Could we build a roof over the Grand Canyon?

Question

(or some other Lesser Canyon)

I'm playing with story ideas for a videogame that takes place in the post-post-apocalyptic US. Everything is overrun with monsters and ocean horrors and whatnot, you've heard this story before. What happened to humanity isn't important, I just need a place to stick the remainder of it to start the game from. Initially this was going to just be a typical Last Remaining City but since it's essentially only exists to supply you while the entirety of the world is completely destroyed, I'm considering just shoving the entire thing out of sight and out of mind rather than trying to explain why this one city was somehow not leveled like everything else. The basic version of this is well-trod territory here, but I haven't seen anyone considering alternative methods to create shelter underground.

The Grand Canyon is (at its max) 277miles long, 18mi wide, ~1mi deep
Manhattan is 13mi long, 2mi wide, and 1WTC is 1,776 feet tall

So we can obviously fit several cities in there, but could we enclose them? The Grand Canyon is a bit excessive, but perhaps some smaller canyons/valleys that have flatter bottoms with no rivers running through them. Obviously this world is filled with all kinds of unobtanium tech, but I'm trying to start as much as possible grounded in reality and only apply phlebotinum as needed to scale things up. So would it not be easier to enclose a valley than to dig an entirely new underground area?

Research

• The largest suspension bridge in the world is the Akashi Kaikyō Bridge at 2.4 miles with an uninterrupted central span of 1.2 miles. Impressive! Of course a suspension bridge would not make a great roof.
• The longest arch bridge and cantilever bridge are both about 1800 feet. Also probably not a great roof!
• If reading underground city questions here taught me anything, it's that you need a dome. The largest dome in the world is 310m. Not huge, but nothing to sneeze at! Perhaps a bunch of intersecting domes supported by columns?
• On the other hand, there are always massive salt mines, though those are limited by the salt deposit size. There's also a massive underground city large enough to house 20,000 people built in the Bronze Age, so maybe building underground isn't that unreasonable after all!
• Another alternative would be to modify a large existing cave complex like Jewel Cave in South Dakota

NOTE: In terms of the story, nothing is specifically trying to destroy this city, either during construction or after. It's more of a "we're not the top of the food chain anymore" situation.

I guess it's just really funny to put a roof on the Grand Canyon instead of yet another underground city so I want to know more about how unrealistic it is!

Answer 1

A single arch from rim to rim with no supports? No way.

Dallas Cowboys Stadium has the longest single-span roof in the world at 373m. You're talking about arches 29,000m long, or over 77 times as far. And not just two of them (like the stadium) but thousands along the entire length of the canyon. That is not remotely within the realm of what we will be able to do within the foreseeable future, if ever.

Answer 2

Is it possible?

Yes, build enough suspension bridges like you say and it will be covered

Is it feasible? No, a post-post-apocalyptic society is not building technological marvels for minor protection, especially if a wall with guards will provide the same benefit. So it is more likely that this covering would be built pre-apocalypse. However, currently the Grand Canyon is a tourist location and a national park. People go to see the majesty and scale of this giant canyon. Covering it would mean no tourism, since no one wants to see that 50,000 suspension bridges built to cover the amazing canyon. Because the grand canyon is a park it is not even legal to develop that unless the grand canyon loses it's status as a national park. Therefore it is unlikely that it will be built post or pre-apocalypse.

Is it necessary?

No, enclosing the Grand Canyon is difficult and offers little benefit. building several thousand wonders of engineering to cover the grand canyon will not be better than alternative that would achieve the same thing.

why was the Grand Canyon Not leveled?

As nice as the grand canyon is, it is not of great strategic value. The closest military base to it is 50 miles away, so unless a Tsar Bomba is used it won't get caught in that explosion.It is a national park, a part of the country where people are told not to develop and stay out of except for tourism. this means that it won't be high on the list of targets to kill civilians. there is the standard radiation to deal with, but as long as there are high winds the radiation might blow right over some parts of the canyon, but even if it doesn't you can construct a fallout shelter to augment the canyon's natural defense.

Why covering the Grand Canyon won't stop humans from harming your city

First off, you city would need to spend a massive amount of energy to be spent on maintaining the bridges. Sure, it's not like maintaining a bridge take a lot of work, but there are a lot of bridges. Also, if any humans want to destroy your city the have the key to your destruction right over your heads. There are dozens of ways to bring down a suspension bridge, that don't take incredibly large explosives to do. sure, to take out a bridge would be a group effort of a large tribe, but doing so would collapse a unrepeatable bridge that could crush thousands.

So you probably want to have guards around the bridge at all times. but that can be done with the same efficiency with out the massive bridges.

The canyon is already defensible, but only against dumb monsters

Lets say there is no bridge and the monsters and attacking human want to get from the top of your canyon to the bottom to get to your city. They have two choices.

A) attack one of the known entry points to the canyon base, and face heavy resistance at that natural choke point.

B) Climb sheer cliffs down for a mile while snipers shoot at you.

If you want to defend the grand canyon, you can just turn all the parts of the grand canyon that are not sheer cliffs into sheer cliff except for on heavily fortified entrance. You can now inhabit the entirety of the grand canyon with relatively minor defenses. There is still the problem of people tossing explosives from the edge of the cliff. This can be solve with point defense weapons or by dealing with the attacks like air raids. Also, while this doesn't render you entirely immune to humans, it does make you entirely immune to dumb monsters.

In conclusion

Don't cover the grand canyon, make the walls not climbable and then defend on entrance; Or build a wall.