What is the largest underground space that we could build?

Factual Questions
1

Another crazy idea that came to me in the shower…

I got to thinking about underground lairs and whatnot à la Blofeldt. How big an underground space could we build? There seem to be at least three possibilities:[ol][li]Roofing over an existing hole.[/li]I’m imagining a dome over an open-pit mine. How big a dome could we built, and still cover it with dirt to make the ground level? If this is doable, I have some ideas involving the Vortex of Doom.
[li]Building a structure in existing earth and rock.[/li]How big a concrete (or whatever) structure can we build to maintain an opening in regular earth and rock?
[li]Digging a hole in hard rock.[/li]How strong is, say, granite? I’ve heard of power stations that have been built in artificial caves, and seen metros dug that way, with the rock showing; How big a hole is possible that way? Do we reach a point where the rock flows to fill it back in, or does it just collapse?[/ol]

2

If you’re building inside solid rock, you’re really only limited by the size of the rock, assuming your walls and ceiling are big enough to support the structure. Consider Grand Central Terminal, which consists not just of the underground station but 67 tracks on two underground levels, and the four-track-wide underground Metro-North corridor from 42nd Street to 96th Street. That’s pretty big.

As for roofing over an existing hole, huge parts of the NYC subway system are built as “cut and cover” tunnels. That is, dig a trench in the street, put a roof on the trench, and put a street on top of the roof.

For #3, how about underground aquaducts. This thing, when complete, will be nearly 60 miles long. It’s expected to be completed in 2020, and is actually ahead of schedule, an amazing feat for a public works project.

3

We’d have to embark on some fairly major work to beat some of the naturally occurring underground spaces that already exist.

The Postonja Caves in Slovenia are said to be some of the deepest and most extensive caves in Europe, and maybe the world. One of the main Postonja caves is half-jokingly referred to as ‘the Concert Hall’ and you can see a photo on the Wiki link. It is used for live orchestral concerts and similar events, and it is claimed the ‘Hall’ can accommodate 10,000 people. The cave itself was formed entirely naturally, and only a few modifications have been made to make it feasible for an audience to sit there and watch a concert.

The central chamber of the Batu Caves north of Kuala Lumpur is also vast, and did remind me of a James Bond villain’s lair when I went there. Technically you wouldn’t say it was ‘underground’, because it’s inside a small mountain and you climb up 272 steps to get to the entrance; it is, however, a vast naturally-occurring enclosed space with a ‘ceiling’.

I expect the answer to your question is going to depend on how you define your terms. If you covered over a section of the Grand Canyon, you would create a truly vast ‘underground’ area.

It may also be worth mentioning that a very large percentage of the entire Disney World complex in Florida has an underground level, and you can go on a guided tour of it. If you regard this as a single underground ‘entity’, it would constitute a very large underground space.

4

Since nothing in the OP precludes roof supports, I’d say there is no practical limit to the area of an underground space.

5

Er, my apologies. I was indeed thinking of a space without intermediate supports. I was thinking of a roughly cubical or spherical space.

ISTR reading of a nuclear explosion test that was intended to try to create a cave, but I don’t know whether the cave collapsed after the blast.

6

Well, there’s this place in Pennsylvania. They don’t mention the size, but I’ve been inside of it and it’s substantial. For instance, there is a fresh-water reservoir inside that has a boat dock for the maintenance person to tie up his rowboat. The place is like something out of a science fiction movie.

7

Could we build something like that big chamber in the Mines of Moria scene in the first Lord of the Rings movie?

8

Actually, I was thinking of the monster containment facility in Monsters vs. Aliens, but same idea…

9

You guys might want to check out Carlsbad Caverns Carlsbad Caverns National Park - Wikipedia

It is really cool. IIRC you can have lunch more than 800 feet below the surface and then take the elevator up.

10

Wow! That link references the Sarawak Chamber as the largest (natural?) underground space in the world, at “700m (2,300 feet) long, 400m (1,300 feet) wide and at least 70m (230 feet) high”. That is much larger than I expected. And I would have thought that the largest chambers would be in igneous rock, not serimentary. Though I can understand that limestone is more easily eroded.

So maybe it’s possible to mine out a larger chamber in granite?

11

I’ve heard it claimed that a properly designed tensegrity structure, such as a geodesic dome, can be limitlessly large- a claim I have trouble accepting unreservedly. But if true, then you could roof over a hole of almost any size short of the walls flowing under their own weight.

12

As I recall, the Russians did that several times to produce underground caverns for waste disposal. So it appears to work.

