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Unexploded ordnance from British and Allied forces can be categorised into the following areas:
An average of 8 air dropped weapons that originate from the United Kingdom or United States, are destroyed by the Army every year. This has included high explosive bombs, up to 1000lb in weight. It should be remembered that this type of weapon may have been abandoned, not dropped and is unlikely to be encountered on most construction sites, unless it was a former airfield or aircraft crash site.
This ordnance will probably have an explosive content, but perhaps only 1kg to 4kg in weight, which is much less than found in high explosive air dropped bombs. Anti-aircraft artillery rounds could be found anywhere in the UK and are encountered on construction sites reasonably frequently, as they could be buried in the ground, if they they fell without exploding. Mortar shells are unlikely to be found on a construction site, unless the land had previous military use, such as these examples on parkland in Suffolk and Cumbria and could date from WW1. The largest WW2 mortar round had a calibre (diameter) of 4.2 inches.
Allied ordnance could be found anywhere when retained as souvenirs.
Numerous types of grenades and infantry anti tank weapons have been used by British and Allied Forces. A Blacker Bombard (pictured) could fire a 20lb projectile 100 yards. They were often used by the Home Guard; follow the link to read a news report on the discovery of an unexploded Blacker Bombard round during an archaeological dig, also grenades found in a Cornish roof during renovation work and at an Oxfordshire nature reserve. More recently, a PIAT (Projector, Infantry, Anti-Tank) round was found in northern Wales by magnet fishers.
Anti-personnel and anti-tank land mines were used extensively during the early part of WW2. The number and location of the mines were meticulously recorded, so they could be cleared later, although there were still many deaths removing them and not all the mines were located. They are unlikely to be found on urban construction sites, although one was found on a former airfield near Norwich.
The discovery of ammunition on construction sites, especially refurbishment projects, is entirely possible. They are unlikely to pose a significant risk, but should be handed to the local Police immediately.
The lack of available weapons and ammunition in 1940 meant that improvised weapons were issued to the Home Guard. The photograph shows the construction of a sticky bomb, a type of hand grenade. A spherical glass flask containing explosive was coated in an adhesive, which would be exposed when the protective casing was removed, with the intention of attaching it to a vehicle. Over 2.5 million were produced.
A further 6 million self igniting phosphorus (SIP) grenades were produced. These white coloured, half pint bottles were sometimes buried by the Home Guard and could still be discovered. Follow the links to read about the discovery of SIPs in Brecon, Cambridge, Plymouth and Wiltshire. These grenades will still be dangerous and the Police must be informed if they are found.
It is worth noting that these improvised weapons are unlikely to be identified by an intrusive site survey, as they contain no, or very little, metal.
Several types of anti-aircraft artillery were used during WW2, but the
3.7 inch quick fire guns were the main provider of protection. They could fire a 28lb shell to an altitude of 50,000 feet (15km). Hundreds of thousands of shells were fired at enemy aircraft, but many did not explode and returned to earth. Discovery of these shells is relatively common and can be very disruptive, as they are sometimes mistaken for high explosive bombs, as this news report demonstrated. Four shells were found during construction works in Sheffield, which are thought to have been brought to the site as back fill. During WW2, these shells were not considered to be a danger to the public and were collected, then stored for later disposal. Sometimes they were not destroyed and there have been occasions in Romford and Liverpool, where these stores have been discovered during construction works. The photo above shows three types of anti-aircraft round: 40mm Bofors, 6 pounder and 3.7 inch.
Another type of ordnance that could be discovered on construction sites is grenades. They are likely to be close to the surface and are sometimes found in groups. The most common grenades were known as Mills bombs and the Number 36 (which is pictured), was in use during WW2. They had an explosive content of 70g and were considered to have a lethal range of about 30 metres. Follow the links to read news reports about the unexpected discovery of grenades in Northamptonshire, Lancashire, Leicestershire and Norfolk.
Many thousand explosive land mines were placed at strategic locations around the country, especially the coastline. Some mines that were laid in sand close to the coast would move and are continue to be discovered. Follow the link for more information about a anti-tank mine found in Suffolk. The cylindrical mark II anti tank mine was 7.5 inches in diameter and 5 inches high, containing 4 lb of explosive. Mushroom mines referred to naval beach mines which contained 20lb of explosive.
These mines consisted of 3 inch diameter pipes that were bored into the ground, at a slight angle, perhaps a metre under the surface and contained large amounts of gelignite. They are known to have been deployed at airfields close to the coast and many roads, especially near bridges. The mines were filled with explosives and could be electrically detonated at very short notice. This could have happened if an airfield was about to fall into enemy hands or to create a surprise anti-tank obstacle for invading troops. The photo shows a trench that was created by a pipe mine during tests; it was intended that they would be 10 feet deep and 20 feet wide.
Following WW2, the Canadian engineers tasked with removal of these pipes failed to complete the work and there have been numerous discoveries at airfields since then. This included at Detling, Hawkinge, Manston and Gravesend. At Lympne in 1962, a former Ukranian SS soldier was killed when clearing them. In 1989, Operation Crabstick was initiated to ensure that they had all been removed, however even this was not entirely successful. During WW2, 265 pipe mines were installed at HMS Daedalus; follow these links to read media reports about discovery of twenty in 2006, with another pipe mine found in 2017. A pipe bomb was also found at the former RAF Manston in 2019.