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Comment by mc32 | Hacker News Reader

The Millennium Tower in SF has been sinking[1] since it was erected. Tenants have sued and now there is a fix proposed.

It all happened because the builder didn't want to go 200' to hit bedrock and instead cheapened out.

[1]https://www.sfgate.com/news/editorspicks/article/SF-s-sinkin...

I watched them build a 22-story high rise on landfill (mud/sand) in the SOMA district of San Francisco—it was right next to my building. After demolition of the old structure (a 3-story apartment) and excavation, they did the following:

- Pump water out of the excavated ground. It was a slow drain, maybe 50 gallons per minute, but it ran for months.

- Drive dozens of 90 foot steel pylons into the site with an enormous pile driver.

- Pour the foundation over the pylons.

The Millenium Tower is more than double the height of that, at 46 stories, and it’s right by the water. But they used the same length of pylon. Mind-boggling.

Can you build a small replica of this with a bucket of mud, sand, etc…?

This is good point. If you are willing to go deep enough, can you build anywhere?

Technically, probably yes, but the math could get ridiculous. Essentially you want to go into solid ground, and if you're in the ocean, solid ground might be miles below sea level. And reclaimed land doesn't change that. Also, if the material higher up shifts, your supports need to be able to survive the ground shifting around it, which means you need not just depth, but girth.

It would be very unusual for a city's headlands to be literally miles above solid ground, especially for the carribean, which is extremely shallow (hence the hurricanes).

Take a look at the Florida keys, where you can find bedrock after a couple of meters. You can even go out a mile into the sea and hit bedrock after 6-8 meters:

https://pubs.usgs.gov/pp/2007/1751/professional-paper/figure...

I wouldn’t know whether it’s very unusual, but https://blogs.egu.eu/divisions/ts/2019/06/17/in-search-of-th...:

“Digging down in Amsterdam, it takes about 1000 meters until we find the first solid rock”.

There are more or less solid sand layers at much lower depths, though. That’s what buildings stand on.

It would surprise me if other cities in the west of the Netherlands were different.

Keep in mind that bedrock in this context is limestone and oolite.

This rock is not an ideal foundation material. http://www.mgs.md.gov/geology/geohazards/engineering_problem...

Harder rock is deep, around 1,300m. (4,000ft). https://ufdc.ufl.edu/UF00000245/00001/20x

> especially for the carribean, which is extremely shallow (hence the hurricanes)

The Caribbean is nowhere near shallow, take a look at a marine chart or just the seafloor topography on google maps. The Caribbean is thousands (often over 10K) of feet deep in most areas.

Also, this has nothing to do with hurricanes, which originate off the west coast of Africa.

Also, the Florida Keys are not in the Caribbean.

There are many points south of SF against the Bay where the solid ground is down far enough that they instead drive piles until they find enough resistance.

Oracle's HQ is still standing. But the (East) Palo Alto 4 Seasons Hotel, whoops.

Although it was reported that it had experienced more than expected settling, didn't the 4 Seasons open? Are there still problems?

I vaguely remember that they went back and drove more piles, somehow, until the building was stable that the exterior glass facade no longer shattered on a regular basis. However they did the work, it wasn't visible from the outside.

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