Tunnels, spalling and liquefaction

Back when I was a sproutling engineer, it was possible to go to the library in the Engineering building and pull interesting books off the shelf and give yourself an education outside of the official curriculum.

For instance, there are some phenomena that seem to fall in the cracks between Civil, Mining and Mechanical engineering.

Photo credit https://www.instagram.com/williamsphotography_nz/

 

The mechanism whereby rock faces are turned into gravel is that the explosive material creates an intense sound-wave through the media as illustrated in the second image. The velocity of the rock face creates tension in the rock which exceeds the tensile strength of the rock. Fractures propagate along pre-existing cracks.

More intense explosives impart higher-frequency pulses which result in smaller gravel. Lower intensity (i.e. slower) explosives like blackpowder produce boulders and slabs.

Spalling is one of the mechanisms for defeating armor in tanks. A HE round does not need to breach the armor if it can detach projectiles from the inside surface to bounce around and puree the passengers. Heavy fabric is draped inside of the armor to contain spalling and that fabric is appropriately named "spalling curtains".

Why this might be important

Tunnels can be destroyed via spalling and the attendant liquification of the earth

If you look back at the second image you will see dense areas that are under compression and less dense areas that are in tension.

Dried earth and poor quality rock are held together by friction or weakly bonded by dried carbonates and other salts. The weight of the overburden causes friction between the particles which holds them in place.

A shock-wave that is traveling horizontally first makes a tunnel wider as the side-wall closest to the explosion spalls into the vacant space (and simultaneously knocks away vertical shoring material. Then the shock-wave liquifies the earth the (now larger) upper span as the tensile portions of the wave pass by.

Remember the Cold War?

A great deal of effort and science went into figuring out how to destroy weapons that were buried in silos.

A silo is merely a vertical tunnel.

Bombs were developed that could be dropped from height and they were designed to not detonate with the initial shock of impact but to penetrate reinforced concrete and detonate at a predetermined depth.

Shaping the HE in the bomb in a long, cylindrical form created a shaped charge that caused most of the energy to radiate radially, parallel to the ground with the expectation that spalling and liquefaction would neutralize the silo and the contents therein.

Exotic tactics create exotic vulnerabilities

Hamas is on record as saying that there are no civilians in the tunnels. That makes them pure military targets.

Tunnels have vulnerabilities.

We are starting to see massive protests. As normal people, we see the threat they present to us BUT massive numbers of highly emotional people who are tightly packed together have their own vulnerabilities. There are reasons that professional military do not fight this way any more.

Can you imagine the effect of a cheap fog machine and a couple of Dollar General dummies popping up wearing "Chemical Warfare" gear.

Or consider some  fireworks enthusiast launching some loud, star-burst rounds overhead.

You can't shout "FIRE!!!" in a crowded theater for a reason. The stampede kills.

Does anybody doubt that there are elements of Hamas who would NOT launch rockets at their own supporters in the US as a false-flag to stop aid to Israel?

Don't be stupid. Stay away from crowds. And don't expect a tunnel to save you.