Large enough to fit in one or more shipping containers. Really depends on how big you want your bunker to be.
Easiest way to dig the hole is by using a tractor, backhoe, or similar machinery.
Make the hole with a large ramp so that the shipping container(s) can easily be taken down the ramp and placed in the position you want them to be in.
Once done, cement a floor at the bottom of the hole.
Note - Before choosing where to dig the hole, make sure the location is not on a flood plain. Last thing you need is your bunker to flood. You should also try to avoid any area that is swampy or has a water table.
STEP TWO, BUY SHIPPING CONTAINERS
Search for used shipping containers online that you can buy locally and have delivered. As-is containers starting at around $1,500 and like-new or modified containers will cost between $5,000 and $8,000. Make sure whomever you are buying from also delivers.
One shipping container itself will probably not be enough unless it is meant to be a short term bunker only.
If it is long term bunker, for 2 or more people, then you need to be thinking a lot bigger. Possibly as big as 3000 square feet, including a 400 sq foot greenhouse which will supplement the food supply.
Once you get them, tow the shipping containers down the ramp and into position.
STEP THREE, ADD ANYTHING ELSE YOU NEED
You will need air vents, water, sewage disposal, and possibly electricity unless you are okay with using candles/lamp oil.
Build everything you need during this step, including:
- Secondary Exit
- Air Vents with air filtration, multiple.
- Water source and filtration
- Optional - 2nd shipping container with all your electricity and utilities.
- Optional - Underground Greenhouse for growing food.
- Everything else you think you need.
If it is meant to be a long term bunker with a supply of food, you need to determine the logistics of how much food you need, for how much time, and how you disposing of your bodily fluids.
If you are building an underground greenhouse (and possibly using your waste as fertilizer) then you need to work out the logistics of that and get it working properly, and solve any problems first.
You will also need to store everything from light bulbs to food to cooking oil to anything else you might need, and storing all that stuff requires a lot of space.
STEP FOUR, STRUCTURAL REINFORCEMENT
You don't want your shipping container to collapse under the weight of any dirt or rocks you place on top of it. Thus you need to reinforce the structure so it can take extra weight.
Dirt weighs about 100 lbs per cubic foot. The amount of dirt above the shipping container can add a significant weight to the structure, enough weight that even 18 inches of dirt on top is sufficient to cause the roof of the shipping container to start caving in over time (see image below).
So how much structural reinforcement does it need?
Well it depends... how far down are you burying this bunker?
If you are burying it under 10 feet of dirt (1000 lbs per sq foot), then you should aim to reinforce it for 3 times that amount so that each square foot can support 3000 lbs if it needs to.
Even if you only bury it under 6 inches of dirt, you should still be reinforcing it in case any vehicles are being driven over the top or parked on top of the structure.
STEP FIVE, PROTECTING THE STRUCTURE FROM DIRT
So here is the thing about dirt: Dirt is acidic, caustic, and wet - all of which causes steel to rust faster.
Wet, acidic and caustic dirt will slowly rust your shipping container away over a long period of time, causing it to eventually structurally fail. To prevent that eventual failure, what you need to do is place something in between the dirt and the steel.
Something like cement.
Now the good news here is that the cement will also serve to reinforce the structure, so you should take this into account when building it and determining how to reinforce the structure.
The bad news is that you better be done STEP THREE completely and everything can be repaired or replaced without needing to rip apart tonnes of cement, because one the cement goes in it will not be easy to bring back out if you need to conduct repairs.
STEP SIX, THE DIRT
Once everything else is done, everything works fine, then comes the dirt. Finally. This is the easy part. Just pile dirt on top of the structure and hope you did everything correctly...
Because the last thing you need is for the bunker ceiling to collapse in the middle of the night and kill you.
Can you make a shipping container bunker? Yes, but it is not ideal.
Should you? Not unless you are willing to put a lot of effort and money into it, because they don't actually make ideal bunkers. It would almost be easier to make a submarine than to turn one of these into a safe-to-live-in bunker.
So what about alternatives?
So you just used tonnes of cement to bury a shipping container, because by itself the shipping container cannot withstand the weight of the dirt, and the dirt and water will make it rust easily. When you really think about it, the shipping container is really a waste of time as a bunker. It is not ideal, as it is too small, not structurally sound, and rusts easily.
So why not just SKIP the shipping container entirely?
Option A. Just Use Cement
Build a round cement bunker. Or a square one. Whatever.
Design it with the greenhouse and living area in the middle and chambers around the sides for utilities like water, electricity, sewage, air filtration, extra living quarters, kitchen, bathrooms, machine shop, etc. For a family of 4 you will want at least 3,000 sq feet, including 400 sq feet for the greenhouse.
Option B. Buy An Old Nuclear Silo or Bunker
So in Northern Ontario there are a number of old nuclear silos dating back to the Cold War which have since been ripped apart and the missiles moved elsewhere. The properties have since been sold off and a variety of people now own such old missile silos.
Thus it is possible to just purchase an old missile silo or bunker. You don't need to build one.
Option C. Buy A Cave, Cavern or Mine
There are a number of places in Ontario which are privately owned caves. As well as old abandoned mine shafts. Buy the property, renovate and reinforce the cave or mine shafts to suit your needs.
Option D. The Ready-Made Bunker
There are companies that make and sell ready-made bunkers. More expensive? Yes. But this is also the fastest and easiest way to have a bunker made, and they could certainly build a giant underground home if that is really what you wanted.
Does your "bunker" really have to be underground?
Actually it does not.
If you are not near any place that is likely to be a nuclear missile target, then your "bunker" could be above ground and simply camouflaged and well hidden. You don't need to bury it.
You could paint it to blend in with foliage...
You could bury only part of it so that most people from a distance cannot see it, and add lots of trees, flowers, bushes, etc.
Or you could just build a normal "home" with them and trust that the trees on the property will keep away prying eyes. Why make it harder if it doesn't have to be?
The Final Say
Maybe just skip the bunker and build a home instead.
And if you are that worried about intruders or whatever, build booby traps and bear pits.