Thread by @andrew_barr

How do you dispose of a huge WW2 bomb in the middle of a town while keeping everyone safe? Sand!
The discovery of a 250kg bomb in #GreatYarmouth last week provided a great example of how the Army responds to these threats. 🧵

Last Tuesday a WW2 bomb was discovered during dredging work in the River Yare. It's common to unearth old bombs during construction work in the UK, and they still pose a real danger 80 years later.
This appears to have been a German SC 250.

SC 250s have a total mass of 250kg of which 130kg is a high explosive such as TNT.
Detonating 130kg of TNT in an urban area is a bad idea [citation required] due to the risks of death/injury and damage from:
💥Fragmentation of the bomb casing
💥Shock waves in air
💥Ground shock

Ideally, all of these threats are eliminated by an Explosive Ordnance Disposal (EOD) team removing the device and destroying it far away from the public.
But when the device is too dangerous to move, the alternative is to deactivate or destroy the device in place.

That brings us to sand. Sand is fantastic at slowing down fast-moving projectiles and absorbing shocks, thanks to friction between the sand particles and the energy required to move it around (it's heavy).
Of course, sandbags are not a recent invention!

Surrounding an explosive device with a sand-filled structure is an effective way to capture fragments and absorb some of the energy of the explosion, significantly reducing the risk to the public.
But building large structures with sandbags requires a lot of time and labour.

Instead, the EOD team made use of Hesco MIL units and excavators to rapidly build a hefty sand enclosure around the bomb, with more sand on the roof supported by steel sheet piles.

Hesco MIL units are like a conventional gabion steel mesh, but lined with a geotextile material that allows them to be filled with finer soils like sand. Machinery can be used to do all the heavy lifting, then some compaction by foot and you're done.
So easy even I can do it...

The design of the enclosure will vary depending on factors such as the type of explosive device and the proximity of other structures: in this case it looks like an extra L-shaped wall has been added to protect nearby houses.

As an explosion would also result in a shock transmitted into the ground, nearby buried utilities were also taken into account, such as the presence of gas mains. The EOD team, Dstl, local authorities and utility providers will have all worked together to finalise a solution.

Once the protective structure was in place the work to deactivate the bomb could begin. An initial plan to cut into the bomb and render it safe to transport had to be abandoned, after water used in the cutting process threatened the stability of the sand walls.

An attempt was then made to destroy the explosive by controlled burning, which is riskier as there is a chance of a transition to detonation. This did indeed happen, but luckily we have drone footage to see the sand enclosure in action! 💥

The blast is channelled upwards through the sand on top of the enclosure, resulting in the jets of sand visible in this image. Kinetic energy in the sand is kinetic energy that isn't available to the air shock, so full marks there. What about fragments?

The sheet piles that formed supported the sand on the enclosure roof are perforated with holes from the bomb casing fragments. These fragments were then captured by the sand in the walls and roof - no fragment damage has been reported.

In the video you can see that the sheet piles themselves become projectiles, but mostly fall back within the area of the enclosure. The scaffolding struck by a pile will need repair, a few car windows are broken, and there's some damage to the river wall.

Even though the EOD team was unable to disarm the bomb without detonation on site, the mitigation measures seem to have worked perfectly, without any injuries or serious damage caused. All thanks to sand (and a lot of clever people). 👏

The #GreatYarmouth bomb received a lot of media coverage, but it isn't an isolated incident - @11_EOD_Regiment expertly deal with unexploded ordnance like this on a weekly basis. Give them a follow to see what they get up to!