Construction sites benefit from early adoption of UXO plans, says explosives expert

A shortened version of this article was originally published in October by Construction News.

The discovery of possible unexploded ordnance (UXO) on a construction site is a major event, requiring an emergency response and, crucially, a complete stop to work. RSK Ordnance Management Managing Director Richard Holmes makes the point that a thorough investigation as part of a clear UXO plan is essential for site safety and to prevent project delays.

More than 80 years after the Blitz, it is still not uncommon to discover UXO, also called explosive remnants of war (ERW), during construction, trenching or dredging activities. The Ministry of Defence receives approximately 2500 calls per year to respond to suspected UXO, or roughly five every single day. Around three or four make headlines per year, such as last year’s Second World War bomb in Plymouth that saw 10,000 people evacuated.

A Second World War bomb discovered recently in Cologne, Germany, saw some 20,000 people evacuated, with the safe disposal taking nearly five days. If this halt to work happens at a time when plant, equipment, engineers and contractors are on-site, it can present a serious safety risk, and it can become extremely costly.

UXO risks and a plan for dealing with any discoveries need to be established as soon as possible in the construction process, at the preliminary site risk assessment. During this assessment, a literature review that cross-references the site and examines historical records and archives, including the air raid patrol warden reports and aerial photographs, should be carried out. This can be used to gauge the likelihood of an encounter with unexploded ordnance on a particular site and can inform further work or risk mitigation strategies.

Former industrial city centres, where brownfield sites are increasingly utilised for new housing and developments, are well-known for their UXO risks, but this review should cover more than simply World War Two bomb sites. Former Home Guard, Territorial Army or Special Operations Executive training sites that might have stored live ammo and explosives should be considered. Most people do not consider remote Scottish lochs to have a UXO risk, but several were used for D-Day practice by the allies. In our experience, approximately 40% of sites in the UK have some connection to military activity, and this risk assessment and literature review must be appropriately in-depth to properly inform what further investigation or mitigation is required.

On a site with a relatively high risk of encountering unexploded ordnance, further investigations before work starts should be carried out. We would typically start with a non-intrusive survey using electromagnetic scanning rigs to identify metallic items underground.

These readings will help us decide what action is taken next. If something that could be a bomb, grenade or other ordnance is found at a shallow depth, experts will likely investigate further and have a UXO engineer safely remove it. They can then ascertain if it is an explosive, whether it’s safe to move or needs to be disposed of on-site and whether we can do that through a thermite charge that burns out the fuse or whether it requires a full explosive demolition. If an anomaly is found buried five metres or deeper, then more detailed, intrusive investigations are carried out with cone-penetrating testing rigs or borehole tests.

The decision on what to do after this varies significantly depending on the results of the surveys, the depth of excavations and the surrounding infrastructure. In many cases, contractors may choose a live service clearance, where a UXO specialist will be present on-site during works, carrying out one-metre stage clearance, systematically investigating and clearing a site to ensure machinery and drilling equipment are safe. In some cases, on-site training on the recognition of possible UXO might be given to site workers, with a plan of action established.

If the site does not have a historical link to the military or appear to have been the site of any bombing and the risk of encountering UXO is low, that doesn’t mean that no further thought should be given to UXO. We have been called to sites where a grenade has been found buried in a garden that turned out to be a relic from the Second World War that got lost during a redevelopment. It is always possible, and although further investigations may not be required owing to the low risk at the site, it is still crucial for site workers and contractors to have a plan for the event of a possible UXO discovery. On some sites, simple actions such as deciding in advance which UXO specialist the site staff should call if anything is found will be sufficient.

Not all sites will require UXO specialists from the outset, nor will every discovery result in a high-profile evacuation, cordon and demolition. Most will not, but an expert assessment of the risk, how it will be managed and who will be contacted in the event of a discovery should be done at the earliest stages of any project. This will reduce the risk of injury for workers and the length of any project delays that result from UXO discovery.

Richard Holmes is managing director at RSK Ordnance Management. He has over 20 years’ experience working with explosives and bomb disposal and has a particular focus in the field of innovation. He has also worked on many projects involving mine clearance abroad. Richard is a Member of the Institute of Explosive Engineers and has recently led the team to win the Ministry of Defence’s flagship UXO Clearance project in the UK, delivering live demolitions on a weekly basis.