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.
I used to tease the civil engineers in undergrad for studying dirt, but this guys content has consistently been awesome.
The most soothing android I've ever learnt from.
Whoa, good channel. Thank you.
Honestly, if this guy was one of my profs in university, I would have done so much better
I watched the whole video and I don't know what to do with this information.
Although to be fair, he said you normally wouldn't use this system with gravel because it's not really feasible and he just did it for demo purposes. Just thought that was kinda funny.
Potash miner here. I do rock bolting all the time. Nice to see it shown from an open angle.
Excellent Video. I Learned something today. Plus, when I go through a tunnel, and see a huge bolt that seems to just go straight into the rock, I now know what it is!
Learning is fun.
i saw him standing on top of that and imagined his demo failing and him falling through and jamming one of those bolts up through the bottom of his shoe and into his foot. He should have had steel shank shoes on at the very least.
That looks like an awesome project to do with my kids.