[Note that this article is a transcript of the video embedded above.]
When we bought our house several years ago, we fell in love with every part of it except one: the foundation. At 75 years old, we knew these old piers were just about finished holding this old house up. This year we finally bit the bullet to have them replaced. Any homeowner who’s had foundation work done can commiserate with us on the cost and disruption of a project like this. But homes aren’t the only structures with foundations. It is both a gravitational necessity and a source of job stability to structural and geotechnical engineers that all construction – great and small – sits upon the ground. And the ways in which we accomplish such a seemingly unexceptional feat are full of fascinating and unexpected details. I’m Grady and this is Practical Engineering. Today, we’re talking about foundations.
There’s really just one rule for structural and geotechnical engineers designing foundations: when you put something on the ground, it should not move. That seems like a pretty straightforward directive. You can put a lot of stuff on the ground and have it stay there. For example, several years ago I optimistically stacked these pavers behind my shed with the false hope that I would use them in a landscaping project someday, but their most likely future is to sit here in this shady purgatory for all of eternity. Unfortunately, buildings and other structures are a little different. Mainly, they are large enough that one part could move relative to the other parts, a phenomenon we call differential movement. When you move one piece of anyTHING relative to the rest of it, you introduce stress. And if that stress is greater than the inherent strength of the thing, that thing will pull itself apart. It happens all the time, all around the world, including right here in my own house. When one of these piers settles or heaves more than the others, all the stuff it supports tries to move too. But doorframes, drywall, and ceramic tile work much better and last much longer when the surrounding structure stays put.
There are many kinds of foundations used for the various structures in our built environment, but before we dive into how they work, I think it will be helpful to first talk about what they’re up against, or actually down against. Of course, buildings are heavy, and one of the most important jobs of a foundation is to evenly distribute that weight into the subsurface as downward pressure. Soil isn’t infinitely strong against vertical loads. It can fail just like any other component of a structural system. When the forces are high enough to shear through soil particles, we call it a bearing failure. The soil directly below the load is forced downward, pushing the rest of the soil to either side, eventually bulging up around the edges.
Even if the subsurface doesn’t full-on shear, it can still settle. This happens when the particles are compressed more closely together, and it usually takes place over a longer period of time. (I have a post all about settlement that you can check out after this.) So, job number 1 of a foundation is to distribute the downward force of a structure over a large enough area to reduce the bearing pressure and avoid shear failures or excessive settlement.
Structural loads don’t just come from gravity. Wind can exert tremendous and rapidly-fluctuating pressure on a large structure pushing it horizontally and even creating uplift like the wing of an airplane. Earthquakes also create loads on structures, shifting and shaking them with very little warning. Just like the normal weight of a structure, these loads must also be resisted by a foundation to prevent it from lifting or sliding along the ground. That’s job number 2.
Speaking of the ground, it’s not the most hospitable place for many building materials. It has bugs, like termites, that can eat away at wooden members over time, reducing their strength. It also has moisture that can lead to mold and rot. My house was built in the 1940s on top of cedar piers. This is a wood species that is naturally resistant to bugs and fungi, but not completely immune to them. So, job number 3 of a foundation is to resist the effects of long-term degradation and decay that come from our tiny biological neighbors.
Another problem with the ground is that soil isn’t really as static as we think. Freezing isn’t usually a problem for me in central Texas, but many places in the world see temperatures that rise and fall below the freezing point of water tens or hundreds of times per year. We all know water expands when it freezes, and it can do so with prodigious force. When this happens to subsurface water below a structure, it can behave like a jack to lift it up. Over time, these cycles of freeze and thaw can slowly shift or raise parts of a structure more than others, creating issues. Similarly, some kinds of soil expand when exposed to moisture. I also have a post on this phenomenon, so you have two to read after this one. Expansive clay soil can create the same type of damage as cycles of freeze and thaw by subtly moving a structure in small amounts with each cycle of wet and dry. So job number 4 of a foundation is to reach a deep enough layer that can’t freeze or that doesn’t experience major fluctuations in moisture content to avoid these problems that come with water in the subgrade below a structure.
Job number 5 isn’t necessarily applicable to most buildings, but there are many types of structures (like bridges and retaining walls) that are regularly subject to flowing water. Over time (or sometimes over the course of a single flood), that water can create erosion, undermining the structure. Many foundations are specifically designed to combat erosion, either with hard armoring or by simply being installed so deep into the earth that they can’t be undermined by quickly flowing water.
Job number 6 really applies to all of engineering: foundations have to be cost effective. Could the contractor who built my house in the 1940s have driven twice as many piers, each one to three times the depth? Of course it can be done, but (with some minor maintenance and repairs), this one lasted 75 years before needing to be replaced. With the median length of homeownership somewhere between 5 and 15 years, few people would be willing to pay more for a house with 500 years of remaining life in the foundation than they would for one with 30. I could have paid this contractor to build me a foundation that will last hundreds of years… but I didn’t. Engineering is a job of balancing constraints, and many of the decisions in foundation engineering come down to the question of “How can we achieve all of the first 5 jobs I mentioned without overdoing it and wasting a bunch of money in the process?” Let’s look at a few ways.
