oversite capping or blinding and in ground venting layer (in accordance with CIRIA C665/ BS8485).
"Ramboll have consulted with a leading contractor in the design and installation of ground gas protection systems. Based on the maximum concentrations of methane and carbon dioxide and the maximum flow rate recorded on site a high performance passively ventilated system has been designed. This will be revisited once the further ground gas monitoring results are made available."
8.9 By their letter of 11 October 2012, the installers (Landline) confirmed that they were signing off the installation of the gas monitoring system at the school, saying "The basic active gas monitoring system on this project is now fully operational and the building is fully protected from ground gas ingress by the whole system incorporated in the project".
Validation of work
8.10 As noted in other respects, the remediation work was peer reviewed by WSP, instructed for that purpose. The letter from WSP on their Report Review of 6 December 2012 said:
"Gas Protection Measures
- A classification of Characteristic Situation 4 was reported to be considered as appropriate for the site based on reported concentrations of ground gas during previous site investigations and Landline Containment Solutions (LCS) were contracted to design and install the protection system for the high school building.
- The specification detail for this is noted to include a 1.0mm HDPE gas barrier membrane and 25mm geocomposite void former attached to ground level venting outlets and a system of extraction fans as part of the LCS 'Basic Active' Gas Monitoring System. This system is also indicated to include a network of underfloor sensors, which sample the air beneath the building on a regular basis. Should these sensors detect elevated ground gas, an active fan system is switched on to introduce more air into the space beneath the building and dilute the concentrations of gas beneath it.
- Detail drawings illustrating the system installed are provided by way of appendix, which also highlight the presence of a reinforced concrete slab.
- LCS installed the system and CQA certificates and photographs prepared/taken by them are provided by way of appendix to detail inspection sign off. Ramboll also report that while they were not present full-time during the installation, checks were made during their watching brief visits to ensure all penetrations and joints had been appropriately sealed and that the installation work was being undertaken by qualified LCS engineers."
Expert assessment of the membrane
8.11 Mindful of the issues causing concern which we too do not have the expertise to understand, we asked specialist experts in this field the following questions:
- Can you help us understand in general terms the purpose, specification and public safety security features of methane membranes used for purposes such as those in the present case?
- When would it typically be installed?
- Why would it typically be installed?
- Please provide installation details
- Can you explain the site context and the gas protection design?
- Can you comment on building settlement concerns expressed to the Review that inter-act with points raised about the methane membrane?
8.12 Those specialists (Dr Geoff Card and David Mason) provided us with their report on 5 August 2019 and we refer to that Report for its full terms.
8.13 Inevitably that Report is of a technical nature, but we draw out three aspects.
8.14 Firstly, at paragraph 34 of the Report, it states that "the gas protection measures …are more than adequate for the gas regime identified at the school campus" and concluded that, while it was prudent to proceed on the basis of Characteristic Situation 4, a lower assessment would have been sufficient.
8.15 It concludes at paragraph 48 that "the ground gas protection system has been designed for a higher gas regime, [Characteristic Situation 4]. I consider that this is overly conservative for the gas regime and nature of the development. I consider that the gas protection as designed, specified and installed is more than adequate to resist ground gas migration into the building(s) and adversely affect indoor air quality".
8.16 Secondly, on the matter of sensors, paragraphs 38 to 41 of the Report states that-
"38. In addition to the gas membrane and concrete slab an active gas venting system has also been specified to remove and ground gas beneath the ground slab. The active ventilation system has a gas detection and fan control system for methane and carbon dioxide. Commissioning records have been provided and are dated 16th March 2012 to 8th March 2012. The manual for the system states that 'maintenance of this system is essential to ensure reliable operation. The system utilises mechanical components that run continuously, and these require overhaul on an annual basis and inspection every six months. Maintenance should only be carried out by a trained technician.
"39. I have not received evidence of any service records after the initial calibration. I understand the monitoring system alarm has been activated on occasions. The alarm does not mean that hazardous gas concentrations have been detected beneath the ground slab. In my experience the alarm to the ventilation system has been triggered when:
a) a concentration of ground gas reaches a percentage of the lower explosive limit for methane which is 5% methane in air. It is normal to set the alarm criteria at <1% methane in air, i.e. at least 1/20th of the lower explosive limit; or
b) a gas monitoring sensor in the ventilation system Is not working and needs servicing or requires replacement.
