[Note that this article is a transcript of the video embedded above.]

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. 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. 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. Ask me how I know. 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.