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Even though it’s a favorite vacation destination,
the beach is surprisingly dangerous. Consider the
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lifeguard: There aren’t that many recreational
activities in our lives that have explicit staff
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whose only job is to keep an eye on us, make
sure we stay safe, and rescue us if we get into
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trouble. There are just a lot of hazards on the
beach. Heavy waves, rip currents, heat stress,
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sunburn, jellyfish stings, sharks, and even
algae can threaten the safety of beachgoers.
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But there’s a whole other hazard, this one usually
self-inflicted, that usually doesn’t make the list
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of warnings, even though it takes, on average, 2-3
lives per year just in the United States. If you
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know me, you know I would never discourage that
act of playing with soil and sand. It’s basically
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what I was put on this earth to do. But I do have
one exception. Because just about every year,
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the news reports that someone was buried when
a hole they dug collapsed on top of them.
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There’s no central database of sandhole collapse
incidents, but from the numbers we do have,
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about twice as many people die this
way than from shark attacks in the US.
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It might seem like common sense not to
dig a big, unsupported hole at the beach
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and then go inside it, but sand has
some really interesting geotechnical
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properties that can provide a false sense of
security. So, let’s use some engineering and
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garage demonstrations to explain why. I’m
Grady and this is Practical Engineering.
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In some ways, geotechnical engineering
might as well be called slope engineering,
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because it’s a huge part of what they do.
So many aspects of our built environment
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rely on the stability of sloped earth. Many
dams are built from soil or rock fill using
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embankments. Roads, highways, and bridges rely
on embankments to ascend or descend smoothly.
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Excavations for foundations, tunnels, and
other structures have to be stable for the
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people working inside. Mines carefully monitor
slopes to make sure their workers are safe. Even
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protecting against natural hazards like landslides
requires a strong understanding of geotechnical
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engineering. Because of all that, the science
of slope stability is really deeply understood.
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There’s a well-developed professional consensus
around the science of soil, how it behaves,
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and how to design around its limitations as a
construction material. And I think a peek into
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that world will really help us understand
this hazard of digging holes on the beach.
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Like many parts of engineering, analyzing
the stability of a slope has two basic parts:
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the strengths and the loads. The job of a
geotechnical engineer is to compare the two.
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The load, in this case, is kind of obvious:
it’s just the weight of the soil itself. We
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can complicate that a bit by adding loads at the
top of a slope, called surcharges, and no doubt
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surcharge loads have contributed to at least
a few of these dangerous collapses from people
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standing at the edge of a hole. But for now, let’s
keep it simple with just the soil’s own weight.
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On a flat surface, soils are generally
stable. But when you introduce a slope,
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the weight of the soil above can create a
shear failure. These failures often happen
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along a circular arc, because an arc minimizes
the resisting forces in the soil while maximizing
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the driving forces. We can manually solve for
the shear forces at any point in a soil mass,
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but that would be a fairly tedious engineering
exercise, so most slope stability analyses use
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software. One of the simplest methods is
just to let the software draw hundreds of
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circular arcs that represent failure planes,
compute the stresses along each plane based
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on the weight of the soil, and then figure
out if the strength of the soil is enough
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to withstand the stress. But what does it
really mean for a soil to have strength?
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If you can imagine a sample of soil floating
in space, and you apply a shear stress,
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those particles are going to slide apart from
each other in the direction of the stress. The
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amount of force required to do it is usually
expressed as an angle, and I can show you
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why. You may have done this simple experiment in
high school physics where you drag a block along
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a flat surface and measure the force required
to overcome the friction. If you add weight,
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you increase the force between the surfaces,
called the normal force, which creates additional
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friction. The same is true with soils. The
harder you press the particles of soil together,
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the better they are at resisting a shear
force. In a simplified force diagram,
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we can draw a normal force and the resulting
friction, or shear strength, that results. And the
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angle that hypotenuse makes with the normal force
is what we call the friction angle. Under certain
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conditions, it’s equal to the angle of repose, the
steepest angle that a soil will naturally stand.
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If I let sand pour out of this funnel onto the
table, you can see, even as the pile gets higher,
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the angle of the slope of the sides never
really changes. And this illustrates the
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complexity of slope stability really nicely.
Gravity is what holds the particles together,
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creating friction, but it’s also what pulls
them apart. And the angle of repose is kind
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of a line between gravity’s stabilizing
and destabilizing effects on the soil.
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But things get more complicated
when you add water to the mix.