"40. Methane and carbon dioxide readings have been provided to me from late 2012 which show a peak methane concentration of 0.43% and a peak carbon dioxide concentration of 3% in the active ventilation system. These concentrations confirm non-hazardous gas concentrations detected beneath the ground slab.
"41. In order to assess the ongoing effectiveness of the gas protection measures it would be useful to obtain the current data relating to the gas concentrations recorded within the void as well as indoor air quality monitoring at various points throughout the school building including confined spaces such as cupboards".
8.17 Thirdly, on the implications of potential settlement (see Chapter 9), the Report says:
"42. In theory, any differential settlement between building components such as the foundations to the school buildings and the ground slab will affect the risk of membrane failure. In my experience such failures have occurred to the gas membrane where excessive differential ground settlement has occurred causing the floor slab to settle relative to the deep foundations which have remained fixed.
"43. For the Buchanan and Ambrose school buildings it appears from review of the architect's drawings that the buildings are supported on piled foundations. The piles will have been designed and constructed to transfer building loads to competent natural ground beneath any Made Ground or Fill and zone of potential significant ground settlement.
"44. The ground slab is designed as a suspended slab such that if the ground settlement occurs, for whatever reason, the slab will remain fully supported by the piled foundations and will not deflect or crack. If there is significant ground settlement beneath the slab there is the possibility that the gas membrane will also drop resulting in a void being produced between the membrane and the underside of the slab. For a small degree of settlement, typically up to 100mm, the membrane will stretch and accommodate the movement. For settlements of greater magnitude there is the possibility that the membrane will tear along joints or at fixed points where it is held up by the slab and/or foundation. If this occurs, then ground gas could migrate to the underside of the slab. The integrity of the membrane and concrete slab to resist ground gas cab [sic] be tested by carrying out a smoke or gas tracer test as set I describe in paragraph 23 of this report.
"45. It has been claimed that parts of the school hardstanding areas are "bubbling upwards" and that this could that be a sign of an accumulation of gas under pressure forcing the overlying ground and hardstanding to rise. In my opinion this is not as a result of ground gas accumulating under pressure. This is because the gas monitoring records to date show no evidence of high gas pressures in the boreholes. I consider that these apparent "bubbles" are probably the results of localised settlement in the surrounding Made Ground creating the impression of a localised area of rising ground at its centre."
8.18 In light of the comments set out at paragraphs 41 and 44 of the Report and because of the points we make at paragraph 8.20, it had seemed to us in discussion with the commissioned experts that in order to be fully satisfied in these respects, it would be prudent for them to carry out certain gas as referenced at paragraph 23 of the Report. This would assist in giving confidence about the ongoing integrity of the design.
8.19 David Mason arranged for tests for methane and CO₂ via the gas monitor and hydrocarbons via the PID to be carried out on 26 July 2019 and the results were set out in an email from his firm of 5 August 2019 saying (by the tester):
"I attended St Ambrose High school on the morning of July 26th, and met Eric Hislop and the Head Teacher. I had with me a MiniRae Lite photoionisation detector (PID), a Gas Data GFM 430 portable gas monitor, as well as and MEP gas monitoring technician to allow each of us to operate the two pieces of equipment independently. Gas monitoring was undertaken continuously during the visit, as Eric Hislop took us on a tour of the building. Particular attention was given to smaller rooms such a cleaning cupboards, individual bathrooms and storage cupboards, as well as rooms with multiple service penetrations such as larger bathrooms, and changing rooms (with shower facilities/drains).
"We were on site for approximately one hour, a while photographic record of the visit was taken and can be provided upon request.
"Throughout the visit the PID monitor registered 0 ppm and did not waiver throughout the visit, similarly the gas monitor failed to detect any methane during the whole of the visit.
