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Comment by swebs | Hacker News Reader

How would you build to withstand against soil liquefaction? It could be likely that Tokyo is built on more stable bedrock.

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My understanding (probably naive) is the footings driven into the ground are deep enough to go below this area and rest on stable rock.

That’s how it was explained in a documentary about mission bay (south of AT&T).

edit: For those interested, this documentary explains the approach https://www.youtube.com/watch?v=SyjijQS2dAk&t=1219s

Do you remember the name of this documentary? A quick search didn't turn up anything. I live in the area and would love to learn more about this.

I recall seeing some computer animation to this effect in a documentary (in the UK) about the Millennium tower tilting - searching on that brought up some confirmation in the form of a press release by the TJPA:

http://tjpa.org/uploads/2016/08/301-Mission-Press-Release-Au...

The 60-story Millennium Tower is made of concrete rather than steel, resulting in a very heavy building. This heavy structure rests on layers of soft, compressible soil. The foundation of the Tower, however, consists only of a concrete slab supported by short piles that fail to reach the bedrock below. That foundation is inadequate to prevent settlement of a building with the weight of the Tower. In contrast, the Salesforce Tower and 181 Fremont Tower, also adjacent to the Transit Center, are supported on piles drilled down to bedrock. Millennium Partners’ poor design decision is the cause of the tilt and excessive vertical settlement of the Millennium Tower.

Sure, at about the 20:00 is when they discuss Mission Bay specifically. The whole documentary is solid though if you have more time. I learned a lot and found it very fascinating.

https://www.youtube.com/watch?v=SyjijQS2dAk&t=1219s

https://www.sciencedirect.com/science/article/pii/S003808061...

> The 2011 Great East Japan Earthquake caused the severe liquefaction of reclaimed lands in the Tokyo Bay area, from Shinkiba in Tokyo through Urayasu, Ichikawa and Narashino Cities to Chiba City.

Those aren't the parts of Tokyo with skyscrapers.

Most of Tokyo comprises low-rise (1-10 story) buildings.

Which parts have the tallest buildings?

Either go deep enough to hit bedrock or design your building to float. They both work fine, it's just a question of cost.

Well, option #1 is to build somewhere else. Is every square foot of SF subject to liquifaction? Or is it just the cheapest piece of ground? Is it that the local government has zoning rules that simply prevent development and density in the places that would be safer? To be clear, I don't blame developers for finding the cheapest option- that's how capitalism works. It's up to governments to make the rules such that the cheapest options are also the best ones.

Second, is there anything that can be done such that the soil won't suffer from liquifaction or will to a lesser degree? Probably expensive to do, yes, but surely there are means.

And there's probably a million other ideas that might work that I don't even know of. It's silicon valley, that's where ideas come from, right? "Impossible" usual gets San Francisco types all excited.

There's a downvoted reply to my original comment saying that I haven't actually tried to build these- and they are correct. I'm not a civil engineer. But I am a firm believer that if you make strong regulations and enforce them, clever engineers always find a way, at a cost. But the hard part of that isn't the engineering, it's having the political willpower to enforce strong rules.

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