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Soil particles, like all things that take
up space, have buoyancy. Just like lifting a
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weight under water is easier, soil particles
seem to weigh less when they’re saturated,
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so they have less friction between them. I can
demonstrate this pretty easily by just moving
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my angle of repose setup to a water tank. It’s
a subtle difference, but the angle of repose
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has gone down underwater. It’s just because
the particle’s effective weight goes down,
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so the shear strength of the soil mass goes down
too. And this doesn’t just happen under lakes
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and oceans. Soil holds water - I’ve covered
a lot of topics on groundwater if you want to
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learn more. There’s this concept of the “water
table” below which, the soils are saturated,
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and they behave in the same way as my little
demonstration. The water between the particles,
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called “pore water” exerts pressure, pushing them
away from one another and reducing the friction
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between them. Shear strength usually goes down for
saturated soils. But, if you’ve played with sand,
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you might be thinking: “This doesn’t really track
with my intuitions.” When you build a sand castle,
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you know, the dry sand falls apart,
and the wet sand holds together.
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So let’s dive a little deeper. Friction
actually isn’t the only factor that
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contributes to shear strength in a soil.
For example, I can try to shear this clay,
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and there’s some resistance there, even
though there is no confining force pushing
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the particles together. In finer-grained
soils like clay, the particles themselves
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have molecular-level attractions that make
them, basically, sticky. The geotechnical
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engineers call this cohesion. And
it’s where sand gets a little sneaky.
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Water pressure in the pores between particles can
push them away from each other, but it can also
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do the opposite. In this demo, I have some dry
sand in a container with a riser pipe to show the
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water table connected to the side. And I’ve dyed
my water black to make it easier to see. When I
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pour the water into the riser, what do you think
is going to happen? Will the water table in the
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soil be higher, lower, or exactly the same as the
level in the riser? Let’s try it out.
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Pretty much right away, you can see what happens. The sand
essentially sucks the water out of the riser,
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lifting it higher than the level outside the
sand. If I let this settle out for a while,
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you can see that there’s a pretty big difference
in levels, and this is largely due to capillary
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action. Just like a paper towel, water wicks
up into the sand against the force of gravity.
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This capillary action actually creates
negative pressure within the soil (compared
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to the ambient air pressure). In other words,
it pulls the particles against each other,
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increasing the strength of the soil.
It basically gives the sand cohesion,
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additional shear strength that doesn’t
require any confining pressure. And again,
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if you’ve played with sand, you know there’s
a sweet spot when it comes to water.
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Too dry, and it won’t hold together.
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Too wet, same thing.
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But if there’s just enough water,
you get this strengthening effect.
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However, unlike clay that has real cohesion, that
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suction pressure can be temporary. And it’s
not the only factor that makes sand tricky.
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The shear strength of sand also depends on
how well-packed those particles are. Beach
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sand is usually well-consolidated because of the
constant crashing waves. Let’s zoom in on that
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a bit. If the particles are packed together,
they essentially lock together. You can see
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that to shear them apart doesn’t just look like
a sliding motion, but also a slight expansion
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in volume. Engineers call this dilatancy, and
you don’t need a microscope to see it. In fact,
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you’ve probably noticed this walking around on
the beach, especially when the water table is
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close to the surface. Even a small amount
of movement causes the sand to expand,
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and it’s easy to see like this because it
expands above the surface of the water. The
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practical result of this dilatant property
is that sand gets stronger as it moves,
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but only up to a point. Once the sand
expands enough that the particles are no
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longer interlocked together, there’s a lot less
friction between them. If you plot movement,
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called strain, against shear strength, you
get a peak and then a sudden loss of strength.
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Hopefully you’re starting to see how all
this material science adds up to a real
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problem. The shear strength of a soil,
basically its ability to avoid collapse,
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is not an inherent property:
It depends on a lot of factors;
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It can change pretty quickly; And this behavior
is not really intuitive. Most of us don’t have a
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ton of experience with excavations. That’s part
of the reason it’s so fun to go on the beach and
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dig a hole in the first place. We just don’t get
to excavate that much in our everyday lives. So,
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at least for a lot of us, it’s just a natural
instinct to do some recreational digging. You
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excavate a small hole. It’s fun. It’s interesting.
The wet sand is holding up around the edges,
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so you dig deeper. Some people give up after the
novelty wears off. Some get their friends or their
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kids involved to keep going. Eventually, the hole
gets big enough that you have to get inside it to
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keep digging. With the suction pressure from
the water and the shear strengthening through
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dilatancy, the walls have been holding the
entire time, so there’s no reason to assume
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that they won’t just keep holding. But inside
the surrounding sand, things are changing.
