What is the largest underground space that we could build?

[ralph124c]

Interesting-I wonder if extremely large caves (like the “Big Room”) in Carlsbad Caverns is beyond the limits for limestone-I wonder if it could be close to collapse.

[LSLGuy]

@runningdude

Recrunching my numbers, I actually got a dome that would be 1000 feet across. Such a stone dome would be impossible; due to the sheering force of the weight, a 300 foot diameter disk would come crashing down from the top center, and without the key stone, the rest would come crumbling down as well.

In perfectly solid rock, such a void might be possible, although I think it rare a void that size would exist without interruption.

Since it sounds like you’re a CE kinda guy and I am not, nor do I play one on TV …

Ref my post #52: Am I all wet? If so, why? If not, what does that mean for your calcs?

As I understand this stuff, your calcs would work fine for a hollow cylinder of granite with a specific wall thickness with a hemispherical dome of the same wall thickness sitting atop it, all of this sitting on a hard flat surface in the open air.

But a cave is not in the open air. For simplicity assume the same granite extends for miles in every direction: NESW, plus maybe above the top of the dome-shaped underground hollow.

Now what?

[Leo_Bloom]

A good thread.

—a passerby

[gnoitall]

@Chefguy

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.

I’ve worked there.

It’s more impressive in idea than execution. There aren’t any huge wide-open expanses. Large tunnels, really; parallel and cross tunnels about 50 feet in diameter. Enough to build long, narrow, 3-storie buildings in.

And sometimes scary as hell. It wasn’t particularly rare to hear chunks of rock coming loose from the wall or ceiling and crash to the paved tunnel floor, or to see fist-sized or larger rocks lying there. I certainly wouldn’t trust that kind of geology to support a SPECTRE secret headquarters, for damn sure.

But a fairly amazing accomplishment for early-1950s mining technology.

[runningdude]

@LSLGuy

IMO there’s a fundamental error in using standard CE dome calculations for this: What is the outer diameter of a “dome” which is really just (more or less) solid rock extending for miles away in every direction from our (more or less) dome-shaped hollow?

Which also explains billfish678’s point about much larger natural hollows.

You make a valid point. In Civil Engineering, we tend to be widely conservative. I started out with a twenty foot thick dome to get a lower limit to an underground void size, particularly a human dug one.

The final numbers that I’ve gotten, a theoretic 1000 foot diameter dome (about 600 foot high at center), would be impossible to build with granite blocks due to shearing forces, but plausible if carved/naturally formed from solid solid stone.

I would image though that even voids of such size are not perfectly contiguous, with numerous “support columns” dividing the space. Unsupported spaces would simply end up at least partially filled by debris knocked loose by earthquakes, etc.

[billfish678]

Why don’t you just change the calculations? Assume something more along the lines of a strong limestone (since all the natural big voids are in that), and then assume something much, much thicker than 20 feet?

That should approximate an excavated void in a solid block of material as a reasonable first stab at it.

Once that calculation matches what seems to be the limit in nature (limestone caves) then you can do calculations for voids in granite, high strength concrete, or other materials that you can consider reasonable.