"A maximum concentration of 0.2% carbon dioxide was noted in the changing rooms in the eastern most and south eastern most changing room facilities. The concentration of carbon dioxide in this area was observed to fluctuate between 0% and 0.2% and was not the result of any direct observation (the sweeping of a drain or confined area) rather the 0.2% readings were noted in more open parts of the changing room area."
8.20 In the time available, there are two matters that we have not been able to complete consideration of. These are in relation to being satisfied that the periodic testing results since installation are satisfactory and to be fully satisfied as to the integrity of the membrane. We were unable to obtain recent recordings of the methane membrane in operation. Our independent experts considered the membrane was not a requirement to manage the risk of gases entering the building (a characteristic situation 2 rather than 4) and we undertook precautionary testing in the building to rule out the risk of methane, other hydrocarbons or carbon dioxide.
8.21 The test results at paragraph 8.19 satisfy us as to ongoing current safety, but we consider it important that more detailed material is made public about the records of previous periodic testing. Associated with the work of the Site Recovery Group, we recommend that this be done as soon as possible. We believe that this would be helpful for reassurance on the site if there were more up-to-date information from methane membrane monitoring to be made available.
8.20 The review team are content that the gas membrane was put in place on a precautionary basis to lower the risk of possible inhalation of gases by site users. It is worth pointing out the gas membrane was recommended as one of three precautionary measures to protect site users. The others were an underground ventilation system and a concrete slab on which the school was built. What the gas membrane adds is an active process of ejecting gas via extractor fans should the concentration of gas (methane and carbon dioxide) reach a certain level. This concentration level is set many times lower than what would constitute a health risk.
8.21 For the system to operate, a network of sensors sample the air above ground level (beneath the concrete slab) on a regular basis. Should these sensors detect elevated ground gas, an active fan system is switched on to introduce more air into the space beneath the ground and the concrete slab and dilute the concentrations of gas beneath it. The ventilation vents beneath the membrane release this air/gas mixture into the atmosphere at a distance from the building.
8.22 There are sensors in the membrane to detect gases – methane and carbon dioxide. These sensors transmit data for monitoring purposes to a recording device to demonstrate the frequency with which the ventilation system has been activated. As part of this system, there is an alarm to signal if a sensor becomes activated when it detects ground gas to initiate active ventilation or because it is faulty. This is not the same as a fire alarm which indicates immediate evacuation is required. The membrane itself continues to function if the alarm goes off and this can be demonstrated by its output in terms of the monitoring record. A graphic illustration of the concept of the membrane is provided below.
8.25 As it is relevant to this Chapter, we should at this point address the calls made on us to require testing of the air both outside the buildings and within.
8.26 We had this in mind when we initiated soil and water testing but needed the expert advice we were commissioning to guide us further. Equally in relation to the methane membrane, we were conscious that issues concerning methane might suggest that air testing might be valuable. Subject to our recommendation to provide future reassurance and to paragraph 8.29 below, we consider that the work in that regard is sufficient without further air testing.
8.27 Air testing requires a logical approach – where to look and what to look for. To begin with, our soil testing included an assessment of volatile organic compounds (VOCs) which could have been a gaseous hazard on the site. As chapter 7 reports, there were no significant levels of VOCs found at ground level from the sampling pits.
8.28 Following the review of the methane membrane a simple test of air in small spaces in the schools provided an assurance that no methane or other hydrocarbons were an issue.
8.29 This left only one aspect of indoor air quality to assess, which could only be of value when it was being used by pupils and staff – namely temperature, humidity and concentrations of carbon dioxide. As we report in Chapter 5 on health concerns, this is an area worth investigating further.
8.35 While we are generally assured in respect of the gas membrane installed as a precautionary measure on construction, we are conscious both of the information unable to be considered in time and the proposals we make at Chapter 10 concerning Site Recovery.
8.36 Continued confidence in the methane membrane is essential. While, as at the date of this Report, we consider that parents and staff can have a high level of confidence in it, it would seem remiss not to include the taking of appropriate steps to monitor its continued effectiveness over the period ahead.
8.37 We therefore (a) recommend that more detailed material is made public about the records of previous periodic testing as soon as possible and (b) include the methane membrane and ongoing assurance as to its integrity as an element of the work of the proposed Site Recovery Group.
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