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Talking Point - Mike Sainsbury calls for more clarity from guidance on methods used to detect unexploded ordnance.

Developers, contractors and other stakeholders in the construction industry are receiving conflicting advice on methodologies used to ensure the safety of ground works from the risk of unexploded ordnance (UXO).
It is increasingly frustrating that some UXO specialists give the impression of being professional, while giving ambiguous and incorrect advice on potential risks. This is often followed up by inappropriate methods of mitigating those risks.

The reputation of a professional industry with a long track record on safety is being tarnished.

This year has seen some of the worst desk study and risk assessments produced by the UXO industry. This has been combined with geophysical surveys involving basic equipment designed for detecting only one metal type – UXO comprise a range of metal types.

Inappropriate methods of geophysical detection will mean the post-survey clearance certificate is not worth the paper it is written on.

One issue arises with the procurement of surface-based detection methods for potential unexploded bombs (UXB), which could be at significant depth below ground level in soft soils.

For these sites, a surface-based technique, no matter how good the equipment, will never overcome the laws of physics and is almost certainly not going to provide the detection limits required to make the site safe.

For the majority of sites in the UK that have a potential UXB risk, activities such as piling or site investigation boreholes can only be made safe by methods that enable detection of UXB to required depths.

Using geophysics to locate UXO is, in some way, analogous to mobile phone reception. If the signal is weak you may not be able to distinguish enough of a conversation to make it understandable or you may lose the connection altogether. So it is with searching for buried UXO.

In good reception areas signal-noise ratio is high and UXO may be found up to the theoretical limits of detection of a particular instrument. In bad reception areas, such as made ground that comprises material that interferes with the signal, the signal-noise ratio is low and it may not be possible to find any UXO.

Unless you are scanning over a sand box or other non-magnetic or non-conducting material that has no effect on the response of UXO then the theoretical limit of detectability will always be compromised.

UXO specialists are paid to certify that an area scanned is clear of UXO before construction activities can begin. It is therefore essential to quantify a detection assurance level – the distance from a sensor, less that which a UXO of a certain size can be detected, with absolute confidence.

This measure will vary from site to site and within a site depending on the composition of made ground or geology and target size. For example, on relatively clean sites, a 50kg Second World War bomb buried at 3m may be detectable from the surface, but on noisier sites only to 1.5m .

The detection assurance level at which target-size UXO can be detected is thus essential.

Stakeholders should question why they are being offered apparently the same service as a more professional company, but for only 10% of the price. In these situations, UXO specialist need to be asked to provide detection assurance statements along with clearance status certificates for UXO of expected size and estimated depth of burial.

And if you are still not sure, ask for proof of capability on site, it may be surprising what they cannot achieve.

Mike Sainsbury is managing director of site investigation firm Zetica

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