Continuous Monitoring and Statistical Approach to Ground Gas Risk Assessment at a Substation Site by Atkins
Shortlisted Brownfield Awards Category 6 - Best Project Closure/Verification
The site is an electrical substation compound in the south of England. Historical maps indicated that a landfill crossed the site boundary near the substation. The ground investigations undertaken by Atkins and others at the site had identified elevated concentrations of ground gas (particularly methane and carbon dioxide) in the vicinity of and directly beneath an electrical relay room.
The concentrations of methane recorded within the ground had been previously assessed as representing an unacceptable explosion risk. Consequently, a temporary gas alarm was installed and operational.
Atkins reassessed this site using additional ground gas data (including gas flow) collected using the latest available technology and by applying a statistical approach involving probability density functions (PDFs) of gas data as recommended in the Ground Gas Handbook. This has reduced the level of conservatism and increased confidence in the estimated risk level such that the ground gas risk has been demonstrated as acceptably low. As a consequence, the requirement for ongoing gas control and mitigation measures has been shown to be unnecessary enabling closure of the project and the lifting of operational controls.
The site is underlain by Made Ground (including a historically infilled gravel pit) and Superficial Deposits over the Holywell Nodular Chalk Formation. Gravel pits were identified both on site and extending to the north of the site on historical maps between 1960 - 1964 and were infilled with landfill waste prior to construction of the substation in 1972 -1973.
Biodegradable materials such as domestic refuse, wood and paper which had been identified within the infill materials are capable of generating landfill gas including methane. Therefore, there was potential for ground gas to migrate through the ground and gaps or cracks in the floor and accumulate in the buildings on site.
The site had been characterised by a series of historical ground investigations and assessments undertaken since 1966. Monitoring locations within the historical landfill had reported methane concentrations between 41.8 and 97.2 % v/v. Boreholes located next to the relay room of the 132kV substation had reported lower methane concentrations, but still above the Lower Explosive Limit (LEL).
As a result of the potential risk identified from ground gas, a gas alarm was installed within the sub-floor void of the relay room on a precautionary basis and an emergency plan was implemented; this arrangement remained operational for over five years since May 2014. In addition, four venting cowls connected to gas wells constructed in boreholes were installed around the perimeter of the relay room building in 2017 to mitigate the risk from elevated concentrations of ground gas.
The challenge was to understand better the risks to site users and to reduce or remove the operational costs, disruption and administration complexities involved in maintaining the gas alarm, either through physical remediation or further detailed risk assessment.
Figure 1 – Sub-slab monitoring (Indoor)
To resolve the challenge, Atkins undertook a programme of innovative continuous gas and flow monitoring, to inform an unconventional detailed quantitative risk assessment (DQRA).
The continuous monitoring was undertaken using Ambisense GasfluX™, which is the world’s first device able to monitor real time continuous gas and flow including ambient conditions such as barometric pressure, temperature and humidity.
A total of seven weeks of continuous monitoring was carried out at two locations on site. The outdoor unit was attached directly to an existing monitoring location (borehole) along the potential ground gas pathway. The indoor unit was attached to sub-slab monitoring location (core hole) at the receptor.
The in-built technology of telemetry in the device was used to check when two critical pressure drops had been achieved in accordance with the latest CL:AIRE guidance.
The investigation also included conventional spot monitoring across the site using a hand-held GA5000 gas analyser unit during the same period as the continuous monitoring was undertaken. Validation of the continuous monitoring data was undertaken by comparison with the spot monitoring data which showed that the two data sets were in good agreement.
Continuous gas monitoring results and gas screening values (GSV) were calculated as per the modified Wilson and Card methodology for both the outdoor (borehole) and indoor (sub-slab) monitoring locations. The rating of characteristic situation (CS) based on the calculated continuous GSV for new developments demonstrated that at least 12 % of the monitoring period was CS2, which implies low risk but still requires gas protection measures.
Unconventional Risk Assessment
While the calculated GSVs are useful for initial screening of potential gas risks, GSVs do not assess quantitatively the risk posed by extreme events which are unlikely to occur in a given monitoring period.
The most commonly used method of quantitatively assessing risk on gassing sites is that described in the CIRIA Report 152. This uses a combination of forward modelling (to evaluate the gas concentration at the receptor) and fault tree analysis to provide a numerical estimate of the risk (i.e. a probability that an adverse effect will occur in any year or other specified period).
The method used in this assessment was a variation on the above and used backward modelling (to evaluate the gas flow required for ventilation failure) and the statistical approach of considering the PDFs for gas flow based on the Ground Gas Handbook and Atkins’ extensive experience of undertaking gas risk assessments.
Probability of Methane Explosion Event
The probability of failure to ventilate the building adequately was derived using a statistical approach made possible by the collection of continuous gas flow and methane concentration data collected from the sub-slab monitoring location at the receptor. Under this approach, a set of PDFs were fitted to the methane flow rate.
Comparing a set of five PDFs with a histogram and empirical density function of the observed data, the gamma distribution was identified as the best fit.
Based on the best‑fitting PDF, the probability of exceeding the critical flow rate of methane was calculated as 1.1 x 10 and the annual probability of a methane explosion event that would result in harm to human health was calculated as 2.7 x 10 (1 in 3.6 x 1062 years). According to the CIRIA C665, this level of risk is of no concern.
Figure 2 - Indoor (sub-slab) methane GSV histogram and probability density functions.
Assessment of Residual Gas Risk
The combination of using statistical and fault tree analysis approach and use of the innovative continuous gas monitoring technology conclusively demonstrated that there was not an unacceptable risk of methane explosion within the relay‑room building. Therefore, Atkins was able to recommend that the existing monitoring activities and other control measures including a gas alarm system could safely be discontinued at the site.
Throughout the project, Atkins worked closely with the client to achieve a cost effective and technically robust solution to reduce the operational costs, disruption and administrative complexities involved in maintaining the gas alarm and monitoring system. Some specific benefits the project achieved were:
The innovative technology in the Ambisense GasfluX™ (i.e. telemetry and real time monitoring) was utilised successfully to collect continuous gas and flow data within a short time frame for use in the DQRA;
The outcome of the assessment demonstrated no requirement to undertake further costly remediation;
There will be a saving on the operational and maintenance costs of a highly specialised gas alarm and emergency plan;
Increased confidence that site users and critical national infrastructure is not being put at risk; and
Carbon footprint reduced as the need for regular site visits to maintain the gas alarm or to undertake remediation works is removed.
Application of Best Practice
All works have been undertaken with care and diligence in relation to current best practice and legislation as follows:
Supplementary gas monitoring was carried out from both the sub-slab location and gas from wells constructed according to the current best practice given in BS8576:2013 and with full compliance with critical health and safety requirements. To date there have been no reportable incidents or disruption of the current site operation;
Best practice was applied in the continuous gas monitoring as recommended in the latest CL:AIRE guidance both to collect sufficient data and to make sure that the critical pressure drop was captured;
The continuous gas monitoring data collected using the latest available technology were verified against conventional monitoring data, which showed good agreement; and
The risk assessment was undertaken using the statistical approach recommended in the Ground Gas Handbook and compared the results of assessments undertaken in accordance with the CIRIA C665.
To discount the residual risk from ground gas at the site, conventional approaches to monitoring were not sufficient due to the site constraints and the conservatism of conventional gas risk assessments. Consequently, an innovative monitoring scheme involving a novel statistical approach and fault tree analysis was adopted which provided robust scientific evidence to justify that no further gas monitoring or gas alarm was required. The adoption of this approach resulted in a sustainable and cost-effective outcome.