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In my fictional setting, albeit close to real world, there is a research facility built on a remote island in a form an underground multi-floor bunker.

This bunker does not look like a mine - it is, in fact, a concrete-steel building with living rooms, laboratories, elevators, storage areas, etc - just built below ground, not above.

It seems for me, that construction of new floors further below an already operational bunker is not practically possible without:

  1. Making loud noises and other inconveniences for people, who work in already built areas above.
  2. Risking entire areas above collapsing into bottom due to new tunnels digged below.

So once bunker is built and is manned by staff and equipment - it's not possible to expand it further below vertically.

Am I right with this assumption, or not? Question is only about vertical expansion of bunker, not horizontal. Consider, that:

  1. In my fictional world only tech of real world's 2024 year is available - no science-fictional tech. This applies to mining equipment as well.
  2. It's on planet Earth.
  3. Real world physics and difficulties when building underground - are applied too.

Thanks for your activity and responses! Upd (based on questions below):

  1. The fictional setting is not post-apocalyptic. This underground research facility is a government/corporations funded installation, where research and development, subjected to commercial/military secrets, occurs.
  2. The height between each two vertically adjacent floors is 3-6 meters (9-18 feet).
  3. Regarding to geology, within which bunker is built - I'm still choosing the rock, which will make the dynamic expansion of the bunker plausible, but still strong enough compliment concrete-steel outer walls of bunker to support the entire structure.
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    $\begingroup$ Why are people in the bunker? What are they sheltering from? Can they enlarge their living space, and still retreat to the original steel and concrete bunker in times of need? And where are they getting their materials from? If they are on chalk or sandstone, they may just be able to dig new rooms, or use natural caverns. It could be easy or very hard, depending on the backstory. $\endgroup$
    – Richard Kirk
    Commented Jul 8 at 17:19
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    $\begingroup$ The feasibility of expanding an existing underground bunker vertically with modern technology, while mitigating the risks of structural collapse and operational disruption, would likely require careful planning and advanced engineering techniques. This process could involve significant noise and inconvenience for the current occupants, as well as potential structural risks. It would be important to modify your question to be easier for me to understand in underground construction and structural engineering to evaluate the specific challenges and solutions for such an expansion. $\endgroup$
    – Sam Biswas
    Commented Jul 8 at 19:09
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    $\begingroup$ How is this different from adding deeper drifts in an underground mine? Which is done fairly often. $\endgroup$
    – Gault Drakkor
    Commented Jul 8 at 21:54
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    $\begingroup$ Biggest question - where does the excavated stuff go? Digging out rock makes chunks of rock that take up more volume than the original, so unless you have some place to dump them outside of the shelter you're just reducing the livable space. $\endgroup$
    – IronEagle
    Commented Jul 9 at 2:31
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    $\begingroup$ Bingle the The Mayfair Hotel Megabuild. It's a documentary about how Claridge's Hotel in London was significantly expanded, including a five-storey downward extension to the basement, without significant disruption to the operation of the hotel. Watching that, you'd believe anything is possible with good enough engineers. $\endgroup$
    – BWFC
    Commented Jul 9 at 11:00

5 Answers 5

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People expand downwards all the time

People build tunnels out of prisons, expand mines downwards, build deeper basements. You can dig down and reinforce the tunnels. So long as the tunnels are reinforced, they won't collapse and there's no risk of the areas above collapsing.

You can mine quietly but it's slow

If you are worried about noise pollution you can mine slowly and slowly remove dirt and crack rocks and such. It's much slower, but you can do so if needed.

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    $\begingroup$ Yes, so long as your tunnels are stable, the above floors will be fine. Concrete is very stable, and you can regularly check it for cracks and replace it if needed. $\endgroup$
    – Nepene Nep
    Commented Jul 8 at 17:47
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    $\begingroup$ For two very different examples take a look at the New York City Subway system (old and new parts), and the tunnels built in Gaza. Both are relatively recent and under existing construction. $\endgroup$
    – Adam
    Commented Jul 10 at 19:40
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    $\begingroup$ It would also help greatly if the original bunker system was also designed with the possibility of further expansion in mind. That way ease of access for mining equipment equipment, structural reinforcement of the lower levels and possible future power and utility requirements etc would be engineered into the original design making any expansion later on a LOT easier. CONTINUED BELOW $\endgroup$
    – Mon
    Commented Jul 11 at 5:52
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    $\begingroup$ For example if a descending spiral road bed was built around the outer circumference of the bunker when it was first constructed and this road goes right down to the lowest level & ends in a blank concrete wall? You can just cut though that wall and continue the spiral downwards as far as you need to go. Every new level can then be excavated laterally off the spiral at the right depth below the levels above it. All the excavation waste is transported up and out via the spiral while all construction materials etc are trucked down and in the same way. $\endgroup$
    – Mon
    Commented Jul 11 at 5:58
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    $\begingroup$ If your worried about what can be done without disturbing existing tunneling, look at some of the projects done in London as part of the Thameslink and Elizabeth line (previously crossrail) projects, where very limited surface level space was available, and they were building interconnections into existing deep tunneled stations. $\endgroup$
    – user1937198
    Commented Jul 11 at 15:19
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Reinforced concrete is a lot stronger than you're assuming, especially since this is meant to be a bunker so I'd expect it to have thicker walls with higher density concrete to maintain rigidity.