Foundations are generally divided into two classes: deep and shallow. Most buildings with only a few stories, including nearly all homes, are built on shallow foundations. That means they transfer the structure’s weight to the surface of the earth (or just below it). Maybe the most basic of these is how my house was originally built. They cut down cedar trees, hammered those logs into the ground as piles, layed wooden beams across the top of those piers, and then built the rest of the house atop the beams. Pier and beam foundations are pretty common, at least in my neck of the woods, and they have an added benefit of creating a crawlspace below the structure in which utilities like plumbing, drains, and electric lines can be installed and maintained. However, all these individual, unconnected points of contact with the earth leave quite a bit of room for differential movement.
Another basic type of shallow foundation is the strip footing, which generally consists of a ribbon or strip of concrete upon which walls can sit. In some cases the floor is isolated from the walls and sits directly on concrete slab atop the subgrade, but strip footings can also support floor joists, making room for a crawlspace below. For sites with strong soils, this is a great option because it’s simple and cheap, but if the subgrade soils are poor, strip footings can still allow differential movement because all the walls aren’t rigidly connected together. In that case, it makes sense to use a raft foundation – a completely solid concrete slab that extends across the entire structure. Raft foundations are typically concrete slabs placed directly on the ground (usually with some thickened areas to provide extra rigidity). They distribute the loads across a larger area, reducing the pressure on the subgrade, and they can accommodate some movement of the ground without transferring the movement into a structure, essentially riding the waves of the earth like a raft on the ocean (hence the name). However, they don’t have a crawlspace which makes plumbing repairs much more challenging.
One issue with all shallow foundations is that you still need to install them below the frost line – that is the maximum depth to which water in the soil might freeze during the harshest part of the winter – in order to avoid frost heaving. In some parts of the contiguous United States, the frost line can be upwards of 8 feet or nearly two-and-a-half meters. If you’re going to dig that deep to install a foundation anyway, you might as well just add an extra floor to your structure below the ground. That’s usually called a basement, and it can be considered a building’s foundation (although the walls are usually constructed on a raft or strip footings as described above).
As a structure’s size increases, so do the loads it imposes on the ground, and eventually it becomes infeasible to rely only on soils near the surface of the earth. Tall buildings, elevated roadways, bridges, and coastal structures often rely on deep foundations for support. This is especially true when the soils at the surface are not as firm as the layers farther below the ground. Deep foundations almost always rely on piles, which are vertical structural elements that are driven or drilled into the earth, often down to a stronger layer of soil or bedrock, and there are way more types than I could ever cover in a single video. Piles not only transfer loads at the bottom (called end bearing), but they can also be supported along their length through a phenomenon called skin friction. This makes it possible for a foundation to resist much more significant loads – whether downward, upward or horizontal – within a given footprint of a structure.
One of the benefits of driven piles is that you install them in somewhat the same way that they’ll be loaded in their final configuration. There’s some efficiency there because you can just stop pushing the pile into the ground once it’s able to resist the design loads. There’s a problem with this though. Let me show you what I mean. This hydraulic press has more than enough power to push this steel rod into the ground. And at first, it does just that. But eventually, it reaches a point where the weight of the press is less than the bearing capacity of the pile, and it just lifts itself up. Easy… (you might think). Just add more weight. But consider that these piles might be designed to support the weight of an entire structure. It’s not feasible to bring in or build some massive weight just to react against to drive a pile into the ground. Instead, we usually use hammers, which can deliver significantly more force to drive a pile with only a relatively small weight.
The problem with hammered piles is that the dynamic loading they undergo during installation is different from the static loading they see once in service. In other words, buildings don’t usually hammer on their foundations. For example, if a pile can withstand the force of a 5-ton weight dropped from 16 feet or 5 meters without moving, what’s the equivalent static load it can withstand? That turns out to be a pretty complicated question, and even though there are published equivalencies between static and dynamic loads, their accuracy can vary widely depending on soil conditions. That’s especially true for long piles where the pressure wave generated by a hammer might not even travel fast enough to load the entire member at the same moment in time. Static tests are more reliable, but also much more expensive because you either have to bring in a ton (or thousands of tons) of weight to put on top, or you have to build additional piles with a beam across them to give the test rig something to react against.
One interesting solution to this problem is called statnamic testing of piles. In this method, a mass is accelerated upward using explosives, creating an equal and opposite force on the pile to be tested. It’s kind of like a reverse hammer, except unlike a hammer where the force on the pile lasts only for a few milliseconds, the duration of loading in a statnamic test is often upwards of 100 or 200 milliseconds. That makes it much more similar to a static force on the pile without having to bring in tons and tons of weight or build expensive reaction piers just to conduct a test.
I’m only scratching the surface (or subsurface) of a topic that fills hundreds of engineering textbooks and the careers of thousands of contractors and engineers. If all the earth was solid rock, life would be a lot simpler, but maybe a lot less interesting too. If there are topics in foundations that you’d like to learn more about, add a comment or send me an email, and I’ll try to address it in a future post , but I hope this one gives you some appreciation of those innocuous bits of structural and geotechnical engineering below our feet.
Watch Video At: Practical Engineering.
Source: The Paradise News