The essence of a bridge is not
just that it goes over something, but that there’s clear space underneath
for a river, railway, or road. Maybe this is already obvious to you, but bridges present a
unique structural challenge. In a regular road, the forces are transferred directly into
the ground. On a bridge, all those forces on the span get concentrated into the piers
or abutments on either side. Because of that, bridge substructures are among the strongest
engineered systems on the planet. And yet, bridge foundations are built in some of the least ideal
places for heavy loading. Rivers and oceans have soft, mucky soils that can’t hold much weight.
Plus, obviously, a lot of them are underwater. What happens when you overload soil with a weight
it can’t handle? In engineering-speak, it’s called a bearing failure, but it’s as simple as stepping
in the mud. The foundation just sinks into the ground. But, what if you just keep loading
it and causing it to sink deeper and deeper? Congratulations! You just invented one of the
most widely used structural members on earth: the humble foundation pile. How do they work, and how
can you install them underwater? I’m Grady, and this is Practical Engineering. Today we’re having
piles of fun talking about deep foundations. I did a video all about the different
types of foundations used in engineering, but I didn’t go too deep into piles. A
pile is a fairly simple structural member, just a long pole driven or drilled into the
ground. But, behind that simplicity is a lot of terrifically complex engineering. Volume 1
of the Federal Highway Administration’s manual on the Design and Construction of Driven Pile
Foundations is over 500 pages long. There are 11 pages of symbols, 2 pages of acronyms, and
you don’t even get to the introduction until page 46. And just a little further than that,
you get some history of driven piles. Namely that the history has been lost to time. Humans
have been hammering sticks into the ground since way before we knew how to write about it.
And that’s pretty much all a driven pile is. The first piles were made from timber, and wood
is still used all these years around the world. Timber piles are cheap, resilient to driving
forces, and easy to install. But, wood rots, it has an upper limit on length from the size of
the tree, and it’s not that strong compared to the alternatives. Concrete piles solve a lot
of those problems. They come in a variety of sizes and shapes, and again, are widely used for
deep foundations. One disadvantage of concrete piles is that they have to be pretty big to
withstand the force required to drive them into ground. Some concrete piles can be upwards
of 30 inches or 75 centimeters wide. It is hard to hit something that big hard enough to drive
it downward into soil, and a lot of ground has to either get out of the way or compress in place
to make room. Steel piles solve that problem since they can be a lot more slender. Pipe piles are
just what they sound like, and the other major alternative is an H-pile. Your guess is as good as
mine why the same steel shape is an I-beam but an H-pile. But, no matter the material, all driven
piles are installed in basically the same way. Newton’s third law applies to piles like
everything else. To push one deep into the ground creates an equal and opposite reaction.
You would need either an enormous weight to take advantage of gravity or some other strong
structure attached to the ground to react against and develop the pushing force required to
drive it downward. Instead of those two options, we usually just use a hammer. By dropping
a comparatively small weight from a height, we convert the potential energy of the weight
at that height into kinetic energy. The force required to stop the hammer as it falls gets
transferred into the pile. Hopefully this is intuitive. It’s pretty hard to push a nail
into wood, but it’s pretty easy to hammer it in ... well, it’s a little bit easier to hammer it
in. "Perfect!" There are quite a few types of pile drivers, but most of them use a large hammer or vibratory head to create the forces required. Maybe it goes without saying, but the main goal
of a foundation is to not move. When you apply a load, you want it to stay put. Luckily, piles
have two ways to do that (at least for vertical loads). The first is end-bearing. The end, or
toe, of a pile can be driven down to a layer of strong soil or hard rock, making it able
to withstand greater loads. But there’s not always a firm stratum at a reasonable depth
below the ground. Quote-unquote “bedrock” is a simple idea, but in practice, geology
is more complicated than that. Luckily, piles have a second type of resistance: skin
friction, also known as shaft resistance. When you drive a pile, it compacts
and densifies the surrounding soil, not only adding strength to the soil itself, but
creating friction along the walls of the pile that hold it in place. The deeper you go, the more
friction you get. Let me show you what I mean. I have my own pipe pile in the backyard that
I’ve marked with an arbitrary scale. When I drop the hammer at a prescribed height, the pile
is driven a certain distance into the ground. Do this enough times, and eventually, you reach a
point where the pile kind of stops moving with each successive hammer blow. In technical terms,
the pile has reached refusal. I can graph the blow count required to drive the pile to each
depth, and you get a pretty nice curve. It’s easy to see how it got stronger against vertical
loads the deeper I drove it in. Toward the end, it barely moved with each hit. This is a really
nice aspect of driven piles, you install them in a similar way to how they’ll be loaded by the final
design. Of course, bridges and buildings don’t hammer on their foundations, but they do impose
vertical loads. The tagline of the Pile Driving Contractors Association is “A Driven Pile is a
Tested Pile” because, just by installing them, you’ve verified that they can withstand
a certain amount of force. After all, you had to overcome that force to get
them in the ground. And if you’re not seeing enough resistance, in most cases, you
can just keep driving downward until you do! But piles don’t just resist downward forces.
