30Jan / 2015
Continuous ground-gas monitoring can be a powerful tool in better understanding the ground-gas regime. However, it is not always necessary and sometimes we can figure out what we need to know through comparing periodic monitoring data with our Conceptual Site Model.
This post sets out some of the situations where continuous ground-gas monitoring may be a good option.
When your data does not match expectations: Concentrations and flow rates collected through spot monitoring are sporadic and do not fit your conceptual site model. This means you need to risk assess to the highest concentrations and flow rates recorded. The result is over-designing protection measures and sometimes installing measures where they are not necessary.
When you need to capture worst case: Ground-gas, by nature, is highly variable and worst case conditions happen only occasionally. Rapid pressure drops like the one which caused the Loscoe event of 1986 (below) are very rare, but we can hope to capture something close with sufficient monitoring data. Continuous ground-gas monitoring greatly increases the likelihood that significant events, which approach worst case conditions, will be captured.
Monitoring migration pathways: Ground-gas will only migrate through a pathway under specific environmental conditions; unmanned continuous monitoring vastly increases the probability of capturing these events. Where new housing developments border existing landfills, these migration events must be designed for unless there is evidence that rules them out. Continuous ground-gas monitoring can improve confidence that landfill gas migration does not pose a risk to future receptors.
When time is not on your side: Continuous ground-gas monitoring increases the frequency of monitoring; this can reduce monitoring periods from months or years, to weeks. When development delay costs are unacceptable, continuous ground-gas monitoring can provide high quality data in a short period of time. Regulators are increasingly accepting of shortened monitoring periods using this method as the graphed data is easy to understand.
Highly sensitive sites: Where the end receptor is highly sensitive, or where future conditions compound the risk of ground-gas explosion, risk of under-protection must be avoided. Continuous ground-gas monitoring increases the probability of seeing the full ground-gas regime and allows for truly risk-specific protection.
Post development verification: In those cases where development has progressed and evidence of ground-gas protection measures is lacking, continuous ground-gas monitoring of internal rooms or subfloor voids can indicate the risk to occupants. In some cases this will reduce the need for destructive works to replace gas protection measures.
If you have any comments, or know of other scenarios where continuous ground-gas monitoring would be beneficial, please leave a comment below.
If you have any questions regarding continuous ground-gas monitoring please contact me or one of my colleagues on 0161 232 7465.