Why Tunnels Don’t Collapse


Tunnels play an important role in our constructed
environment as passageways for mines, conveyance for utilities, and routes for transportation.
But, excavating a tunnel underground in unstable material can lead to some dangerous situations,
like the 2010 mining accident in Chile when 33 men were trapped deep in subsurface for
more than 2 months. Hey I’m Grady, and this is Practical Engineering. On today’s episode
we’re talking how engineers stabilize tunnel excavations to keep them from collapsing. This video is sponsored by Dollar Shave Club – more on that later. Rocks are heavy. That may seem self-evident,
like many fundamental principles of civil engineering. But when you build things underground,
it starts to become a major consideration. Just like atmospheric pressure is created
by the weight of air molecules pressing down on each other, pressure exists in the subsurface
of the Earth from the weight of the soil and rock above. This pressure compresses the material
in the subsurface more and more the further down you go. Building a horizontal passageway,
or a tunnel, through this material, interrupts the flow of these compressive forces. Just
like if you remove a column from a building, excavating a tunnel takes away the support
from the material above. Where you once had compression throughout the subsurface, now
you’ve created a zone of tensile stress, where the material above the tunnel is trying
to pull away from itself. Many materials react differently to tension
than they do to compression, and soil and rock are no different. You can imagine soil
as a collection of individual particles. The only reason a soil mass has any strength at
all is because of the friction between those particles. But friction is a function of the
force pressing the particles together. So, if you instead reverse that force and pull
the particles apart, the soil loses all its strength. Some soils, like clay, do have a
certain amount of natural attraction between the particles, called cohesion, but it’s
not enough on its own to resist significant forces. In other words, you can’t make a
rope out of soil – it has no strength against tension. If you build a tunnel in soil, you
have to replace the support you removed with some other way to transfer the load of the
soil above. This is why many tunnels are lined with materials like steel or concrete, to
provide support to the tunnel walls and transfer the stresses in the subsurface around the
tunnel. These lining systems add a major cost to the tunnel construction. Rock, on the other hand, behaves a little
bit differently in that it does have some tensile strength. You could make a rope from
it. Not that it would be particularly useful, but it’s a good way to imagine the difference
between soil and rock mechanics. In fact rock generally has more strength than soil for
all types of stress. This additional strength gives rock the ability to transfer forces
around a tunnel just like the lining discussed before. But, it’s not as simple as saying
tunnels in soil require support and tunnels in rock don’t. Geologists use the term “massive”
to describe rock that is uniform without layers or joints. Unfortunately, not all rock is
massive. In fact, most geologic units of rock in the subsurface have at least some amount
of jointing, or natural breaks. In many cases, the jointing of rock follows specific patterns
that can be observed and mapped. But, the problem with joints is that they have no tensile
strength, and so no ability to transfer tensile stress. You can see that jointed rock starts
to behave more like a soil just with much larger particles. So, even tunnels through
rock often require some type of support to prevent collapse. But, what if there was a way to take advantage
of the superior strength of rock without going to the added trouble and expense of lining
the tunnel to provide support? Well it turns out there is. Rock bolts are a type of reinforcement
for stabilizing rock excavations, usually made from steel bars or bolts. I built this
demonstration to show how they work. This is essentially the frame of a table, but the
top is completely open. I attached a bottom to the frame to represent temporary shoring
of a tunnel roof. Even though our permanent support system doesn’t rely on this, it’s
necessary until we get the rock bolts installed. My rock bolts are just actual bolts with large
fender washers to spread out the load. You can see that I spaced them out in a nice grid
pattern. Actual rock bolts are similarly installed in a pattern along a tunnel. For the rock material of the tunnel roof,
I’m using gravel. Of course, there are a few differences from the real world and my
demonstration here. First, in the real world, the rock is there first. We don’t get the
convenience of adding the rock after the tunnel is already in place. Real rock bolts are installed
by drilling into the native material. The other difference is the scale. Although there
isn’t a fine line between soil and rock mechanics, gravel really falls into the soil
side. It would never be feasible to use this many rock bolts just to stabilize a gravel
mass. Rock bolts are most feasible when you’re tunneling through jointed rock where you can
put a little more space between the bolts, but this demo is just to show that it can
be done. To tension the rock bolts, I tightened washers
and nuts onto each one. Another obvious difference between my demo and the real world is that
we don’t normally having access to the top of the bolts to add nuts and washers. Instead,
the rock bolts are secured at their ends by some other method. Two of the most common
methods of anchoring are a wedge device and pumping in grout. It’s very similar to putting
an anchor in concrete or even hanging a picture frame in drywall. Once the bolts were tensioned,
it was time to remove the temporary bottom. You can see I lost a little bit of gravel
between the rock bolts, but the majority of the rock is spanning gap. I’ve essentially
created a bridge made from gravel. But you know that supporting its own weight isn’t
exciting enough for this channel. So I decided to put my own safety on the line as a test
subject. The rockbolted gravel could support my weight, even with a few hops. You can see
things flexing a bit underneath, but the simulated tunnel ceiling held strong. There are lot
of ways to conceptualize what’s happening here. At the most basic level, the bolts are
creating a continuous zone of compression in the gravel. I’ve taken a fractured rock
mass and knitted it back together, giving it the ability to resist tensile stress. This
is very similar to post-tensioned reinforcement used in some concrete structures. Like I mentioned before, trying to support
a gravel ceiling using rockbolts isn’t the most appropriate use. They do have their limitations.
But, this simple construction method dramatically reduces the cost of making tunnels through
rock safe from collapse. And public safety is priority number one for civil engineers.
Do you have questions about tunnels or any other topic in engineering? If so, post it
in the comments below. Thank you for watching and let me know what you think. Thanks to Dollar Shave Club for sponsoring
this video. Just about every video topic on this channel was first conceived in the shower.
It’s the perfect combination of monotony and absence of distractions that my brain
needs to be creative. The very last thing I want to try and remember in the shower is
whether I need more soap or shaving cream. Dollar Shave Club is pretty much a a one-stop
shop for grooming products: everything you need to look, feel, and smell your best delivered
right to your doorstep at a frequency that you choose, and for much cheaper than the
grocery store prices. This is the stuff I got in my first box – it’s not just razors.
Support Practical Engineering and get your Daily Essentials Starter Set from Dollar Shave
Club for only $5 by visiting the link in the description below. Get it for yourself, or
give it as gift to a friend. That’s Dollarshaveclub.com/practicalengineering. Again, thank you for watching, and let me
know what you think.

