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I have a colony of humans living underground in man-made catacombs on another planet. They are, essentially, digging/blasting as they go; they did not create a complete underground city first and then move in. Thus, I need for them to be able to expand "in place", without harming themselves with either falling debris or seismic instability caused by their actions. And I need them to be able to do it practically; pick-axes probably won't do the job alone. Their technology level is near-future. They have access to the surface (with proper protection), so they can conduct geological surveys instead of digging blind, and can also vent debris/heat/fumes/etc there.

It occurred to me, perhaps incorrectly, that a softer stone like limestone might allow them to do this more easily -- that they wouldn't need explosives as significant as for (say) granite, and that they might even be able to hand-dig in especially-sensitive areas (like right outside where people are living). But that led me to wonder how stable an underground habitation would be if carved out of soft stone -- at what point, either depth or size, would it become too unstable to risk living in? Or is that not how stone works?

How should I be thinking about this? Can a settlement of at least hundreds (perhaps thousands) of people be safely built in a soft rock with low-impact methods of digging/blasting? (And what would those methods be?) Or, alternatively, is there a way to expand such a settlement in "regular" hard rock without causing cave-ins and other hazards? Do they need to dig deep rather than shallow?

I am in the beginning stages of building this world and haven't worked out details of what this planet is like, beyond that the surface environment has become inhospitable. So if particular properties of the planet would make this easier (volcanic? mostly water, or alternatively mostly arid? etc), that's fine. (The nature of the inhospitable environment doesn't matter a lot either, in case that matters. I just need to keep my people underground and expanding.)

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    $\begingroup$ I would suggest the place to start to answer this is the Normandy trenches, they were regularly dug while under attack with their friends sleeping up the hall! $\endgroup$ – Liath Sep 30 '14 at 10:11
  • $\begingroup$ @MonicaCellio - Thematically, in general terms without spoilers, what role does the underground play in the story? Knowing that, we can narrow down what kind of geology and conditions would best serve the needs of the plot. $\endgroup$ – TechZen Oct 19 '14 at 22:54
  • $\begingroup$ The depths of the planet are a place of gloominess, fear, and danger that even the strongest walls can't make people feel completely safe about. (There is a non-physical aspect to the threat.) $\endgroup$ – Monica Cellio Oct 19 '14 at 22:57
  • $\begingroup$ Can you also consider that any development should include an Environmental Impact Assessment (EIA) in order to document what is being destroyed, as well as a Preliminary Cultural Review (PCR) in case there might be paleontological or archaeological artifacts where they're drilling? I add this only half jokingly, because I'm a nerdy urban planner and any digging we do requires this. I assume it will continue to happen in the near future, and may add a bit of practicality to the story (PCR might not be necessary, but EIA should even for asteroid mining today!) $\endgroup$ – Mikey Dec 22 '14 at 5:54
  • $\begingroup$ @Mikey I suspect the colonists' crankiness at being driven underground trumps any concerns they might have had for the impact of their actions. :-) $\endgroup$ – Monica Cellio Dec 22 '14 at 13:27
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The main problem actually is where do you put the rock that you dig out. You would either need access to the surface to dump it somewhere or some way to compress it and then use the compressed rock to make the walls of the tunnel.

For digging through rock close to habitation you would definitely not be using explosives. Modern tunneling machines are quite capable of doing so without explosives and do not have so many associated risks.

http://en.wikipedia.org/wiki/Tunnel_boring_machine

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Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional "hand mining" in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less—this is because tunneling with TBMs is much more efficient and results in a shorter project.

It's entirely plausible that a society such as you describe would have one or more TBMs running continuously, and a substantial number of people would be involved in the maintenance and operation of the machines. The main problems would be getting hold of the raw materials required to run the machines and line the resulting tunnels and (as already mentioned) getting rid of the waste.

Depending on just how "future" their tech is the TBMs themselves may be very similar to our current ones, or significantly more advanced. For examples ones able to use force fields or ultrasound or similar to do the digging would need far fewer replacement parts.

