Science! in Equestria 509 members · 542 stories
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Could someone with more physics knowledge than me tell me what would happen if a tunnel network underneath a castle exploded?

6513470
The tunnels would want to be filled and everything above the tunnels will fall to do just that. It would ruin the city and most of mount Canterhorn; killing thousands and it would be seen as a change in political climate.

jxj

6513470
It depends on a whole bunch of things. It could be something like mocha said, or nothing. Or anything in between.

6513470
Disregard the two previous answers. They don't actually address your question. Allow me.

A tunnel is an empty space underground. If you mean something else, feel free to clarify.

An explosion within the tunnel is a rapid increase of pressure. It will generally seek to conform to the space it is in, applying pressure to all interior surfaces of the tunnel. That force will, in essence, "push" in all directions.

Presumably "down" is a whole lot of solid earth. Force in that direction, depending on the looseness of the material, may compress and absorbed that force, or reflect it. Same presumably to the sides, and several other angles. These parts may result in the tunnel becoming wider, etc. Whatever supports there are for the tunnel may be weakened, particularly by the sideways forces.

The more interesting part is the upward forces. Presumably, that is the thinnest amount of material surrounding the tunnel, and therefore the direction to have the most impact. If there is sufficient force to move that material, it may "burst" and scatter to the surface. This would also allow the pressure to escape the enclosed space.

Insufficient force to cause that might merely bulge towards the surface. Less force still will make no difference at all, as the upward force merely compresses some material and is absorbed, or reflected.

Most, possibly all, of the pressure of the explosion is likely to simply extend the length of the tunnel though. If there isn't anything containing a segment of tunnel into basically an underground room, there is a probably a whole lot of space for the pressure to expand into in that direction. Open air offers the least resistance to the increase of pressure of the explosion, and all of the outward pressure to the walls will be mostly spread out across the length of the tunnel, making the pressure in any particular spot along the tunnel walls much less.

So in order to "explode the tunnel" you need either a sufficiently huge explosion to overpressure the entire tunnel by a whole lot, or a way to contain a section of the tunnel, and the material between the surface and the tunnel be the weak point rather than whatever you cap your tunnel section off with.

Without reaching that, and with sufficiently weak/loose material to the sides of the tunnel, deforming the tunnel sideways could critically break the supports, which could allow the tunnel to collapse after the explosion pressure normalizes fully and escapes.

Failing to achieve that, you might cause some superficial damage inside the tunnel.

6513470

In simplist terms of what 6513534 said-


Tiny boom, little rumble


Big boom, bigish rumble


Super bigga boom, city fall maybe


Super duper bigga boom, city probably fall

6513564
lol

6513470
Part two of the answer:
Assuming some situation where there are either enough explosives within the tunnel network, it is sealed off, or so on. The tunnel network itself "explodes." This would be due to the overpressure within the tunnels exerting enough force on the "roof" of the tunnels to burst them, as the pressure seeks to normalize to the open atmosphere and overwhelms the barrier (the tunnel roof and material between to the ground surface).

The earth/stone that previously formed the flooring of the castle and the roof of the tunnel network that the ground between would be pushed upwards violently. Depending on how the tunnels and castle were specifically oriented to each other, that could directly push on walls of the castle from below. That said, ground+walls is more material to push on than just ground alone, so the upward motion would be less overall at the walls than elsewhere. They might still "bulge up" for a bit, but quite likely they wouldn't fly into the air. Most pressure that would do so would likely rupture the tunnels in less weighty areas of floor-roof.

Some of the material that goes up would fall back down. Some might go up and sideways from the tunnels at an angle, then fall down. For the most part, the tunnels themselves would be "collapsed" once things settle. Sections of tunnel underneath walls that collapse would create sections of wall that sag down by as much as the height of the tunnels. That's probably pretty damaging to the structual integrity of said walls. They may collapse sideways as a result, depending on how much material, how far it moved, and so on.

So, for the most part, explosions sufficient to rupture your tunnels may sag or even collapse some castle walls. Will probably render portions of the floorplan into incredibly rough terrain, rubble, sinkholes, and so on.

While this isn't exactly the same scenario as you asked about, there are some similarities:

While the missile starts from above the surface, it does penetrate into the ground a ways before detonating. It, in essence, creates a bit of a tunnel before exploding that tunnel. You'll notice that a lot of ground material gets thrown upwards and away, and it leaves behind a big crater.

More than likely, whatever scenario you have in mind would be far less destructive than this. Tunnels are likely deeper, much more internal space to dissipate force, much less force, and so on.