100 thoughts on “Why Tunnels Don’t Collapse

  1. I pinned this comment just to say thanks for watching my video, and I hope you are having a very nice summer.

  2. Standing on top of it was lot more dangerous that it looked. Thanks for the demonstration! Great video 🙂

  3. Thanks for the great demo! I always appreciate a relatively simple way of showing some basic engineering/physics concepts.

  4. Why does elon musk say its easier to build deeper and deeper tunnels than build higher and higher bridges??

  5. Some see a bald head. I see a blank canvass. PLEASE play with wigs! I'm sure it'll fit SOMEWHERE…. maybe soon!

  6. Very informative video. Can you make video on boiler and steam turbine efficieny calculation. Also heat mass balance diagram of boiler and steam turbines.

  7. Wow! I did not expect the rocks to stay up, and then when they did stay up, I did not expect you to be able to stand on it. Incredible!

  8. Dump the ADs Your not making enough, ya gotta push for more and more. The biggest problem in the world today is greed.

  9. I don't care a lot about engineering, but this video was informative, juicy and entertaining. Great job!

  10. I was taken on a tour of Bristol Cathedral some years ago, and we got to see above the stone-vaulted roof of the nave. It is packed with rubble above it just like this to stabilise it. So next time you are in a medieval building with a vaulted ceiling, remember that above you are many tons of stabilising rocks.

  11. That was really cool to see… had no idea it was so powerful. I'm an artisanal miner myself, but we don't use rock bolts in our mine. We still use timber, which also is impressively effective.

  12. Thank you so much for this amazing video, it would be nice if you make more videos in detail on tunnels. Thank you

  13. But you CAN make a useful rope out of at least one kind of rock… basalt. Basalt fiber ropes, yarn, and roving have double to triple the tensile strength of steel cable while being much lighter. The stuff is even stronger and more durable than Kevlar, as basalt fibers don't melt until you get up to about 1200 degrees, and UV doesn't degrade them.