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    $\begingroup$ Real world tunnelling encounters all sorts of different material - hard rock, soft rock and various minerals. If you had enough mixed strata (maybe due to seismic activity?) then you could potentially encounter all the minerals you might need as raw materials for your machines, and maybe natural caverns you could use as living space or for disposal of tunneling waste. $\endgroup$ – Colin Pickard Sep 30 '14 at 13:55
  • $\begingroup$ Great answer. As long as the world becomes slowly inhospitable, enough to allow building a few of these before moving entirely underground, it's quite plausible. $\endgroup$ – mechalynx Sep 30 '14 at 19:56
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There are tables, created for mining engineers that list types of stone e.g. sandstone, granite, limestone etc and tell exactly how large a tunnel they will support, how big galleries and where to leave pillars etc. I've seen them in old WWII combat engineering manuals as well. Most of those are available on the Internet Archive and other similar resources. These will tell you the size and shape of underground structures in particular stones as well as the relative merits of each.

Each material has its own properties, strengths and limitations. These parameters are based on centuries of expense and testing. Giving how many lives were paid for the information, we should probably treat the tables as sacred.

In main, the major restriction in subterranean construction is width. It's the compressive load that a section of unsupported ceiling can carry that defines the maximum width. This can be mitigated with an arch shape but speaking generally it's easier to support a narrow deep passage than a wide shallow one. In many large mines you see a shaft maybe 3m wide but as much as 20m tall, divided top to bottom in stories with one way traffic on each level.

Soft stones support less width and support less shallow arcs. In the stone city of Petra, there are several large interior spaces carves as a series of connected domes with pillars at their vertices. Hard stones support larger spaces but their rectilinear crystalline structure makes arch hard to carve, instead the tunnels are triangle shaped at the top

Soft stones support less weight but are less brittle and fail more gracefully i.e. they give you warning. Hard stones are brittle and appear fine until they crack and the cracks can propagate a long way.

Soft stone often have pockets of sand, gasses or water that can cause serious and unexpected problems. Soft stones are more likely to be associated with organic materials like coal and methane that can cause explosions. Hard stones are safer in those regards but contain preexisting cracks and strain faults whose pressure mining can suddenly release.

The ideal material for tunneling and building extensive structures in is salt domes. These are thick bubbles of salt, many dozens of kilometers in expanse ofter a kilometer or more in thickness that remain from the evaporation of ancient oceans. They are easy to mine, support large dome shaped chambers and fairly wide and tall tunnels. They are used to store just about anything from the legendary government cheese to strategic oil reserves to seed banks.

During the Cold War extensive designs were made up for facilities supporting several thousand to built in large domes in Michigan and Utah.


If your looking for gloom and oppression, I suggest the colonist must dig through relatively soft but "rotten" rock which has lots faults, pockets of water and gas, perhaps mixed strata as well if digging into an uplifted hill or mountain side with lots of mixed strata.

Soft stone means smaller chambers with lower roofs, giving a claustrophobic feel. Ventilation is more difficult. It's harder for find places for large groups to communicate leading to paranoia. With mixed strata and pockets of water and gas, mining dangerous, collapses common and overall stress raised.

There is a place in Turkey where people dug extensively into the local limestone to create havens for generations of attacks. Though very expensive and clever, they are rather small and gloomy even with modern lighting. Might want to check on them.

Of course for gloom and grim, it's hard to beat a coal mine. A colony might have to tunnel in for both shelter and fuel but the walls are dark,the dust gets everywhere and in everyone. The softer coals are mined in very small shafts (that's why surface mining became so popular, that and less water pollution.)

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  • $\begingroup$ Strip mining coal is also nice because of the reduced explosions, mine collapses, death by oxygen deprivation, and long-running fires. $\endgroup$ – Gary Walker Dec 22 '15 at 1:49
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First off, I believe this documentary will prove beneficial for you: https://www.youtube.com/watch?v=yVkyfC_AOLI

To answer all your questions, given near unlimited resources you can build a construction to house an arbitrary number of people underground. However, if your resources are limited, it depends on a bunch of things, namely the following:

1- How do you intend to configure the underground areas to house everyone. What type of geometry, living spaces and what not are you picturing. This will be a major influence on whether it's feasible or not.

2- At what rate do you want to expand the habitation at.

3- How far underground you want your colony to be.

4- The geology which you are mining through.

In response to your initial questions:

It occurred to me, perhaps incorrectly, that a softer stone like limestone might allow them to do this more easily

This ones a double edge thing, mining in soft rock is much easier than in hard rock, although the major downside is that soft rock isn't as strong as hard rock. The trade off would be: harder to dig but much bigger/safer rooms or easier to dig but necessarily smaller/slightly less safe rooms.

