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 aboutsinkholes.
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
风化 滑坡 磨蚀和冲刷
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 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 verycreative name for just such a process:
If just the right factors come together in the subsurface,
Some interesting things can happen, including sinkholes.
But let’s look at a non-erosive exampleof groundwater movement first.
This is 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 movementof water is slow.
There’s not a lot of open space between all that sand
so it takes time for water to flow through.
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’swhen sinkholes happen.
Most natural sinkholes occur 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 in the character of groundwater flow.
First, they allow water to flow quickly just
like it would through a pipe, making it more erosion.
Secondly, they create a space for soil to washaway.
With those two conditions,
any soil overlying a dissolution feature runs the risk of eroding away from the inside,
eventually leading toa sinkhole.
But not every sinkhole is formed through naturalprocesses.
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 witha specific geology.
If you could take a look into the subsurface of any open area
you can 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 thiseffect.
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 subsurfacevoid grows.
Depending on all those soil properties we talked about earlier
this process can take days to years before any one 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 causedby 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 failedin 1976.
Maybe I’ll build a piping demonstrationsomeday 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 many 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 whatyou think!
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Again, thank you for watching, and let me know what you think!