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Some of the most complex civil engineering
problems stem from the interaction of water

and the ground.
It sounds mundane but, there’s a good chance
you’ve seen one of these on the news.

How is it possible for the ground to simply
open up and indiscriminately swallow anything

or anyone that happens to be around?
I’m Grady and this is Practical Engineering.
On today's episode, we’re talking about
sinkholes.

This video is sponsored in part by Blue Apron.
More on that later.
We all know about erosion.
This is the process that takes soil and rock
from the earth’s crust and moves it somewhere

else.
And there’s a lot of ways this can happen:
wind, landslides, abrasion, and scour.

But here’s the thing, none of it compares
to just the movement of water.

Water is the great eroder.
If you ever find yourself wondering how did
this particular feature of the earth come

to be here, or why is the ground shaped like
so, or just why are things the way that they

are, more often than not, the the answer is
pretty much just water.

The ability of water to move soil or rock
depends on several factors.

The faster and more turbulent the flow, the
more erosive it is.

Larger particles like gravel and more resistant
to erosion than small particles like silt

or clay.
Finally, rather than physical erosion, some
materials are soluble in water, just like

sugar or salt, and can be eroded just by dissolving
into the groundwater over time.

Most of us think about erosion on the surface
of the earth, but erosion can occur in the

subsurface as well.
In fact, scientist and engineers have a very
creative name for just such a process: internal

erosion.
If just the right factors come together in
the subsurface, some very interesting things

can occur, including sinkholes.
But let’s look at a non-erosive example
of groundwater movement first.

This is a from a video I made before the channel
was even called Practical Engineering.

Water is flowing from the left side of the
demo under an obstruction and over to the

right.
Notice two important things: first, the movement
of water is slow.

There’s not a lot of open space between
all that sand, so it takes time for water

to flow through it.
Second, the sand is confined.
Even if it wanted to move, there would be
nowhere for it to go.

If those two conditions go away, that’s
when sinkholes happen.

Most natural sinkholes happen in areas with
large deposits of carbonate rocks, like limestone.

Over long periods of time, groundwater flowing
through the subsurface can dissolve the rock,

creating voids and open tunnels.
In fact, this is how most caves are formed.
These tunnels and voids create a significant
change the character of groundwater flow.

First, they allow water to flow quickly just
like it would through a pipe, making it more

erosive.
Second, they create a space for soil to wash
away.

With those two conditions, any soil overlying
a dissolution feature runs the risk of eroding

away from the inside, eventually leading to
a sinkhole.

But not every sinkhole is formed through natural
processes.

In fact, many of the most famous sinkholes
in recent times have been human-created.

Just like a cave dissolved into the bedrock
can act like a pipe and allow groundwater

to carry away soil, an actual pipe can do
the same thing.

And actual pipes aren't limited to areas with
a specific geology.

If you could take a look into the subsurface
of any urban area, you'd see miles and miles

of water, sewer, and storm water drainage
pipes.

Unfortunately we can't see into the ground,
so I built this demonstration so we can see

for ourselves how this works.
All it takes is a little bit of settlement
or shifting to create an opening in one of

these pipes and allow internal erosion to
start.

I added a gap in my pipe to simulate this
effect.

Water moving through the pipe is able to dislodge
the adjacent soil and carry it away.

Notice that there's no signal on the surface
that anything is awry.

As more soil is washed away, the subsurface
void grows.

Depending on all those soil properties we
talked about earlier, this process can take

days to years before anyone notices.
Many of our subsurface utilities are placed
directly below roadways, and the paving often

acts as a final bridge above the sinkhole,
hiding the void below.

It's only a matter of time before anything
above is swallowed up.

Sinkholes aren’t the only problem caused
by internal erosion.

A specific type of internal erosion called
piping is the most common cause of failure

for earthen levees and dams, including Teton
Dam in Idaho which killed 11 people and caused

billions of dollars of damage when it failed
in 1976.

Maybe I’ll build a piping demonstration
someday for a separate video.

Internal erosion can be a natural process,
but sometimes sinkholes can form due to bad

decisions, bad construction, or just bad luck
with human-made infrastructure as well.

It’s just one of the complex failure modes
that civil engineers must consider when designing

a structure that might interact with water,
the great eroder.

Thank you for watching, and let me know what
you think!

Big thanks to Blue Apron for sponsoring this
video.

We’re in the process of moving and just
starting to get unpacked in the new house.

The last thing on my mind is going out to
buy groceries.

Blue Apron delivers all the fresh ingredients
you need, right to your doorstep, in exactly

the right proportions to create delicious
recipes at home.

It’s essentially just the fun parts of cooking
with none of the chore, which is exactly what

we needed this week.
If that sounds like something you’d be interested,
you can find out for free.

The first hundred people that click the link
in the description will get 3 blue apron meals

with no commitment whatsoever.
If you like it, you can sign up for a subscription,
and if you don’t, hey, you just got three

free meals shipped right to your doorstep.
Again, thank you for watching, and let me
know what you think!

コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

How Do Sinkholes Form?

338 タグ追加 保存
Amy.Lin 2017 年 9 月 19 日 に公開
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