Structures experience loads in other directions too. Buildings have horizontal, or lateral, loads
from wind. Bridges see lateral loads from flowing water, and even ice or boats contacting the piers.
Both can experience uplift forces that counteract gravity from floods due to buoyancy or strong
winds. If you’ve ever hammered in a tent stake, you know that piles can withstand loading from
all kinds of directions. And then there’s scour. The soil along a bridge might look like this right
after the bridge is built, but after a few floods, it can look completely different. Engineers
have to try and predict how the soil around a bridge will scour over time, from natural
changes in the streambed and those created by the bridge itself. Then they make sure to design
foundations that can accommodate those changes and stay strong over the long term. This is why
bridge foundations sometimes look kind of funny. Loads transfer from the superstructure down
into the piers. The piers sit on a pile cap that transfers and distributes loads into the
piles themselves. Those piles can be vertical, but if the engineer is expecting serious lateral
loads, some of the piles are often inclined, also called battered piles. Inclined
piles take better advantage of the shaft resistance to make the foundation
stronger against horizontal loads. As important and beneficial as they are, driven
piles have some limitations too. For one, they’re noisy and disruptive to install. Just
last year, I had two friends on separate trips to Seattle who sent me a video of the exact same
pile-driving operation. It’s good to have friends who know how much you like construction. But my
point is, this type of construction is pretty much impossible to ignore. In dense urban areas,
most people are just not willing to put up with the constant banging. Plus the vibrations
from installing them can disrupt surrounding infrastructure. Pile driving is crude; in many
cases, the piles aren’t designed to withstand the forces of the structure they’ll support but
rather the forces they’ll have to experience during installation which are much higher. They
can’t easily go through hard geological layers, cobbles, or boulders; they can wander off path,
since you can’t really see where you’re going, and they can cause the ground to heave because
you’re not removing any soil while you force them into the subsurface. The second major
category of piles solves a lot of these problems. And, wouldn’t you know it? There’s an FHWA manual
that has all the juicy details - Drilled Shafts: Construction Procedures and Design Methods.
This one a whopping 747 pages long. A drilled shaft is also exactly what it sounds like.
The basic process is pretty simple. Drill a long hole into the ground. Place reinforcing
steel in the hole. Then fill the whole thing with concrete. But, bridge piers
are often, as you probably know, installed underwater. Pouring concrete
underwater is a little tricky. Imagine trying to pour a smoothie at the bottom
of a pool! Let me show you what I mean. This is my garage-special bridge foundation
simulator. It has transparent soil in the form of superabsorbent polymer beads… and you know we
have to add some blue water too. You can probably imagine how easy it might be to drill a hole
in this soil. It’s just going to collapse in on itself. We need a way to keep the hole open so the
rebar and concrete can be installed. "Oh, it's making a huge mess." So, drilled shafts installed in soft soils or wet conditions
usually rely on a casing to support the walls. Installing a casing usually happens while the hole
is drilled, following the auger downward. I tried that myself, but I only have two hands, and it
was pretty unwieldy. So, just for the sake of the demo, I’m advancing the casing into the soil ahead
of time. Now I can drill out the soil to open the shaft. And now I’m realizing the limitations of
my soil simulant. It was still pretty hard to do, even with the casing in place. It took a few
tries, but I managed to get most of it out. So now I have an open hole, but it’s still full
of water. Even if your casing runs above the water surface, and you try to pump it out, you
can still have water leaking in from the bottom. In ideal conditions, you can get a nice seal
between the bottom of the casing and the soil, but even then, it’s pretty hard to keep water
out of the hole, and luckily it doesn’t matter. Instead of concrete, I’m using bentonite clay
as a substitute. It’s got a similar density, and it’s perfect for this demo because you can
push it through a small tube… if you get the proportions right. This is me pondering the life decisions that led up to me holding a gigantic syringe full of
bentonite slurry in my garage. You can’t just drop this stuff through the water. It mixes
and dilutes, just turning into a mess. Same is true for concrete. The ratio of water to
cement in a concrete mix is essential to its strength and performance, so you can’t do
anything that would add water to the mix. The trick is a little device called a tremie.
