# How fast can giant underground shelters be built?

Detailed Synopsis: I started writing a story wherein advanced technology slowly stop working (starting from most advanced, and progressing until only handmade items function), while the global climate simultaneously cools.

In response, governments (and other wealthy/powerful organizations) begin to build massive underground cities/shelters so people can survive the cold. The problem I'm running into is that the degradation of technology must be slow enough for humanity to react...so I need to know how fast that reaction can reasonably be.

Question: Roughly how much time would it take for a government (such as the US) to build a single, self-sustaining, underground shelter capable of supporting 10,000 people?

Constraints:

1. The shelter can be designed and constructed using all modern capabilities (such as using a computer for engineering simulations, automated construction vehicles, and 3D printing), but the shelter itself must be relatively low-tech: no electronics or complex moving parts, no steam power, etc. Essentially, tech no more advanced that ancient Egypt.

2. The shelter must be self-sustaining for 10,000 people. This includes being able to grow food which can survive without direct sunlight (such as mushrooms and insects, most likely). It can use things like coal or natural gas to provide heat (those substances will still burn), but any attempt to use those substances as fuel for an engine will fail (the engine will, at best, simply not function).

3. The shelter may be built in an extreme hurry: everyone involved is aware that these shelters may be humanity's last hope for survival. Existing subterranean infrastructure (such as subway tunnels) may be used to expedite the process.

I'm mainly concerned about the rough construction time, ((Would it take months? Years? Decades?), but any suggestions regarding food and heat (or other logistics) would also be welcome.

• "advanced technology slowly stop working" How? "10,000 people" That's a very small city!!! – RonJohn Dec 9 '17 at 18:24

# Regarding the time and cost to build underground shelters

Since you are looking for survivability, I assume, a good model for construction costs would be a missile silo. Looking at some articles about missile silo sales and conversions; the Titan II silos can be effectively converted into about 50,000 square feet of living space. Silos are oriented vertically and not horizontally, so I think you might be able to get even more space with a better orientation.

This history of construction in South Dakota indicates that 150 underground silos were built scattered over an area of 15,000 square miles around Ellsworth AFB in just under 2 years, at a cost of $75 million (about$600 million in todays money). If we assume that we can squeeze 100,000 square feet of living space out of each dig, then that is 15 million square feet of space in two years.

According the HUD, 24% of Americans live with 250-500 sq ft per person in their residence; 39% live with 500-1000 sqft. So it isn't unreasonable to assume that you could jam people in a survival situation in at 250 sqft per person. A general rule of thumb for shipbuilding is to double the area of berthing for common spaces and galleys and such. So now we are at 500 sqft per person.

The real trick is how much it takes to feed these people. I don't know that growing mushrooms and bugs will feed people effectively. I'm not going to dwell on that, and just go in at 9500 sqft of vertical hydroponic farming setup per person (a little under a quarter acre); for an even 10000 sqft per person.

At that rate, we can fit 1500 people into our 150 silo cluster (presumably built close to each other, not spread out all over whereever)

Those silos were more or less build in a hurry, since they finished construction right before the Cuban Missile Crisis, so I imagine there isn't that much speed improvement to be had. So the powers that be would want to start 5 or 10 of these construction jobs at the same time, to the tune of a few billion dollars, to get their desired capacity in two years time.

Power, Heat flow and Air Flow Issues for Low-Tech Underground Shelters

Your constraints look like they run into some tough, tough problems regarding power, heat flow and air flow, as follows:

I came to a similar area for per person's portion of the underground farm. (I ballparked it at 4 people/acre or close to 10 people per hectare 10^4 m^2.

So 1e4 ft^2 or 1e3 m^2. I prefer MKS units.

Mushroom farming in the dark won't work, without total handwavium; you need green plants that photosynthesize to create the food calories, protein and fat (directly or by feeding fish or chickens, etc.) But those green plants need light, lots of light. Peak sun is roughly 1.3e3 Watts for each m^2. So for each person, we need 1e3 * 1.3e3 = 1.3e6 Watts!!! to simulate full noon sunlight. OK, we only need it half the time, but that's still a whopping 6.5e5 Watts, continuous, for each and every one of your 10,000 people. Do the math, and you're talking 6.5e9 W -- 6.5 Gigawatts -- that's some serious power. And this is just to grow the basic food.