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Sand is permeable to water, meaning water moves
through it pretty freely. It doesn’t take a big
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change to upset that delicate balance of wetness
that gives sand its stability. The tide could
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be going out, lowering the water table and thus
drying the soil at the surface out. Alternatively,
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a wave or the tide could add water to the
surface sand, reducing the suction pressure.
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At the same time, tiny movements within the
slopes are strengthening the sand as it tries
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to dilate in volume. But each little movement
pushes toward that peak strength, after which
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it suddenly goes away. We call this a brittle
failure because there’s little deformation to
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warn you that there’s going to be a collapse.
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It happens suddenly, and if you happen to be
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inside a deep hole when it does, you might be just
fine, like our little friend here, but if a bigger
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section of the wall collapses, your chance of
surviving is slim. Soil is heavy. Sand has about
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two-and-a-half times the density of water. It just
doesn’t take that much of it to trap a person.
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This is not just something that happens to people
on vacations, by the way. Collapsing trenches and
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excavations are one of the most common causes
of fatal construction incidents. In fact,
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if you live in a country with workplace
health and safety laws, it’s pretty much
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guaranteed that within those laws are rules about
working in trenches and excavations. In the US,
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OSHA has a detailed set of guidelines on how to
stay safe when working at the bottom of a hole,
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including how steep slopes can be
depending on the types of soil,
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and the devices used to shore up an excavation to
keep it from collapsing while people are inside.
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And for certain circumstances where the risks
get high enough or the excavation doesn’t fit
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neatly into these simplified categories, they
require a professional engineer be involved.
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So does all this mean that anyone who’s not an
engineer just shouldn’t dig holes at the beach.
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If you know me, you know I would never agree with
that. I don’t want to come off too earnest here,
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but we learn through interaction. Soil and rock
mechanics are incredibly important to every part
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of the built environment, and I think everyone
should have a chance to play with sand, to get
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muddy and dirty, to engage and connect and commune
with the stuff on which everything gets built. So,
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by all means, dig holes at the beach. Just don’t
dig them so deep.
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The typical recommendation I see is to avoid going in a hole deeper than your knees.
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That’s pretty conservative. If you have kids with you,
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it’s really not much
at all. If you want to follow OSHA guidelines,
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you can go a little bigger: up to 20 feet (or
6 meters) in depth, as long as you slope the
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sides of your hole by one-and-a-half to one or
about 34 degrees above horizontal. You know,
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ultimately you have to decide what’s safe
for you and your family. My point is that
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this doesn’t have to be a hazard if you use
a little engineering prudence. And I hope
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understanding some of the sneaky behaviors
of beach sand can help you delight in the
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primitive joy of digging a big hole without
putting your life at risk in the process.
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I was impressed to learn that the training for
many lifeguards and emergency responders now
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includes ways to safely and quickly excavate a
victim from a collapsed sand hole. The general
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procedure is to form two rings of responders
around the collapse, moving sand outward from
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the center. There is a lot of complexity
in rescuing people from unusual situations,
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and actually, my friend Sam at Wendover
Productions produced a video all about The
00:14:28
Logistics of Search and Rescue. This is part
of the Logistics of X series that dives into
00:14:33
the little details of systems that you never
considered before. It’s a really fascinating
00:14:38
peek behind the curtain, and if you want to
check it out, it’s only available on Nebula.
00:14:42
You’ve heard me talk about Nebula before. It’s
a streaming service built by and for independent
00:14:47
creators, including a lot of my favorites like
Neo, Wendover Productions, the Coding Train,
00:14:52
and Branch Education. I don’t know about you,
but independently-produced content is most of
00:14:57
what I watch these days. I just like the
authenticity and thoughtfulness of videos
00:15:01
that haven’t been through ten levels of studio
executives watering the information down to
00:15:06
capture the widest audience possible. I just
think passionate individuals and small teams
00:15:11
make the most compelling work, and
Nebula is the perfect place for it.
00:15:16
Nebula’s totally ad-free, with tons of excellent
channels and lots of original series and specials
00:15:21
like the Logistics of X. It’s also a great gift,
especially because a yearly membership is 40%
00:15:27
of the link in the description. At thirty-six
bucks for a year, that’s pretty tough to beat.
00:15:32
My videos go live on Nebula before they come out
on YouTube. If you’re with me that independent
00:15:37
creators are the future of great video, I
hope you’ll consider subscribing. That’s
00:15:41
go.nebula.tv/Practical-Engineering. Thank you
for watching, and let me know what you think!
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