  14. Can you make a video explaining the tunnel structure used by London Underground?
    I head it goes 7 stories deep… What do they do prevent it from collapsing?
    Are there lines that are stacked one above another? or there would be lines that must be passing underneath others?
    I think it will make a very interesting video (or video series)

  15. Wow…as he was standing on top,the gravel started to fall at a small slow rate…but in a short time here it was obvious that it was gradually getting faster and worst with more and more rocks collapsing.
    I have no doubt that after the video stopped,it eventually collapsed
    to total failure. It amazed me that this guy still put out this video showing the eventual failure that will come but cut it off before it did.
    This guy is dangerous….I hope he doesn't have kids.

  16. My opinion is that you are nuts to stand on compressed gravel for demonstration purposes. I would have never believed this stuff holds.

  17. love your content and I don't often compliment a video. I am a mechanical engineer and I find your videos very interesting and practical. I wish university actually taught me something practical such as this.

  18. i want to the type of rocks bolts use for tunneling works. & also which bolt is use foe alimak raise climber

  19. I was worried about your safety when you demonstrated the strength of these bolt joints by your own body weight. Please stay safe sir. Respect!

  20. All wrong … had he stood and jumped a little bit longer on that gravel connected and held together with the bolts, more stones would have fallen out and there would have been no more compression … he would have fallen through. He is explaining things in a completely wrong way by using foggy wording …

  21. You talk about the pressure of air???? This is not a LIQUID, these are stones, rocks, and you are not mentioning the force of GRAVITY at all … you must be a joker …

  22. sir sir could you please make a video on Microseismic monitoring to predict Rock burst in Under ground tunneling with detailed function. please sir beg you, and i hope you will reply my comment

  23. "showing" how much support strength (NONE, we know) the gravel has prior to engineering, would have been a really nice feature in this video…but very good.

  24. In the beginning, mother earth created chile.
    Fortunately they were IN Chilly so they didnt go hungry while being trapped

  25. Hai sir
    I think there is a good bond between the rocks and spacing is the crucial role and that is the main reason your are standing there correctly without support

  26. I know this video is a little old but how do old tunnels like the ones from the 1800s and older work? also how does blasting to make a tunnel work vs how we do it now?

  27. An engineer , huh??? Be advised the force of gravity is much greater than atmospheric pressure. In fact gravity CAUSES atmospheric pressure by acting on the atmosphere above and around us. Your assertion that "barometric pressure" is the force ~which compresses the earth's crust~ is gratuitous. Gratuitous assertions must be just as gratuitously dismissed according to the rules of debating..

  28. In the Broken Hill zink mine, Australia, they used drilling machines to bolt a steel mesh to the ceiling of the tunnel. When they turned the lights off you could not see your hand in front of your face. Time also seemed to go away at 3,000 feet down.

  29. Standing on your simulated rock mass wasn't very smart. If it had failed, you would have fallen 2.5 feet and landed on (and likely impaled yourself by) dozens of threaded studs! Even if it hadn't impaled you directly through your feet in a cartoon like fashion.. you could have very likely sustained a serious injury when a bolt jammed itself into your calf-meat, or at the very least, you could have landed with all of your weight onto an uneven, loose, pile of rock and thread-rod, and the weight of the fall could have caused your ankle to roll right out from under you, then your ankle would have resembled a large stick, wrapped in ketchup packets, shoved through the center of 2 grape fruits, then broken in half! Then you'd end up with 2 mangled grapefruits, flipping around on a snapped stick, with ketchup and grapefruit juice pouring out between the break! OMG.

  30. How do they prevent the ceiling from collapsing as they attempt to lengthen the tunnel underground since they have to advance before they can brace the tunnel.

  31. I appreciate these videos for their lack of pathos and ethos, instead focusing entirely on logos. It's a nice respite from the real world, and much of the rest of youtube

  32. is mountain region strength are calculated according to young's modules and likewise formula
    how we calculated total strength of mountains across a selected area

  33. Seriously dude, I had countless teachers spend years trying to get me to be interested in the practical applications of science and math, all to no avail. If it worked I would have been a civil engineer clearly

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