Hi

CIRIA Report 152 (O’Riordan & Milloy, 1995, "Risk Assessment for Methane and
Other Gases from the Ground") has not been withdrawn either fully or in
part.  Nor am I aware of any intention to withdraw it either in part or in
full in the future as it provides excellent descriptions of risk assessment
practices.

In particular, CIRIA Report 152 presents Table 5.2 (p48), which is an
excellent tool to classify risk in the initial conceptual model stage of a
risk assessment to establish if any site does pose a risk to a proposed
development.  Table 5.2 should, however, not be used for more detailed risk
assessment procedures or for the development of detailed design options.

The main emphasis of CIRIA Report 152, however, is risk assessment through
FAULT TREE ANALYSIS.  What the authors of the report you are referring to
may be inferring is that there are significant restrictions of this
technique that must be considered.  The primary downfall is the narrow
nature of the result, allowing only one final outcome, and not acknowledging
the presence of numerous alternative outcomes.  It should also be noted that
the use of a fault tree generally results in a highly conservative approach,
with the worst-case scenario often being considered in each step of the
tree.  Fault tree analysis also has many "unknowns" within its application
and, as a result, should only be used by experts, who are generally few and
far between.

Maybe the authors of the report have the CIRIA documents confused?  Through
the extensive and improper use of CIRIA Report 149 (Card, 1995, "Protecting
Development From Methane") it has become common practice to obtain the
required protection measures for a development from the use of Tables 28 and
29 (page 144).  Here, gas concentrations or flow rates for either methane or
carbon dioxide can be referred to Characteristic Situations and protection
measures that were previously used are regularly stated by the industry as
those that are currently required for developments.  However, these tables
are consulted with little regard to the nature of the gassing source,
borehole flow rates and the estimated surface emissions.  Of particular
concern is the fact that some protection measures stated in CIRIA Report 149
should never be used on a low-rise residential developments (i.e. active
measures).  As the author of CIRIA Report 149 freely states in a later paper
(Card & Wilson, February 1999, “Reliability and Risk in Gas Protection
Design” in Ground Engineering) “[the] tables were never intended to be used
as a definitive design tool and were only prepared to show the typical scope
of measures for gas control that were in current use at the time the tables
were produced.”

Within Card & Wilson the Characteristic Situations were combined into a more
robust risk assessment tool.  Table 5 presents characterises gas situations
based on a variety of parameters, including concentration, borehole flow
velocity and borehole gas volume flow.  Comparison is made between the
characteristic gas regimes given in CIRIA Report 149 and the Partners in
Technology regimes.  An important fact to note with this article is that
Table 5 has VERY IMPORTANT MISSING FOOTNOTES regarding the proposed length
of times of gas monitoring.  These were reproduced at the end of the
article, but are not commonly referred to.

There is a new CIRIA ground gas document is being produced by Enviros
Consulting with RSK ESNR and is due for publication shortly (middle of
2006?).  This document aims to collate and draw upon all previous guidance
documents and considered best practice in ground gas investigations and risk
assessment.  In addition, the NHBC have commissioned guidance for internal
and external use that myself and Peter Witherington from RSK ENSR have
produced, which is due for publication before the CIRIA document (first
quarter 2006?).  Both of these documents will present clearer instruction on
risk assessment and will provide two different techniques, one for low-rise
housing using a series of Traffic Lights and another for other developments
using the revised Wilson and Card procedure.

I hope that the above answers your question and clarifies the use of both
CIRIA Reports 152 and 149.

Regards

Dr Richard Boyle
Environmental Scientist

RSK ENSR Limited
172 Chester Road
Helsby, Cheshire
WA6 0AR, UK

A member of the RSK ENSR Group

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