Even though it has a funny name, it’s nothing more than a pipe that runs to the bottom of the
hole. As long as you keep the end of the tremie below the surface of the concrete that you’re
pumping in, or concrete simulant in my case, there’s no chance for it to mix with the water
and dilute. I’m just pushing the clay into the casing with a big syringe, making sure to
keep the end of the tube buried. Because concrete is a lot more dense than water, it
just displaces it upward, out of the hole. In underwater installations, the casing is
often left in place. One advantage is that you can build a floating pile cap. Instead
of building a big cofferdam and drying out the work area to construct a big concrete
structure, sometimes you can raise the pile cap into or above the water surface, reducing
the complexity of its construction. These “high rise” pile caps are used a lot in offshore wind
turbines. But, not all casings are permanent. In some situations, it’s possible to pull
the casing once the hole is full of concrete, saving the sometimes enormous cost of each
gigantic steel tube. I tried to show this in my demo. It’s not beautiful, but it
did work. Again, the concrete is dense, so the pressure it exerts on the walls of the
hole is enough to keep the soil from collapsing. And because drilled shafts can be much larger
than driven piles, sometimes you don’t even need a group of them. Lots of structures,
including wind turbines, highway signs, and more, are built on mono-pile foundations.
Just a single drilled shaft deep in the ground, eliminating the need for a pile cap altogether.
Another interesting aspect of drilled shafts is that you can ream out the bottom, creating an
enlarged base that increases the surface area at the toe. This helps reduce a pile’s tendency to
sink, and it can help with uplift resistance too. Driven piles and drilled shafts are far from the
only types of deep foundation systems. There are tons of variations on the idea that have been
developed over the years to solve specific challenges: Continuous flight auger piles do the
drilling and concreting in essentially one step, using a hollow-stem auger to fill the hole as it’s
removed. Then reinforcement is lowered into the wet concrete. You can fill a hole with compacted
aggregate instead of concrete, called a stone column or tradename Geopier if you’re only worried
about compressive loads. Helical or screw piles twist into the ground, instead of being hammered,
reducing vibrations and disturbance. Micropiles are like tiny drilled shafts used when there
are access restrictions or geologic constraints. And of course, there are sheet piles that aren’t
really used for foundations but are driven piles meant to create a wall or barrier. Let me know if
I forgot to mention your favorite flavor of pile. Even though they’re usually much stronger
than shallow foundations, piles can and do fail. We’ve talked about San Francisco’s
famous Millennium Tower in a previous video. That’s a skyscraper on a pile foundation that
sank into the ground, causing the building to tilt. It seems like they mostly have it fixed
now, but it’s still in the news every so often, so only time will tell. In 2004, a bridge pier
on the Lee Roy Selmon Expressway in Tampa, Florida sank 11 feet (more than 3 meters) while
it was still under construction because of the complicated geology. It cost 90 million dollars
to fix and delayed the project’s completion by a year. These case studies highlight the
complexity of geotechnical engineering when we ask the ground to hold up heavier and
heavier loads. The science and technology that goes into designing deep foundations are
enough to spend an entire career studying, but hopefully, this video gives you
a little insight into how they work. It’s a little hard to see a bridge’s foundation
than its other parts, but if you look closely, you can often get hints about how
they’re secured to the ground. In fact, one of my main goals with these videos
is to connect ideas in engineering to things you can see for yourself out in the
world… like, for example, on a road trip. Bearded Grady here. It’s easy to tell that this
was shot after the main video, and there’s a good reason for that! Today’s sponsor Nebula adds new
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