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For most urban construction sites in the UK, it is the possibility of discovering and potentially detonating a large aerial delivered WW2 bomb, that poses the most significant UXO risk. This is because of the high number of fatalities that could result, should this unlikely event occur.
Between 60-70,000 tonnes of bombs were dropped on the United Kingdom during WW2 and over 60,000 people were killed by the bomb & rocket attacks. In a single night during April 1941, 446 tonnes of bombs were dropped on London; over 58 tonnes (237 bombs) did not explode.
Several types of bombs were dropped by the Luftwaffe on the United Kingdom. Details of their ordnance can be found on our Downloads page. Bombs commonly used included:
250kg and 500kg bombs were sometimes fitted with fuzes which were designed to operate after a predetermined period, or when handled by bomb disposal teams or both. Examples of these devices are the:
Some high explosive sea mines were fitted with acoustic & magnetic sensors, a timer and light sensitive cell.
A key consideration when assessing the likelihood of finding a high explosive bomb is the depth at which they may be found. The penetration is dependent upon the:
Research during WW2 suggested that a 1000kg bomb dropped in clay could theoretically penetrate a vertical depth of 25 metres and 8 metres horizontally. This underground trajectory is known as a J curve and is the reason why bombs can be found under basements that were constructed before WW2.
It should be noted that the maximum actual depth of penetration observed in the research for a 1000kg bomb was 12.5 metres. Contemporary bomb disposal guidance indicated that only 1% of bombs (of 50kg or heavier) penetrated more than 9 metres.
There are three reasons why bombs did not explode:
Analysis after WW2 estimated that about 10% of the high explosive bombs dropped on the United Kingdom failed to explode. Many of them have now been found, but some remain. It is these bombs which are still being discovered on construction sites.
In terms of tonnage, high explosive bombs accounted for the large majority of aerial delivered weapons dropped on the United Kingdom during WW2. Unexploded German high explosive bombs represent the primary UXO hazard that is posed on most construction sites, due to their potential to cause significant harm.
The smallest high explosive bomb in common usage by the Germans was the 50kg SC (blast bomb) and tens of thousands were delivered. The total number of bombs dropped in each weight class reduced, as their weight increased and very few bombs weighing over 1000kg were deployed.
It was estimated that 80% of the high explosive bombs dropped were blast bombs. One reason they were chosen is because of the high number of casualties resulting from glass being blown out of windows.
The diagram shows a 1000kg bomb, which could be up to 3 metres in length and about 650mm in diameter, painted light blue as camouflage when looking up at planes. they were nicknamed "Hermann" after Goering - the Luftwaffe's Nazi leader. 50-60% of its weight was explosive.
In a typical year, the Army will dispose of five German WW2 bombs in the UK, most of which are 50kg SC bombs and on average, one unexploded bomb is discovered in London each year. Not all of these bombs are found on construction sites.
This picture shows the post war recovery of a 1000kg bomb in rural Suffolk. Note the excavator bucket lifting the bomb and the lorry in the background, for scale.
There are many other examples of the discovery of unexploded bombs, some of which are listed below. Click on the bomb weight to read more details from an external website in a new window, noting that the media reports sometimes state their size incorrectly.
These bombs were originally designed for use at sea, however were adapted to be used on land, hence they were sometimes referred to as land mines by the British. As they were dropped by the Luftwaffe, they were called Luftminen by the Germans, meaning air mines.
There were several types of Luftmines, which weighed between 600kg & 1050kg. All had a diameter of about 650mm and were between 2m and 3m in length, with between 200kg to 700kg of explosive in them.
The photo of a Luftmine shows the small parachute used to slow the bomb's descent to about 40mph. When they deployed correctly, they would remain on the surface of the ground, as the huge blast effect of these bombs caused significantly more destruction than a similarly sized high explosive bomb. The safety range for these bombs was usually twice the distance required for high explosive bombs.
If the parachute failed to deploy correctly, and they landed on soft ground, they could penetrate a considerable depth into the ground. If this happened, but they hit hard ground, the mine would usually break up on impact.
The diagram shows a large incendiary bomb used by the Germans, known as the Flam 500 (Flammenbombe). There was also a 250kg version.
The Flam 500 were about 1.8 metres long, with a diameter of 0.5 metres, containing over 150kg of flammable material - an oil incendiary mixture (30% benzine, 70% petroleum) with TNT bursting charge.
These bombs were made of relatively thin material, so did not tend to penetrate deep into the ground, but broke up on impact. Failure was relatively common and the liquid contents would spill without igniting. Due to poor reliability, they were used infrequently after 1940.
The picture shows a Flam 500. Due to the thin walled construction of these bombs and their tendency to break up on impact when hitting hard surfaces, it would be unusual to discover an example of these bombs on an urban construction site.
It would be possible to find these bombs if they landed in softer ground, for example at former airfields.
A contemporary German drawing showing the size and some types of bombs they used. From the left there are:
The SD2 sub-munitions were known to the Germans as devil's eggs and the British as butterfly bombs. They were very early examples of cluster bombs being used in warfare and when utilised by the Germans, they were deployed in large numbers. The diagram shows an SD2 prior to its wings opening.
The individual SD bomblets were delivered using containers holding up to 108 devices (hence they are sub-munitions). The SD2 load was scattered once the container had been released from the aircraft and was opened.
As they fell from the container, the wings would spin, which armed the bomb.
The photo shows an open SD2 above a 300mm ruler. They weighed about 2kg, had a diameter of 80mm, length of 170mm and explosive content of 0.225kg.
Three fuzes were used with the SD2:
Once armed, they could not be defused and had to be blown up. The last known death in the UK due to an SD2 was in 1956, however this type of bomb killed a Maltese man in 1981. On average, the Army deals with about 3 butterfly bombs in the United Kingdom each year.
Hundreds of thousands of incendiary devices were dropped on the UK during WW2 and were dispersed from containers that could hold up to 700 sub-munitions.
Small incendiaries used thermite to create a chemical reaction that produced a great deal of heat. The incendiary most used during the Blitz was known as the Brandbombe (fire bomb) and consisted of a magnesium alloy container with thermite filling.
Some incendiaries were fitted with steel noses to increase their potential depth of penetration, so became known as tile breakers. This maximised the damage that would occur inside a building.
The diagram shows the internal mechanism and components of 1kg incendiary bombs. The inclusion of an explosive charge was relatively unusual in small incendiaries and intended to increase fear within the population, to deter people from dealing with them.
The 1kg & 2kg incendiary devices were both 50mm in diameter. The 1kg device was 350mm long; the 2kg device was over 500mm long and contained 0.1 kg of explosive to burst it. They were ignited by impact to the nose and burnt with sufficient intensity to melt steel.
The photograph shows a 1kg incendiary (left) and a modern training aid with a section removed (right).
The items of unexploded ordnance that are most likely to be encountered today, are these small incendiaries. These devices, or part of them, are usually buried relatively close to the ground level, as the example in this report from Nuneaton shows, but they can sometimes be found in the fabric of buildings, as this news report from Poole demonstrates.
On average, the Army deal with forty five 1kg or 2kg incendiaries in the UK each year.
Whilst they were were described as incendiary bombs during WW2 and the phrase continues to be used when referring to these sub-muntions, their explosive content is a tiny fraction of high explosive bombs and the threat they pose is correspondingly lower.