As to mining quietly, there's one really clever way I've seen this done, look up the youtuber 'Colin Furze', he has an entire series about digging a tunnel under his house using a special tool he made for breaking through bedrock. Its essentially a hydraulic cylinder with a jackhammer bit on the end.

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    $\begingroup$ FWIW Clon furze is largely digging through clay and not bedrock. he has had to take out a jackhammer on numerous occasions $\endgroup$
    – Themoonisacheese
    Commented Jul 11 at 15:05
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Yes, it can be done, and quite quickly, even with crude technology.

For example, consider the Wolf's Den Project, and the Giant Project built by the Nazis in 1940s. Each started as a system of bunkers for the Nazi high command, and then progressively got expanded downwards and to the sides, without much issue. The only reason they stopped was because they lost the war, not the will and ability to dig. We are talking about kilometers of underground corridors, shafts, and halls, big enough for airplane hangars and tank factories, constantly deepened with explosives and shovels, and then supported with concrete masonry. All of this using cheap labor and 40's technology.

The know-how of digging or blowing a tunnel underneath an existing tunnel was already well known for at least 300 years, so there was a minimum risk of collapse from deepening the bunkers. The trick is usually to make the new, deeper tunnels not directly underneath the upper ones, and leave a layer of sheer rock between levels, much thicker than the actual levels were. The noise was not much of an issue either: the sounds carry very well within tunnels and caverns, but don't cross thick rock/concrete/soil walls easily. So unless you were inside the shaft/tunnel that was being extended by explosions, you wouldn't be bothered by noise much.

The main issue, just like with almost every mine, bunker, or any underground structure was water. The deeper you dig, the more water seeps through the walls, and eventually you start hitting whole aquifers. It is next to impossible to make the bunker waterproof, so the only option is to constantly pump the water out, and vent the halls so they would not develop mold all over.

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  • $\begingroup$ Thanks! Regarding not directly underneath, so, let's say, if we have each level of bunker to be 4 meters high - then is digging just 3-6 meters beneath it much a problem? Or if it's 3-6 meters of a sheer rock - then not that much? $\endgroup$
    – Valentin Onishchuk
    Commented Jul 10 at 8:09
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    $\begingroup$ @ValentinOnishchuk that greatly depends on the kind of rock you are digging into. But even if the rock is soft limestone, 3-6meters further reinforced with concrete and steel would work just fine. In any case, before new shafts or tunnels are blasted out, the rock is probed for cracks and instabilities to avoid, so the tunnels would never be on the same level, but flow around problematic places like roots of a tree. Also, all tunnels need to be at an angle, so that water would flow down into gutters and not pool in them. So the bunker system would necessarily be chaotic not orderly. $\endgroup$
    – Going Durden
    Commented Jul 10 at 8:37
  • $\begingroup$ great, I got this - so my underground research facility has to be not strictly ordered cube-like structure, but rather something like roots of tree, where each root may be above or below other root, or may not be. $\endgroup$
    – Valentin Onishchuk
    Commented Jul 10 at 12:21
  • $\begingroup$ Can you clarify which projects you are referring to, possibly with a link? I suppose the second one is en.wikipedia.org/wiki/Project_Riese ? $\endgroup$
    – jcaron
    Commented Jul 10 at 13:25
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    $\begingroup$ @jcaron the second I was referring to is Wolf's Lair (Wolfsschanze) but there were several others across Germany and Poland, some of them entirely underground, but most were a combo of over and underground bunkers. Of course, this is all WW2 stuff, the border region between modern Poland and Germany also has plenty of much older bunkers and hollowed-hill bases going back to at least Napoleonic Wars, often older. The Klodzko Fortress is one example, a giant star-fort atop of a hollowed hill, which sits in the middle of a town riddled with underground tunnels and bunkers like an anthill. $\endgroup$
    – Going Durden
    Commented Jul 11 at 8:36
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In addition to the various examples cited consider subway excavations under existing cities.

Fundamentally, this comes down to the fact that man can build things which are stronger than the natural surface it was sitting on. You remove a small amount of that natural material and replace it with an engineered structure that can take the same load but permits passage. The more reserve strength in the material above and it's ability to take loads in tension as well as in compression (remember, plain concrete is wonderful in compression, horrible in tension) dictate how big a chunk you can take as one bite.

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    $\begingroup$ The channel tunnel crossover boxes are a great example, 10 miles offshore, they are large underground rooms containing crossover points, that where excavated entirely by digging out from the tunnels. $\endgroup$
    – user1937198
    Commented Jul 11 at 15:13
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It will be noisy for the people above (unless it's a case like robbers digging into a bank vault where spending a hundred times longer is worth it).

It's otherwise completely feasible. In London rich people spend millions of pounds on Victorian brick houses with shallow foundations, and then dig downwards to install home cinemas, gyms, even swimming pools. Collapses are rare (but not unheard of, and sometimes its the neighbours' property that falls into the hole! )

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