S A Wilson and G B Card, Card Geotechnics. This paper was first published in Ground Engineering in February 1999
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There is large variance in the scope of gas protection measures provided for developments on gassing sites. This is because of:
- unclear and ambiguous guidance
- subjective interpretation of gas regimes
- poor understanding of gas surface emission rates
In response to incidents of methane explosions in the mid-1980s, guidance documents were produced int eh UK for both planning authorities and the construction industry. Gas protection measures are now routinely included in engineering designs on any perceived high risk sites, eg former landfills, mine working, estuarine and fenland and limestone or Chalk areas.
Frequently, the scope of protection measures is specified solely on the basis of the maximum measured gas concentration, with no consideration given to the spatial distribution and frequency of the readings, borehole flow rates and estimated surface emission rate or the nature of the gassing source. The lack of comprehensive and reliable design method has led to inconsistency in the specification of gas protection systems utilising passive ventilation which can, in some circumstances, result in unsafe design.
This paper illustrates the use of a holistic approach to the characterisation of gassing sites based on gas concentrations, borehole flow rates, estimated surface emissions rate and the nature of the gassing source. It considers all the recently published guidance by the Construction Industry Research and Information Association (Ciria) and the Department of the Environment, Transport and the Regions.
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