Benefits of new ground gas monitoring methods
Introduction:
We have completed several phases of ground investigation for this residential housing project with the Pobl Group and JEHU.
A focus of the Conceptual Ground Model was very high levels of methane and carbon dioxide, potentially limiting residential development options without a comprehensive site-specific risk assessment. Contemporary ground gas monitoring methods using in-situ devices provided suitable evidence to reclassify the site with a reduction in gas protection measures (CIRIA C665).
Details:
The Jehu Group are developing the subject site for the Pobl Group as residential housing within the SA1 Swansea development. Initial ground gas monitoring and assessment of the risks identified very high levels of methane (up to 37.5%) and high levels of carbon dioxide (up to 21%). The maximum recorded levels of methane were above upper bound values for residential development.
We undertook a three-week period of continuous gas monitoring to better inform the Conceptual Site Model and provide the data for a Tier Two assessment of risks to the development from hazardous ground gas, using additional lines of evidence over and above the maximum gas concentrations and flow rates.
Continuous monitoring of gas levels is less common than spot monitoring, and is usually required only where the gas situation needs to be characterised in detail. Monitoring is undertaken within a gas well using in situ data loggers that record time series data at a monoitoring frequency that exceeds the frequency of change of the measured parameter. Data loggers (gas clam and gas sentinel instruments) were installed in three gas wells. During the monitoring period, levels of methane, carbon dioxide, oxygen, trace volatile organic compounds, carbon monoxide and hydrogen sulphide were measures along with atmospheric pressure (a main risk driver), borehole pressure, temperature and groundwater levels.
An example of the concentrations and frequency of data capture is shown below; a major benefit is the accumulation of a larger data set that can be interpreted to provide a more informed characterisation of the site.
Our data suggested microbial respiration of organic materials in the soil and enabled us to rule out the potential influence of nearby landfill materials (Teasdale et al, 2015). Our focussed continuous gas monitoring provided suitable evidence to reclassify the site as suitable for the residential development with a reduction in gas protection measures (CIRIA C665). This was discussed and agreed with the Local Authority and NHBC.