Hacker News Reader

Comment by sharpercoder | Hacker News Reader

I wonder what effect a medium to heavy earthquake would have on this building's foundation.

Probably not good. According to this and other articles, the building's concrete slab foundation sits roughly 80 ft below street level upon sand, while bedrock lies 200 ft down. That means there is 120 ft of sand, possibly susceptible to liquefaction, between the buildings foundation and solid bedrock. That sounds bad enough. But now imagine the building is tipping over slightly. The already potentially worrisome problem compounds dramatically.

The SF Chronicle article that this Curbed article seems to be sourced from provides more information on this.

  And while Stanford’s Deierlein doesn’t consider the sink or tilt a safety issue,
  he did say, “I would be concerned for my investment.” That’s because a shifting
  building can cause walls to crack, elevators to malfunction and all manner of
  other annoyances.

  P.J. Johnston, spokesman for tower builder Millennium Partners and its principal
  owner, Sean Jeffries, said a nine-month, independent structural safety review in
  2014 “determined the settlement has not significantly affected the seismic
  performance of the building, and does not represent a safety risk.”

The first guy is "Professor Greg Deierlein, director of the John A. Blume Earthquake Engineering Center at Stanford University".

I think this link is paywall-free: http://www.sfgate.com/bayarea/article/SF-s-landmark-tower-fo...

How much would it have cost to dig down to bedrock, and pour a concrete foundation from the bedrock up to surface level?

Between the amount of sand to be moved and the amount of concrete to be poured, it sounds like an enormous fortune; I guess that the relevant questions are whether the market could bear it and whether it would be cheaper than the rapidly-developing lawsuits. It may be that the building didn't make economic sense in the first place, if it couldn't be built both safely and profitably -- which has unpleasant implications for the SF housing crunch...

Update: I hadn't known that the usual practice was to build a foundation down to normal depth for a foundation, then set up (large numbers of) pilings from the foundation down to bedrock. That makes sense.

The real problem with digging down to bedrock is destablizing the building. Though there may be enough redundancy in the pilings to do it one at a time.

I think the question was why the builders didn't do it in the first place.

You dig down 200 feet to the bedrock, pour a foundation, and then pour pilings up to the surface to put the building on.

Reply on news.ycombinator.com