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In this alternate metropolis, the sewers of the United States are interconnected via a central sewage hub (probably in New York, Chicago or Omaha) and of uniform size—32 feet from ceiling to base, 160 from left to right and 300 feet below the surface.

Now imagine a global worst-case scenario of high energy and high temperature (like, for example, a volcanic supereruption or a bolide impact) where the energy and heat reach deep into the ground and ignite the methane and raw sewage contained within the tunnels, turning this continent-wide singular maze into a real blaze.

At 300 feet below street level, should anything happen to these sewage tunnels resulting in major structural compromise, will this affect the cities above the tunnels in any way?

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    $\begingroup$ Have you tested to see if that's even possible? Sewers have to be sloped down to operate. Trying to do that through flat regions of the nation may be unreasonably difficult. You may not be able to connect the sewers in that way. (At the very least, it will be impossible to do it across the Rocky Mountains. In an absolute best case scenario you'd need 2 such sewer systems, but I think you'd need quite a lot more) $\endgroup$ – Cort Ammon Aug 9 '16 at 3:53
  • $\begingroup$ Why? How is that at all plausible? $\endgroup$ – JDługosz Aug 9 '16 at 14:51
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In answer to your basic question.

At 300 feet below street level, should anything happen to these sewage tunnels resulting in major structural compromise, will this affect the cities above the tunnels in any way?

Yes. If you have tunnels under something and they collapse, things will happen on the surface, from sinkholes which suddenly appear, to other issues, like problems with buildings, pipelines, roads and the like. I would start with researching towns built on top of old mines.

Favorite quote from that specific article:

''Fellow in town was remodeling his basement last year when this wall gives way and he found himself peering down the throat of a mine shaft, dropping away right below his house,'' said Dick Schmidt, who represents Rock Springs in the Wyoming Legislature. ''I understand it gave him quite a fright.''

Oh and, you might as well research this town in Pennsylvania. Everything under that town is on fire. Sure, it is a bit of a slow burn, but it is interesting as a starting point, since you're playing with fire. Your situation will be more...explosive in places than this, but still worth delving into.

Now, having answered that, I am not sure about the feasibility of sewers this size, with that kind of uniformity and depth, serving the entire US of A. If I were you I would research that and perhaps make it a separate question. This does hit on the Absurdly Spacious Sewer trope, which I'll admit, I've used in my own writing. There are several problems with the design (including the depth) and that it is a centralized system for the entire USA. That's...well there's a lot of reasons why that would not work (especially in New York as a locale) but I won't nitpick 'cause that's not the question, and there is a damn fine tradition of Absurdly Spacious Sewers!

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  • $\begingroup$ I think "would the tunnels collapse is part of the question, not a given... $\endgroup$ – Joanna Marietti Aug 9 '16 at 5:10
  • $\begingroup$ If the author is planning on giant explosions/fires inside these tunnels, structural stresses are a problem, and they would collapse/cause problems on the surface. There's also a town where everything underground is on fire--I'll have to link it in the question. $\endgroup$ – Erin Thursby Aug 9 '16 at 5:27
  • $\begingroup$ @ErinThursby Weren't London's sewers during the Victorian Era "Absurdly Spacious"? $\endgroup$ – JohnWDailey Aug 9 '16 at 5:38
  • $\begingroup$ @JohnWDailey Oh there have been large sewers, Paris--London and the like. The trope exists for a reason. But there are lots of places even there, where they can't fit a person and they are certainly not uniform with ceilings that high. Like I said, damn fine tradition. $\endgroup$ – Erin Thursby Aug 9 '16 at 6:05
  • $\begingroup$ I don’t understand «They do hit». $\endgroup$ – JDługosz Aug 9 '16 at 14:56
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The Mythbusters TV show tried something like this with interesting results.

They built a section of sewer tunnel, including standard concrete pipes, vertical shafts and steel manhole covers, and attempted to blow off the manhole covers by filling the sewer with methane. There was some additional jiggery pokery by filing the tunnel with scrap, for reasons which will soon be clear.

