Why Bridges Don't Sink

An overview of the different types of pile foundations and how they work.

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.

Practical Engineering is a YouTube channel about infrastructure and the human-made world around us. It is hosted, written, and produced by Grady Hillhouse. We have new videos posted regularly, so please subscribe for updates. If you enjoyed the video, hit that ‘like’ button, give us a comment, or watch another of our videos!

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DISCLAIMER
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This is not engineering advice. Everything here is for informational and entertainment purposes only. Contact an engineer licensed to practice in your area if you need professional advice or services. All non-licensed clips used for fair use commentary, criticism, and educational purposes.

SPECIAL THANKS
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This video is sponsored by Nebula.
Stock video and imagery provided by Getty Images, Shutterstock, Pond5, and Videoblocks.
Tonic and Energy by Elexive is licensed under a Creative Commons Attribution License
Video by Grady Hillhouse
Edited by Wesley Crump
Produced by Ralph Crewe
Graphics by Nebula Studios

[danielgreene]

H Beams look exactly like I Beams, but H Beams have the same thickness in the flange and web, and are commonly used in vertical orientations where the loads are applied parallel to the beam length. The thicker web of an H Beam makes them more resistant to twisting. Whereas I-beams are typically horizontal, have thinner webs, because they are designed to only handle loads perpendicular to the length (top in compression, bottom in tension)

[ElectroBOOM]

I didn't know I wanted to know about piles until now!

[westlarper]

grady the only guy who can get me to watch a 17 minute video about bridge foundations

[dandandan]

I'm still a student in civil engineering, and your channel has helped me a lot through the years. It's the only channel I've found almost exclusively for civil engineering making feature videos and content that are high quality and made with the non-engineering audience in mind.

Honestly, I turn to your videos to understand a lot of concepts and principles on civil structures. I can't blame our professors for not delving deep on these aspects because of the limited lecture units/ hours, and it gets so busy that you have to prioritize learning the hard maths over the art of designing structures. It's a shame that my university has one of the biggest and oldest libraries in my country but I can't check all the books I want to consult. So your content are really incredibly helpful and very much a delight to watch.

I hope I'll find somewhere to buy or borrow your book soon. Keep being awesome!

[AppliedCryogenics]

This channel often gets me to appreciate details that I might have missed. It's comforting to know that something as rational as civil engineering is still chugging along as normal. Thanks for that.

[DyslexicMitochondria]

Broken bridges annoy me. I just can't get over them

[CarlosValeraLeon]

"imagine pouring a smoothie at the bottom of a pool. Let me show you what I mean"

I cannot understate my disappointment that there wasn't a smash cut to Grady pouring a smoothie at the bottom of a pool XD

[wingsman]

I’m a Construction Inspector Supervisor in PA and my current project is a 1100’ long, 4 span steel girder structure that spans a deep ravine and creek. We have a little bit of everything on this project, including Micropile, Drilled Caissons and Steel Piling. Each installation method has extensive testing involved such as Tension and Compression testing for the micropiles, CSL or Cross Sonic Logging testing for the Caissons and Dynamic Load Monitoring for the Driven Steel Pile. Pretty cool stuff!

[zachrichardson]

I'm a CAD drafter (6 years rebar detailing and now about a year and a half doing pre cast/stressed) and I've done a lot of shafts & piles during bridgework. Loved them because they're almost always easy, repetitive and made my end of year weight report look good to the bosses.

It's really cool to see different perspectives on stuff like this that I have a decent understanding of but can learn more about the "why" behind the engineering of certain aspects.

Plus the info graphics style animations combined with the at home demo stuff really helps explain things. Great video.

[laura-ann.]

