Unexploded Ordnance (UXO)
Detailed Risk Assessment

Assessing UXO Contamination Risk on Military Related Sites

For the risk of British (or ‘Allied’) UXO, we need to examine whether the site had any history of military use. It is often not obvious whether the military historically utilised land – evidence is often hard to spot or not recorded on historical mapping. Often there is simply no physical evidence remaining of previous use. This is where the wide-ranging knowledge, experience and unique datasets of 1st Line Defence come into play.

Over many years, we have built up extensive maps and databases of military land use and activity in the UK – and this is our first point of call to get an initial idea of any military history in the area. It may be that the site was on an old airfield – but this in itself is not sufficient to determine UXO risk. We would then need to research the specific history of the facility – maps, plans, diaries, record books, photographs and so on.

We need to try and ascertain who used the facility, how long for, what were they doing – where on the facility is the site located, was the site bombed, was explosive ordnance stored there, did training occur, where there aircraft crashes, how was the airfield defended and a whole host of other considerations. From this research, we can then start to consider the risk that these activities could have resulted in items of UXO contaminating the specific area where intrusive works are proposed.

The ‘house-keeping’ of historic military sites was often notoriously poor, and it was not uncommon for unwanted and unused UXO to be lost, burnt, buried or otherwise discarded. The nature of potential contamination is also an important factor to consider – from Small Arms Ammunition to Practice Bombs and Land Service Ammunition – each poses a different potential risk.

Assessing Risk of UXO Contamination from German Air-Delivered Ordnance

Again, the very first consideration is simply ‘was the site area subject to bombing in the first place’. If an area was subject to no bombing, there would be no risk of contamination from the outset. Bomb density figures can be useful here, but it is not enough to rely on these alone. It is well known that many of the UK’s towns and cities were heavily bombed – most central areas will have a ‘high’ density of bombing. But this is not sufficient to determine the risk of UXB contamination. Just because a general area was subject to a high amount of bomb strikes does not mean an individual site within that area is at ‘high’ risk. The bombing has to be researched in much more detail.

The next step is to try and ascertain where individual bombs fell, what damage they caused, what type and calibre they were, whether any unexploded bombs were recorded etc. To do this, we utilise archival material such as bomb maps, written bomb incident records, anecdotal reports, diaries, aerial imagery, historical maps, damage reports and so on. By layering up this information, it is often possible to build up quite a detailed picture of what happened to a site. By combining this bombing information with factors such as groundcover, land use and frequency of access – it is possible to make an assessment of the likelihood that bombs / unexploded bombs fell in the area.

When unexploded bombs are found on construction sites today, it is because evidence of their presence was not noted and reported at the time they were dropped. This is where groundcover, damage and access frequency are key. A 50kg German high explosive bomb (the most common dropped during WWII) could leave an entry hole just 20cm in diameter. Such evidence would likely have been noted, for example on an undamaged house, factory or roadway. However, in an area of serious damage, there is a much higher likelihood that such an entry hole could be overlooked – due to it being obscured by debris and rubble etc. Most of the recent UXB finds in London fell within areas that were subject to bomb damage during the war.

Similarly, areas where ground cover would not have been particularly conducive to spotting entry holes (such as rough, vegetated ground), or areas that received very little / regular access – would also be at an elevated risk of UXBs going unnoticed. It is these areas that can be of concern when we are assessing the risk of UXO contamination. If the historical information is good enough, it will often be possible to ‘zone’ the site into different areas of risk.

Areas where the buildings survived the war intact and undamaged, and which sustained no recorded bomb strikes might be assessed to be at ‘Low’ risk, whereas an area which was subject to very serious damage or clearance may be considered ‘Medium’ risk.