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Comment by Animats | Hacker News Reader

From the article: "To cut costs, Millennium did not drill piles to bedrock." I'm amazed that a building of that height would be built on a slab with friction piles in the mudflat fill zone of the SF bay. How did they get a building permit for that? 16 inches of sinkage in 8 years is huge.

Building sinkage isn't all that unusual, but not for skyscrapers. There was a time when skyscrapers were only built on competent rock, like NYC's. Now they're being built on much more iffy soils. Singapore and some other coastal Asian cities are doing a lot of this.

SF used to be smaller. The outer half mile or so on the bay side is fill. There are buried shipwrecks all through that area.[1]

[1] https://medium.com/the-secret-history-of-america/from-starbo...

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How did it get approved? The developer's lobbyist and PR rep is former mayor Brown's spokesman and personal friend, and at the time the current mayor was Newsom, a product of Brown's machine. Newsom was a bit of a crook in the area of real estate, having some self-dealing while he was on the board of supervisors and making sure all of his friends' problems got cleared.

The same lobbyist also represents the Warriors in their current Mission Bay proposal.

Nobody in SFgov is qualified to review the design of a skyscraper's foundation. That is why everyone involved in the project (city building inspectors, investors, lenders, etc) relie on expert engineering firms. Any firm making a mistake of this magnitude - if it is an engineering error - will be out of business and there insurers writing big checks.

While I agree Brown/Newsome/SFGov is shady but I highly doubt they are responsible for this problem. How did they fool the lenders? How did they get multiple engineering firms to sign off? Why would an investor like Millineum Partners want to take such a big risk in an area they have no advantage?

Sure. I was only pointing out that there was no possibility of this project being blocked on any grounds.

Not drilling piles to bedrock is the key.

From the article too: "-the famed Petronas Twin Towers in Malaysia, for example, sank only a half inch since opening in 1998-"

I have worked and lived in SE Asia for 8 years, and I can tell you that there are always problems like this when they fail to go deep enough on the piles.

I have also been reading up on the shorter-than-expected life expectancy of iron rebar in concrete construction. They were supposed to be good for 100 - 1000 years. Now, there are studies saying remedial works will probably be needed in 20 to 75 years! I wouldn't buy into a building that sank so much so rapidly, is tilting too, and is in a seismic zone like SF. Steel would be better to fight the bending due to tilt. Concrete is good for compressive loading mainly, not bending or shear. I hope they used some sort of non-ferrous reinforcement like fibers or mesh.

On every job I have worked on, we always recommend a survey of the structure for up to 2 years each month. When settling seems to level off after initial construction for 4 months straight, it is passed to the owner to maintain a survey at their discretion.

I am used to seeing 2 inches per year on new construction, but I am amazed the Petronas Towers only have sunk a half inch.

Most of our big coastal cities are like that. Boston is about twice as big as it was in colonial times; all of the Back Bay is landfill. A lot of Cambridge is landfill. And yes, most of the taller buildings cluster around the more solid bedrock areas closer to downtown.

But the engineering principles needed here aren't unknown--cities like Las Vegas and New Orleans have modern buildings that are much taller than were thought possible in earlier times. If the developers didn't do it right because it was expensive, then it is on them.

To add on, Chicago and New Orleans are both built on swamps, and the outer edges of New York are filled in -- making land for cities isn't the issue with the tower.

The tallest buildings in Chicago have foundations drilled down to bedrock. The Sears Tower (now Willits Tower) is built on bedrock. The Trump Tower [1] has pilings down to bedrock. The John Hancock building has foundations down to bedrock, even though they had to go down almost 200 feet.[2]

How unusual is 83 floors in earthquake country without foundations down to bedrock?

[1] http://faculty.arch.tamu.edu/media/cms_page_media/4433/trump... [2] http://khan.princeton.edu/khanHancock.html

As an aside, the land in New Orleans is so flat that the zoo has a 28' hill donated by a benefactor, "so that the children of New Orleans can know what a hill is like" according to the plaque next to it. And when I visited, the kids were going nuts over this little hillock, running up and careening down it, giggling.

As a kid I always wondered why so many of the taller buildings clustered up near DTX and then down that little strip down Boylston. As I got a bit earlier and learned what Boston originally looked like, it all made sense.

Of course, all of this landfill area is going to be the first to go back into the sea when the water rises. I figure our house is going to be riverfront property some day.

My house is one above where they draw the line on the sea rise lines. If I can stick around for a couple of hundred years I'm golden

Part of the clustering is planned: https://en.wikipedia.org/wiki/High_Spine

Years ago I was working for the final company to own the project for Trump Tower Tampa. The reason it didn't get built? It was to be built right next to the river. After the initial site survey the geologist weren't comfortable with the original shoring design, thus they drilled more, and still weren't comfortable. Somewhere along the line the rights to the project were sold off and the incredibly small company I worked for ended up with the rights to the project, which was eventually scrapped. Had the housing boom continued for much longer, I'm afraid it likely would have been built.

It is important to note that one of the chief advantages of building a skyscraper in NYC is that bedrock is within 40 feet of the surface in the downtown district. In other areas of NYC, it is 160 feet to 200 feet below the surface.

Engineers have been building skyscrapers in the mud for a long time. https://www.wbez.org/shows/wbez-news/building-skyscrapers-on...

There's another building that was built on friction piles: http://www.aboutcivil.org/burj-khalifa-design-construction-s...

They write that the chance of ground liquefaction there is zero, since they're not in an earthquake zone. Does not apply to SF.

Am I reading that map right? Looks like the Millenium Tower is not in the fill zone on the shipwreck map? Hard to tell the shipwreck map looks a bit notional.

Probably right. Here's the USGS map of SF's geology.[1] It's mostly dune sand or fill for the flat parts near the bay.

[1] http://www.quake.ca.gov/gmaps/RGM/sfsj/sfsj.html

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