The initial test shot was something of a disappointment, because the explosion was rather small. The end result was the steel manhole covers did get blown off, but only when a stoichiometric air-fuel ratio was achieved, including fans to mix the air/fuel into a uniform blend, and the junk in the tunnel to assist in creating turbulence to ensure mixing. The detonation wave did travel down the tunnel and up the audits, blowing off the manhole covers to a considerable height.

http://www.discovery.com/tv-shows/mythbusters/videos/drain-disaster/

https://www.youtube.com/watch?v=D6l8p45O-hg

This Mythbusters episode demonstrated the absurdities of the concept. First, the entire tunnel system needs to be filed with a stoichiometric air-fuel mixture of methane and air. The mix needs to be uniform throughout and there need to be no weak points for the detonation wave to escape, like the manhole audits in the Mythbusters episode. Considering the effort they needed to do this stunt in a small, artificially ideal tunnel, the idea that a uniform mixture will be present in an active sewer system spanning thousands of miles would require massive preparation. Then, the entire system needs to be sealed to prevent the explosion from escaping. Finally, you need to consider that a massive pipe 300 feet underground is already going to be extremely strong in order to resist the static pressure of all the earth over top of it (not to mention the tamping effect of the overburden).

Effectively, you might blow off manhole covers in select areas where ideal conditions have been created by accident, and possibly damage toilets and other water infrastructure connected to the sewers if the shock wave travels up the water pipes, but aside form some localized damage to the interior finish of the tunnels, the net effect will be "not much".

To develop a large amount of damage, active measures are needed. Terrorists dumping entire tanker trucks of propane into the sewers and igniting it using a timer to achieve the desired stoichiometric air-fuel ratio could collapse some sections of the tunnel. More determined terrorists will simply climb down and fix explosive charges on the tunnel directly. And firing a nuclear device in the tunnel should have interesting effects as the shock wave propagates through the various branches of the tunnel.

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  • $\begingroup$ And what's the example got to do with a supereruption or a bolide impact? $\endgroup$ – JohnWDailey Aug 9 '16 at 20:19
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    $\begingroup$ Except for the damage caused by impact or the eruption, the rest of the tunnels will suffer only minimal damage. $\endgroup$ – Thucydides Aug 9 '16 at 21:01
  • $\begingroup$ @Thucydides: Re "...down the tunnel and up the audits...", I think you might mean adits, though adits are more-or-less horizontal: en.wikipedia.org/wiki/Adit $\endgroup$ – jamesqf Aug 31 at 3:58
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This scenario could make sense, if the world has run out of coal and oil, and the sewage is being collected as a resource for producing bio fuels. Why not gather animal waste too? Doing so in America today would increase the volume of sewage by approximately a factor of 10. Channeling it all into a single location seems difficult to justify, as it would be more expensive than numerous distributed refineries... But maybe an authoritarian government wants total control over fuel production, and they're willing to let their citizens pay the price. In that case, the central processing facility would be truly massive. Giant pipelines of shit would flow like the Mississippi. (In fact, to minimize pumping requirements, following the course of the Mississippi watershed would be the most natural route for the pipelines - you know it's all downhill. In that case, the main refinery would be somewhere in the vicinity of New Orleans.)

If the sewage is being collected for processing as fuel, then this is a very restrictive scarcity economy, because the infrastructure required for doing so would be greater than that needed for crude oil, while the return would be much less. Fuel would be difficult to obtain... private cars would likely be a thing of the past. That aside - the fuel-extraction concept really heightens the possibilities for disaster, because you have both vast pits of raw sewage and tons and tons of explosive, volatile chemicals all right there in one place! Sewers, by themselves, don't generally catch fire unless someone spills something flammable into them - the concentration of methane just isn't high enough. But oil refineries are very much another story. One major, unexpected earthquake - a little spark - and you're off!

Another possibility is that the sewers become contaminated with something... people have been known to dump waste oil and so forth down the drain, and it has caused explosions and fires in the past. It generally isn't enough to collapse the tunnels. The heat would have to be quite extreme for that to happen.

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