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.
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Again, thank you for watching, and let me know what you think.