Great video! I was a draftsman and engineering tech in the CalTrans Office of Structure Design in Sacramento for 15 years, 1991-2006, and spent thousands of hours working on bridge and retaining wall foundation plans and Logs of soil test borings. The only foundations I can think of that have to bear heavier loads than highway and rail bridges are dams, especially the "big guys": Hoover Dam, Glen Canyon, Grand Coulee, etc. And maybe skyscaper office towers built on soils where bedrock is too deep to be reached directly by the building foundation. When the new East Span of the San Francisco-Oakland Bay Bridge was being built a few years ago, I was out at the jobsite one day on a field trip with some of our design section Engineers, just sightseeing (our section didn't have any direct involvement with the project), and we got to watch piles being driven for one of the bents on the Skyway portion of the bridge. The "Skyway" was that part of the bridge that didn't have to be raised high enough for ships to pass under, so it was a cantilevered pre-fab box girder, running from the east abutment on the Oakland shore, to the east end of the new Cable-stayed span. The mud under this part of the new bridge was very deep: the depth from the water surface to bedrock was 350 feet at the center of the channel between the Oakland shore and Yerba Buena Island, so several of the Skyway bents had to be built on pile caps supported by 36" diameter cast-in-steel-shell pipe piles that were 300+ feet long. These consisted of sections of 36" pipe, Schedule 40 I think, that were somewhere between 20 feet to 40 feet long (I don't remember the exact number). The first section to be lowered from the transport barge by a crane had cap plates welded on the ends to keep water and mud out, and was long enough to reach from the mud line to several feet above high-tide water level. This was lowered to the mud and allowed to sink in several feet under it's own weight, then the crane operator would tilt the pile to a pre-determined angle. The "footprint" of the piles as they entered the mud was substantially wider than the pattern of the tops of the piles at the pile cap. I seem to remember there were 12 piles under each pile cap, and 2 pile caps per bent. As the pile reached equilibrium and stopped sinking deeper under it's own weight, it would be driven a bit deeper, then a new section of pipe field-welded on and the driving would continue, with additional sections of pipe added, until the calculated bearing capacity of that pile was obtained, or "refusal" was reached. A rebar cage would be lowered in, and the pile back-filled with concrete. One of the engineers mentioned to me that the cost of the foundations alone - the piles and pile caps - for the Skyway was nearly half the total cost, and maybe a bit more than half. The whole project - the cable-stayed span, the skyway, and the approaches and abutments - eventually totalled out to over $7 billion, I heard, so we're not talking penny-ante level civil engineering here. The Burj Khalifa didn't cost nearly as much as this bridge, I think. The entire original SFOBB, both the east and west spans, only cost $75 million in 1936, so I guess there's no escaping the boogeyman of Inflation even if you are The Federal Government paying the bills.

[mercoid]

Your overall production quality is excellent in nearly every aspect. Animations, voice level and quality, music.. not too loud, well chosen stock footage, editing….. etc. etc… Very impressive.

[fiskurtjorn]

3:34 I once saw a construction site where piles were driven into the ground. There was a peat layer between two layers of sand. It took the machine a good amount of effort to drive the first four meters or so. Then it sank by itself until about four meters left. Those last few meters took a lot of effort again. Watching this for a morning gave a lot of insight into how the peat was undulating on the bottom more than on the top.

[noeraldinkabam]

Amsterdam is build on wooden poles. A lot of older houses in the Netherlands are build like that. Due to groundwater depletion the foundations of those houses are now having problems because the poles that stayed wet and therefor didn’t rot now do (rot)

[tbix]

Thanks for sharing your thoughts, ideas and videos. My favorite memory of a pile installation was of a mall being built when I was in college. They started driving a pile. Every day for about a few weeks they would add another pile in the exact same place. Eventually they took the pile driver away and just left the pile sticking out of the ground. As I drove by every day I eventually noticed it was still going in on its own. After a few weeks it was totally gone. When the mall eventually opened up they stated the reason for the massive parking garage under the mall was for buoyancy since the ground was so unstable that they were unable to put in piles for the foundation. I often pondered after that if the mall ever goes out of business it will be interesting to see if the garage fills with water and sinks the mall.

[davidfalterman]

“skin friction” and “shaft resistance” did give me a giggle, I have to admit….

[mmseng]

"The purpose of a foundation is to _not_ move"

Grady spittin' the hard truths we all need to hear.

[pizzagroom]

I love the thought that Grady set up the hammer and nail demonstration, failed, and just ran with it

[mjt]

I've driven miles of piles over the past 25 years, timber piles, H-piles, prestressed concrete piles, pipe piles (steel and concrete)... Toss in drilled shafts and micropiles, and I've inspected most of the bridge foundations used today. We still use spread footings near my home in SW VA wherever we find competent rock. I know a lot of folks in the business hate pile driving, but other than deck construction, it's my favorite part of bridge inspection.

[TheTech]

Your vocal intonation and texture is comfortingly expressive. I hope you enjoy making these!

[RyanGribble]

Grady, i was a concrete pump operator for several years and worked on many bridges. On company used a basketball to plug their tremme pipe to get concrete under the water in the casings. As the casings filled, one guy would climb down to recover the ball. I did have one critique about the concrete underwater as we use admixtures of bentonite and silica fume to make concrete "water proof" so that it doesn't dissipate underwater.