13

I’m thinking of salt mines that employ the room and pillar method, particularly one that existed on the other side of the lake from Sunspace’s home.

A six thousand acre site, a thousand feet below.

14

Does hollowing out an asteroid count?

15

They recently found something even bigger (significantly) than the Sarawak Chamber. I can’t remember the name. But I do remember it was apparently within hiking distance (a day or three) of the Ho Chi Min (sp?) trail.

The Sarwak is (was) the largest by “floor space/square footage”. I believe the biggest cave room by volume is in the middle east, UAE IIRC.

16

One of the most crappy and expensive lunches (setting a personal record for expense to quality ratio) I ever had was in that lunchroom. On the other hand, some of the cheapest and best mexican meals I’ve ever had were in the above ground cafeteria behind the gift shop there.

Elevator smellevator. Real men and women walk down the natural entrance, tour the big room, then walk back up the natural entrance.

17

Not as an example of great size, but as a bodacious work of art: The Temples of Damanhur. Those Italians!

18

Would that be Hang Son Doong?

19

Hi Sunspace,

Just came across this thread while trawling for more info on man-made underground spaces.

I’m a geologist, and am studying soil and rock mechanics at the moment, so I thought I could add something to the existing replies.

The truth is that the answer to the question is that good old disappointment: “it depends”. I don’t know the absolute upper limit of an underground space that could be man-made in virgin rock, but it would depend on a lot of variables:

  • the kind of rock that you’re excavating;
  • the depth of the excavation below the surface;
  • the major and minor principle regional stresses;
  • the geometry of your excavation;
  • how long you want the excavation to last;
  • most importantly though, the frequency, orientation and geometry of the existing defects within the rock.

In hard rock, defects (like joints, faults, fractures, bedding planes etc) control the effective strength of the rock. You might have the strongest granite as your host rock, but if it’s fractured all over the place in random directions, then blocks of rock are going to fall out of the roof and the walls and make things much more difficult.

Thinking of trying to create a massive open chamber a la the Mines of Moria, it would be quite difficult to achieve. The regional stress fields that I mentioned come into play - there is a major principle stress direction, and a minor one. And they’re not simply a major vertical stress - as you might expect due to the weight of the overlying rock - with a minor horizontal stress. The major principle stress direction can often be closer to the horizontal plane than the vertical, but can be diagonal or anything in between. This is because of the tectonic forces pushing and pulling on the crust of the earth. So, when you open a hole inside a rock that is under imbalanced stresses, those stresses need to flow around this new excavation (they can’t be transmitted through air), and they end up concentrating around the edges of the excavation, which causes rock to burst out of the walls and roofs, and causes floors to heave upwards. The phenomena experienced all depends on the strength of the host rock to resist the concentrated stresses, the magnitude of those stresses in the first place, the shape of the excavation (circular is more stable than square/rectangular), and the use of reinforcing supports.

The cool thing about natural caves/chambers is that they’ve been created fluidly by erosion interacting with the regional stresses, so that the shape we see today is that which is dictated by the excavation moving towards equilibrium between those two forces.

And, then there’s the durability of the excavation - mines and the like might plan for quite a short life-span and cut things really close to the limit, so that they can extract the most material possible. An underground space for a nuclear reactor would be a completely different prospect, and might involve a lot of rock stabilisation techniques which - generally - will cover over the natural surface of the rock so that it looks like a uniform concrete surface (which, in my view, would detract a lot from the feeling of being underground, because it would look more like a bunker).

So, it’s not easy to answer your question definitively, except to say that it’s entirely dependent on the underground space that you want to build, and where you want to build it. While some natural spaces are huge, they’re not usually of the sort of geometry that humans would think of as useful, whereas if you excavate from scratch a nice regular underground rectangle for whatever purpose, be ready to fight the stresses and rockfall hazards, groundwater influx etc. Possibly the best middle ground - if your design allows for it - is to modify an existing cavern and adapt your design to it. It’s already undergone a lot of de-stressing, and might require relatively little reinforcement and support.

Sorry it’s vague, but I hope it’s helped give an appreciation for the sorts of problems controlling the issue.

Baz

P.S. I have a little idea stuck in my head that I’d like to try and create a rock mechanics model for something along the lines of the Great Hall in the Mines of Moria - see just how unrealistic (or otherwise) it might be. IF I ever do get around to it, I’ll post an update on here :slight_smile:

20

A few moments of Googling will reveal that all of the space you see as you meander around Walt Disney World in Florida has at least one level BELOW it. ALL of the space. There is an entire complex under the ground. It may only be one floor deep. But it’s titanic.