UXO Risk Assessments – A Guide To Good Practice

Introduction

A UXO risk assessment is the essential starting point of the UXO risk management framework outlined by the CIRIA C681 ‘Unexploded Ordnance (UXO), a Guide for the Construction Industry’ guidance.  This blog will focus on detailed UXO risk assessments, as we have discussed preliminary UXO risk assessments in a previous blog.

Although there is no direct legislation in the UK about dealing with UXO, the potential risk UXO poses falls under broader regulations for managing risk on construction sites and Health & Safety requirements.

Acquiring a thorough UXO risk assessment is important to ensure the safety of those working on a site by providing confidence and clarity on what the UXO hazard is, as well as covering the appropriate actions required to mitigate any identified risk.

When it comes to costs, commissioning a UXO risk assessment before construction can help avoid unnecessary follow-on mitigation work, ultimately avoiding programme delays, ensuring safety on site, and saving money in the long run.

What makes a thorough UXO risk assessment?

To ensure a UXO risk assessment is thorough, robust, and defendable, there are several key elements, which are discussed below.  The importance of archival research, record corroboration, and historical aerial analysis will be explored in more detail in future blog posts.

Site-specific research

A UXO risk assessment must be based on site-specific research.  Lack of specificity in the research can encourage a reliance on assumptions and generalisations, resulting in overly cautious judgements and unnecessarily elevated hazards.

For instance, not all military sites will have a high risk of encountering UXO.  It is important to understand the operational history of the establishment, the potential UXO hazard sources this may provide, and whether it actually affects your site.

Likewise, assuming that the site has a high risk of encountering Unexploded Bombs (UXB) simply because it is in the East End of London is unhelpful.  There are a range of detailed records available that could demonstrate that your site, despite being in a heavily-bombed area, was not hit.

Too often we see detailed UXO risk assessments assigning a high UXO risk based on generalisations, rather than site-specific records.

Corroboration of sources

A UXO risk assessment should demonstrate that a wide range of sources have been consulted and corroborated.  Archives should be checked for new historical records for every UXO risk assessment undertaken, instead of relying solely on records previously collected and in-house databases.  Reliance on existing research and databases can cause errors or inaccuracies to be replicated and amplified throughout the research, leading to a misinformed understanding of the site and its UXO hazard.

For example, high-level bomb maps may give the impression significant bombing took place in an area.  However, localised bomb maps will typically have more accurately recorded locations, allowing an informed judgement to be made about the bombing hazard for the site.

High level (L) vs localised (R) bomb maps (source: Medway Archives)

Detailed historical aerial photography

Relevant and detailed historical aerial photography should be consulted to gain an accurate depiction of how the site looked in the past, and to track changes over time.  Aerial photography is also an important corroborative source, as it may reveal past activity on the site not recorded otherwise.

Readily available historical aerial photographs not only often cover a very limited date range but also may be subject to doctoring to hide important military established due to wartime censorship, as shown below.

Comparison of readily available (L) and specialist archive (R) historical aerial photography (source: GeoInformationGroup; Historic England)

Competent staff

Analysis of both aerial photography and historical records should be undertaken by demonstrably experienced staff, with appropriate skillsets such as historical research and risk assessment.  Whilst most UXO specialists have Explosive Ordnance Disposal (EOD) expertise, personnel with a background solely in EOD are not typically suitable for undertaking desk-based risk assessment.  The competency of an organisation and relevant staff should be outlined in project proposals, with case studies to demonstrate relevant experience where appropriate.

Human input from experienced risk assessors is essential for interpreting the hazard and risk of a site.  The value of a UXO specialist lies in their relevant experience, expert knowledge, and research and interpretation skills.

Clear and appropriate risk mitigation recommendations

A UXO risk assessment should clearly outline the next steps to ensure any identified hazard is safely and appropriately dealt with.  Of course, the assessment may conclude there is no significant UXO hazard and works may proceed, in which case unnecessary and costly mitigation has been avoided.

Where a potential UXO hazard has been identified, the hazard level should be zoned across the site where possible.  It is rarely the case that a UXO hazard will be site-wide, and a thorough understanding of the hazard will allow for zoning to be done confidently.  A zoned hazard will give the client flexibility when planning their works and reduce mitigation costs for the client.

Where risk mitigation recommendations are given, they should be tailored to both the proposed client works and the type of UXO hazard identified, so that informed decisions can be made about the mitigation techniques to be used.

An EOC Engineer should not be recommended to supervise ‘blind’ works such as piling, as they cannot visually monitor these works for UXO.  Likewise, recommending intrusive UXB surveys when the hazard is from shallow-buried ordnance is inappropriate and can actually reduce safety on a site, as well as inflating costs.

The anticipated ordnance types associated with a particular UXO hazard and the likely burial depths of the UXO should be addressed in the UXO risk assessment in relation to the clients works, as these will all affect what the most appropriate mitigation technique is.

Conclusion

A thorough, robust, and defendable UXO risk assessment is key to managing the UXO risk of a site and ensuring construction works can proceed safely.  The above pointers should give an overview of what to expect from a UXO risk assessment which follows industry good practice guidance.

Zetica’s objective and research-led approach to risk assessment has allowed us to conclude a low UXO hazard on over 80% of sites.  If you would like to enquire about a UXO risk assessment, contact us on UXO@zetica.com.