At what point, either depth or size, would it become too unstable to risk living in? Or is that not how stone works?

That's not how stone works. Given the proper data and knowledge, a suitable habitation can be engineered in any rock mass, however the cost of doing so (resources, time, etc) will differ depending on the circumstances. Given infinite resources you can build whatever you want where-ever you want. Although, if you're colonists are resource limited, then it becomes too costly/risky the deeper the colony and the bigger the rooms you want to make.

Can a settlement of at least hundreds (perhaps thousands) of people be safely built in a soft rock with low-impact methods of digging/blasting?

The safety of your rooms won't be dependent on the digging technique, any technique can be modified so that it's low-impact. When you think about blasting you need to differentiate production blasting and construction blasting. In mining, production blasting is done with the goal of getting as much stuff out as quickly as possible so it ruins the rock, in construction blasting you take a very controlled approach and you do it in such a way that you don't screw with the rocks strength. Example, Hydro Quebec has underground power houses which can be considered pretty massive: https://www.google.ca/search?q=hydro+quebec+underground+power+house&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi56aSKku7JAhUBlB4KHSr0C6IQ_AUICSgD&biw=1184&bih=679#imgrc=XxoK3SQMfdINGM%3A they were made by using blasting techniques and are designed to stay there for many many many years.

Is there a way to expand such a settlement in "regular" hard rock without causing cave-ins and other hazards?

The stability of an underground opening is dependent on two aspects:

  • The intact rock's strength (measured by the UCS (unaxial compressive strength))
  • The quality of the rock mass (how nice the rock is, think slab of counter top granite vs gravel, it's measured using GSI, Q-System or RMR)

The stability of an underground opening will be dependent on these two values, assuming you have a VERY VERY good rock, then you're looking at being able to have a room as wide as 15m by 15m tall by infinitely long while still being on the safe side. There's a number of empirical rules have been developed over the years in order to calculate exactly how big you can make an opening. One of the most used guidelines are from the Norwegian Geotechnical Institute (NGI), it details what type of support system you'll need for a given rock quality and opening size.

Do they need to dig deep rather than shallow?

For geotechnical stability, the answer is always more shallow. The deeper you go the higher the in-situ stresses in the rock. There's a bunch of technical information that goes into this but basically, the deeper you go the smaller your excavations needs to be, too big and you risk failure within the rock itself which can lead to rock burst which can/will eventually kill your settlers.

Mind you all of this information is based on currently knowledge, if the civilization is near future then there's a good probability that would have learned more about rock mechanics. A lot of research is being done in the domain of geotechnical engineering and although we have a good enough understanding to do actual engineering, a lot of this stuff is still pretty black magicqui due to the nature of rocks. To this day, after 150 years of research predicting rock bursts is still akin to magic.

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  • $\begingroup$ Wow, "rockdude" is right! Welcome to Worldbuilding, and thank you for this very-informative answer! That documentary looks like it will be very helpful, judging from the first few minutes. $\endgroup$ – Monica Cellio Dec 22 '15 at 0:52
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According to Wikipedia:

"A cave-in is a collapse of a geologic formation, mine or structure which may occur during mining or tunneling. Geologic structures prone to spontaneous cave-ins include alvar, tsingy and other limestone formations"

You can safely assume that you don't want to dig through limestone. The answer can be found in another question:

What causes cave-in's?

With the answer to that we can know how to prevent a cave-in. Cave-in's are mainly caused by:

• Water causing soil and rock to come loose and fall

• Not enough support in an area resulting in the roof simply falling down

• High pressure (mainly caused by too much weight on top of the roof) causing the roof to fall down

So, to prevent cave-in's in our underground colony we can ensure that there are no area's that are wet/damp. We can also make sure there is enough support. I assume our colonists don't want to go up to the surface constantly even though it is possible. Therefore we must do this:

• Dig out the area using conventional methods, e.g big drills, mining tools, etc.

• Using materials they have mined the colonists can make rooms inside the mine made of metal materials they have found such as iron, aluminium, copper, etc.

We will have to make sure the material we use for making the rooms is strong enough to hold the pressure of the soil and rock above it. A plus side to this method is that we can easily get materials from the minerals we mine. As we mine along we can use support beams made out of the material to hold up the mine. Once the rooms are built in that area we can safely destroy the beams causing a cave-in.

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