Nick O'Riordan is correct when he states that the design of gas protection measures should not be based solely on borehole flow rates (Letters, GE March 1999). To be able to use any classification system the gas regime below the site must be thoroughly investigated and understood.
The basis of any investigation should be a thorough desk study to identify all potential sources of gas, the likely rates of generation and possible migration pathways. This should be followed by a ground investigation which confirms any assumptions, allows characterisation of the gassing source and ground conditions and includes a sufficient period of gas monitoring.
The notes to Table 5 of our paper 'Reliability and risk in gas protection design', which were unfortunately omitted when it appeared in GE February 1999 (but published on the letters page in the following issue), are intended to make these requirements clear.
Once all this information has been collected it should be assessed and a design gas regime chosen which can then be applied using Table 5. If the measured borehole gas volume flow rates are lower than would be expected from a given source, or there are any other anomalies, then the design gas regime should be chosen to take account of this and/or further investigation undertaken.
In addition, we would stress that any gas monitoring should be undertaken by experienced and competent personnel who have the necessary knowledge to interpret any anomalies on site and adjust the monitoring regime in response, for example due to nearby construction activities. Unfortunately this important task is all too often given to untrained staff.
The question of placing housing on sites with borehole gas volume flow rates above 3.5litres/h requires many factors to be evaluated. For example, does the development have individual gardens or does it comprise flats with only communal areas? Is there a management company set up to maintain gas protection measures, are there any covenants applied to the properties, fluctuations in groundwater levels, etc?
Distribution of the gas emissions across a site must also be taken into account. For example, can any highly gassing area be isolated from the housing units? Evaluation of all factors can be complicated and lends itself to the use of statistical analysis using computer techniques. For this reason the authors consider that the use of quantitative risk assessment as proposed in CIRIA 152 should be used to assess the specific requirements on such sites.
We also note Tom Skailes' observation (Letters, GE March 1999) that the characteristic situations provided in Table 5 do not give an exact match between the CIRIA 149 situations and the Partners in Technology gas regimes. The characteristic situations given in the table provide a new basis for classifying sites which is slightly different to the characteristic situations given in Table 4 and also quoted in Partners in Technology. The Partners in Technology classifications are provided only for comparison purposes and no direct relationship is intended in Table 5.
Steve Wilson and Geoff Card
Card Geotechnics, Aldershot