It gets worse, I'm afraid. That's leaf-received optical power, in a spectrum your food plants can use. The only way I can see you generating roughly sun-spectrum light is by burning oil or natural gas to make a lantern mantle glow -- see gaslight. But gas lighting like that has low efficiency, ballpark 1%. (Stand by for link:https://en.wikipedia.org/wiki/Gas_mantle) I can't find an efficiency figure, so I'll stick with the 1% (lower than a decent electrical incandescent, but not an order of magnitude less.)

Now we need 100 times the power input, so 1e2 * 6.5e9 W = 6.5e11 Watts, yikes! Serious yikes!!

Virtually all of that energy is deposited in your farming space's air and your plant and "soil" as heat. If you don't get rid of that heat -- all of it, all the time -- you'll have to choose between starving (with the lights off) or cooking (with the farming lights on.)

Without some sort of mechanical power -- lots of it! -- I just don't see how you're going to move all of that heat and hot air out. You'll need massive, massive air flow, water flow, etc. to move that heat out of your farming space, even if you have a mile thick glacier overhead. (Which would, I admit, be a great heatsink.)

I hope somebody else can see a solution that meets your constraints yet doesn't violate the power and heat/airflow demands.

• This question (also about ventilation) might help: worldbuilding.stackexchange.com/questions/62902/… FYI, I like the idea of taking away technology, as a means to force characters to solve problems (like eating, staying warm) without it. But IMHO, taking away too much may make your storytelling harder. (My story has similar, but no ice age along with -- just a Nuclear Winter to survive.... – Catalyst Dec 2 '16 at 10:31
• A key question is how much do the shelter planners know about what technologies will fail -- and how soon. Imagine if they'd designed in a nuclear reactor (which then fails); then they rush and replace that with a natural gas turbine power plant (which also fails); then the rush and replace it.... They probably can't survive too many big mistakes, so please give them a fighting chance! – Catalyst Dec 2 '16 at 11:39
• Ventilation could work without power using a venturi system. Any wind flow will suck the stale air out and pull new air in all without power or fans of any kind. – Thorne Jul 13 '18 at 1:41

If you want underground space as fast as possible then infrastructure does exist for generating this.

Let me introduce you to the Crossrail Tunnel Boring Machines. Most importantly for your project, they're 7 meters diameter and:

How many metres bored on the fastest day of tunnelling by a single machine?

72 metres by Ellie on 16 April 2014 between Pudding Mill Lane and Stepney Green. Crossrail tunnelling progressed at a collective average of 38 metres per day.

Remember that these are the TBMs from just one project. 8 machines running 24 hours a day is going to give you a lot of concrete lined underground space. It's just a matter of how much you think you need.

Going fully underground isn't necessary. Given that the only reason you give is to excape the cold, then digging a trench 12 feet deep or so, using the bottom 8 feet for human activities, and the greenhouse/roof for growing could work. The greenhouse growing season is stretched somewhat by the the waste heat.

Potatoe have a yield of about 20 T/acre. The Irish would eat 8 lbs/potatoes a day. So a ton, 2000 lbs, = 125 man days, or a third of a year. So an acre supports 7 people on spuds. An acre for 4 people gives you some slack to grow less efficient crops. Having a longer growing season might allow increased produciton too.

To create this, all you need are BIG trenching tools. The rest is standard constuction.

All the other answers offer lots of excellent ideas. But, if technology continues to degrade and later generations need to do repairs, perhaps somehow burrowing animals could be trained to do the digging or supplement it. That's pretty low tech. Perhaps the holes dug could then be shored up with adobe bricks made from the dirt removed from burrowing. Badgers apparently are the fastest burrowing animal, so hordes of badgers might do the trick. That would all be very low tech.

## Yaodong

The type of environment also will play a factor in how quickly people can dig. For example, in China today, over 30 million people already live underground in caves. The caves are called yaodong. But, they are only so easy to build because that region's unusual soil allows for easy digging while maintaining relatively good structural integrity. That being said, the caves are not earthquake proof.

One possible solution might be to build on top of the ground and then cover it in foam (for insulation), then cover that (as an external weatherproof shell), essentially making an artificial underground. That would save you the problems associated with digging, but if covered over well enough, would provide you with the benefits of being underground.

Why go underground? The shelters need an energy source that doesn't require technology that can heat the area.

The answer is simple. Geothermal power. You build greenhouses and habitats on the surface and you heat them by drilling underground to tap geothermal heat.

Hot water rises and as such will pull the cold water back down to heat. No pumps are needed. Control of the temperature is done by taps and windows.

Since the greenhouses are heated, snow will melt off them and sunlight will grow your plants like normal.

A lot of agriculture occurs in greenhouses already in cold European countries. All you need is to drill for the heat source to warm them. No motors or electricity needed.