07Feb / 2018
The introduction of continuous monitoring brought with it a parallel increase in demand for ‘receptor monitoring’. Previous to this, post-construction monitoring was not favoured by designers as it had the possibility of demonstrating that their plan was substandard. What changes will continuous receptor monitoring now bring for the construction industry?
Where a gas protection design has not been followed or has not been verified in line with good practice, then ‘receptor monitoring’ can provide the evidence that a development is safe. This proof is increasingly required to satisfy planning conditions for new developments, and is therefore becoming of increasing concern for developers.
What is ‘receptor monitoring’?
In any ground gas risk assessment, receptors must be identified in the pollutant linkage assessment. These receptors often include future site users, such as office workers and household occupants, as well as the building/construction itself.
‘Receptor monitoring’ is therefore monitoring undertaken either in the environment the receptor will be occupying (that is, any interior space of a building which a person is likely to occupy for any period of time), or within the sub-floor zone of a building (such as a sub-floor void). The most common locations for ‘receptor monitoring’ are the ground floor and/or basement areas of buildings, where potentially harmful ground gases are most likely to accumulate if ingress is occurring, or within the sub-floor void of buildings, which can help establish whether or not the ventilation of the sub-floor void is suitable enough to stop ground gases accumulating to harmful concentrations.
How crucial is ‘receptor monitoring’ in construction?
In 2013 and 2014, 22 people required medical attention due to serious health impacts associated with carbon dioxide poisoning arising from abandoned coal workings beneath a housing estate in Gorebridge. Subsequent continuous ‘receptor monitoring’ within the affected houses demonstrated that the maximum carbon dioxide concentrations were linked to falling atmospheric pressure events (Othieno R. et al., 2017).
The remedial solution chosen by the local authority was the evacuation of the whole housing estate and the demolition of 64 properties.
It is expected that the Gorebridge incident will result in an increase in local authorities requiring verification reports to be submitted on the adequacy of gas protection systems. Where these are not available the only other option open to developers and consultants will be to carry out ‘receptor monitoring’. Therefore, it is very timely that CL:AIRE TB 16 has been published on continuous sub-floor void monitoring – a subset of the ‘receptor monitoring’.
What actions should be taken?
If ‘receptor monitoring’ is required then a systematic approach should be adopted. The first stage should be a full internal survey of the ground-floor and basement of the property using a high resolution hand held monitoring device at ppm level, with particular attention taken around all service entries and confined spaces. This should be carried out during a falling atmospheric pressure event. The results of the survey will inform the choice of locations for subsequent internal continuous monitoring.
Usually these will be at service entry points and smaller rooms with reduced air changes. Continuous monitoring equipment should be installed with appropriate high resolution sensors i.e. 0.05% on a 0 to 5% scale for methane and carbon dioxide where these are the contaminant of concern. Continuous monitoring should then continue to capture two significant driver events that approach worst case conditions.
What’s the best practice approach?
As discussed in CL:AIRE TB 16, sub-floor void monitoring can be used to establish the gas risk under a building by accessing the void beneath the centre of a structure. An alternative to the TB 16 approach is demonstrating that the sub-floor ventilation system is effectively diluting any ground gases that are present.
This tried and tested approach requires the exhaust gas vents from the sub-floor void on the down-wind side of a property to be continuously monitored (see Figures 1a and 1b below). Such monitoring can only be effective on the leeward (down-wind) side of a building and should be accompanied by appropriate local weather information that demonstrates the wind direction and speed during the monitoring period.
It is also important that any sampling line to the void is not kinked or blocked. This can be avoided by using semi-rigid sampling tubes to access the sub-floor void space through the vents.
Figure 1a and 1b. Downwind sub-floor void monitoring.
How can GGS help?
GGS has extensive experience in ensuring sites are accurately monitored and confirmed safe for use. For example, our experts carried out sub-floor monitoring where unacceptable gas concentrations have been encountered (see Figure 2. Continuous monitoring results before (a) and after remediation (b)). These privately owned houses were successfully remediated and demonstrated to be safe by post-remediation sub-floor void monitoring. This showed that the improved ventilation had reduced the gas concentrations back to normal atmospheric conditions.
Figure 2. Continuous monitoring results before (a) and after remediation (b)
Our customisable continuous receptor and sub-floor void monitoring instruments are deployed seamlessly, allowing for the swift acquisition of accurate data. Our dedicated team of specialists then interpret this data and report it to you in a format that meets your requirements. Our best practice approach and market leading technologies ensure that your critical strategies are suitably informed by robust data, helping you make the right decisions.
Our continuous receptor and sub-floor void monitoring services can help you:
Avoid expensive corrective measures
Quicken planning and building control
Achieve cost-efficiency and save time
Access hassle free equipment deployment and data acquisition
Find out more about our sub-floor monitoring services here.