6513534
6513656

This does assume a tunnel that is capable of holding up its' own ceiling.
Relatively small tunnel in rock, this is pretty much what you can expect.

However if say, you have a tunnel in soil, or any other situation where the tunnel requires supports, the situation changes a bit.
(I think: I'm an engineering student, but I have no specific knowledge of tunnels, just materials and physics)
At the start, things would not change significantly from what Cryosite described, up to the point where the explosion extends through the pipe of the tunnel. This could take out supports or damage them, which could lead to collapse of the tunnel.
If the tunnel collapses, it gets filled with material, which has to come from somewhere. Somewhere usually being "up".
In a small tunnel, deep underground, this could probably go unnoticed from the surface. Larger tunnels, or tunnels close to the surface, there would be shaking and some ground collapsing.

Here is an example, from a collapsed shaft in a railway tunnel, taken from this article.
.

The above could also happen in the case of a tunnel that does not require support,as in Cryo's second post, even if the pressure manages to escape through a weak section of wall/ceiling/sealed tunnel entrance etc.
As long as there is bulging, the stone gets deformed, which can lead to cracks forming or growing, which will weaken the integrity of the ground above the tunnel. This may cause parts of the tunnel to collapse even relatively far away from the point where the pressure escaped. Scary part is, this does not need to happen immediately. Could be hours, days or more.

Again, most of this is not likely to happen in case of a small tunnel relatively deep underground.

That said, I think it's likely you're talking about the tunnels beneath canterlot. I went back to look at the look at the beginning of Canterlot Wedding P2. The tunnel system has no wooden supports, nor are there many sections that are poorly supported from what I saw, but the caverns are quite big.
Depending on the cause and location of the explosion you could still be looking at a collapse. Like:
Weakening walls and ceiling of the larger caverns;

Knocking out large sections of wall between parallel tunnels;

Destroying those few more delicate supports (background, upper right corner);

6515212
Whether the supports are natural or artificial doesn't change things. A tunnel with no supports of any kind collapses. If the walls of the tunnel are sufficient to support it, that is still "supported." If it needs additional bracing to support it, that's another thing. In either case, the pressure/forces acting upon the tunnel can damage that support (be it the walls or braces), thus rending the support insufficient for the tunnel.

6513470 So, when are you planning to blow up the tunnel and what kind of a bomb will you use?

Oh, hello there CIA. Nothing to see in this thread. Just move along, please.

6515624
Wasn't my intention to suggest the supports being natural or artificial would change anything, though I imagine most naturally formed caverns/tunnels are pretty stable, as most of them have been along for a pretty long while, and would have collapsed over time otherwise.
But yeah. Only difference would be in the material of the supports.

6516688
I think you were misunderstanding the difference between "supports" and "braces." That was the intent of my reply to you. I remain unsure that this distinction came through, so I will elaborate further.

Supports is a general term. No matter what it doing the supporting or how, the tunnel is supported and remains a tunnel instead of not a tunnel. The walls being rigid and not crumbling/flowing into the open space is "support." Braces are also "support."

Braces are objects specifically put into place to support a tunnel that would otherwise collapse without them. Often things like wooden beams, such as in mining tunnels.

Tunnels come in many configurations and compositions. Not all "natural tunnels" are holes bored into enormous pieces of solid rock. Some are the result of boulders piled in such a way that the space between them forms a continuous hole. That hole might have been created via artificial or natural means also.

It is not just bracing that would be damaged by an explosion, but any form of support. Sufficient explosion pressure can push the boulders that form the tunnel walls, perhaps shifting them from a relatively stable position into one that will settle into the open space of the tunnel. Large, solid rock tunnels might break (or the boulders of more granular tunnels might break), thus permitting more shifting and settling. Braces may be weakened or destroyed and thus no longer prevent said settling.

The list of specifics that could happen are too numerous to list off comprehensively. It is rather pointless to try to, as the generalized answer covers them all pretty well. Strong kinetic force (due to pressure) can shift material around. Because the "ground" is not one immobile solid object, but really a collection of many solid objects of differing hardness, brittleness, density, size, shape, and so on, all of which are acted upon by pressure/force that typically isn't present in a tunnel.

So an emphasis on braces in particular, when they may not even be present in the tunnel under discussion, is rather a moot point. The effects of an explosion are not dependant on the presence of a particular kind of support, just an understanding of why and how support functions to maintain the tunnel "structure" and how the explosive forces can disrupt that. The end result is, as we have both pointed out, the same. Collapse/settling is a possible outcome.

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