Why Bridges Don't Sink
Engineers keep bridges from sinking by transferring loads into deep foundations such as driven piles, which are effectively “tested” as they’re installed into complex layers of soil and rock rather than simple, uniform bedrock. Commenters expand on this with real-world examples—from floating bridges in Seattle and Norway to viaducts built on talus, wooden piles under cities like Venice and Amsterdam, and dramatic drilling mishaps—while also touching on the long-term financing and maintenance of major bridge projects. The thread highlights both the ingenuity and the risks in modern foundation engineering, and points to further resources like documentaries and technical videos for those interested.
Humor, Analogies, and Real-World Swamps
- Commenters connect the “just keep loading it” idea to the Monty Python swamp-castle gag.
- A real analogue is the Lucin Cutoff causeway across the Great Salt Lake, where railroads kept dumping rock into soft sediments, first building trestles and eventually a solid rock causeway that has been repeatedly raised and strengthened.
Floating Bridges and Failures
- Several floating bridges are cited: Nordhordland (Norway) and three on Lake Washington (Seattle), including the world’s longest.
- A former bridge engineer notes one of Seattle’s floating bridges is now on the lake bottom and shares an anecdote about retrofitting.
- There’s light debate on “unsinkable” claims; Seattle’s record of sinking bridges is mentioned with irony, along with the famous Tacoma Narrows footage.
Foundations, Piles, and Geotechnical Complexity
- Deep gravel and talus layers show that “bedrock” can be extremely deep or impractical to reach; some viaducts are founded entirely in loose rock deposits.
- Discussion of pile types: end-bearing vs friction (side resistance). Strong layers can be sand, boulder fields, or other dense strata, not just bedrock.
- Driving piles inherently tests capacity; resistance observed during driving informs design, but some cases (very deep soft soils, caverns, pressurized strata) can be problematic.
- Various geologic hazards are referenced: salt mines and lakes catastrophically connecting, mud volcanoes/diapirs, and anhydrite layers that can swell when hydrated.
- Wood piles can last a long time in fully saturated ground; decay is worst near the air–soil interface. Examples include historically pile-built cities and modern fence-post protection.
Infrastructure Funding and Tolls
- Debate over stopping tolls once a bridge is “paid off” vs. continuing tolls to fund future replacement and maintenance.
- Concerns raised about political inability to preserve dedicated funds and about fairness between current vs future users.
- Some argue for intergenerational investment in infrastructure; others see long-term pre-funding as a step too far.
Structural Shapes and Naming
- Clarification that “I-beams” and “H-piles” are different profiles: piles have thicker webs and flanges for axial load, while wide-flange (W) and S sections are optimized for bending.
- Terminology (I vs H) and pronunciation are playfully debated.
- Explanation of beam anatomy (flanges and web) is shared, along with a graphic reference.
Other Engineering Curiosities and Resources
- Mention of submerged and intentionally submersible bridges, plus a misdesigned floating bridge that sank.
- Interest in a specific animation style used in the video, though the software remains unclear.
- A popular documentary and book on the Brooklyn Bridge are recommended.
- A Japanese company’s “silent” pile-driving tech and novel applications (e.g., lava barriers, underground bike storage) are highlighted.
- The Practical Engineering site’s RSS feed is discovered, and some readers praise the channel’s depth and clarity.