Why Buildings Need Foundations - YouTube

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. In  today’s episode, we’re talking about foundations.

This video is sponsored by CuriosityStream  and Nebula. More on them later.

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 video 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 as you can see.  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 video on this phenomenon, so you  have two videos to watch 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 video,  but I hope this one gives you some appreciation  

of those innocuous bits of structural and  geotechnical engineering below our feet.

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