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