
Satellites Confirm Sinking of San Francisco's Millennium Tower - pmar
http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-1/Satellites_confirm_sinking_of_San_Francisco_tower
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physcab
I'm no civil engineer but I talked to one about this story and the concerns
seem to be overblown. First, the foundation as I am aware is created by
drilling to the point of refusal. Obviously bedrock is ideal, but in a place
like SF where some of the land is based off old landfill, refusal is
acceptable. From there a concrete foundation is laid and the building rests
upon it. Concrete slabs are secure and in the event of an earthquake it will
be allowed to shift and "float" if the ground underneath liquifies. Secondly,
the sinking and leaning can be corrected. Engineers account for some level of
sinkage as the building settles, and if it tilts, they can apply ties to the
sides to straighten it. In the most severe of cases they can shore up the
foundation and build more supports. I'm sure the builders are examining their
options currently so it'll be interesting to see what course of action they
take.

~~~
tomarr
I'm in a similar line of work and don't fully agree with this. Yes everything
he's said is true, but to an extent, and having these rates of settlement per
year is very troubling for a structure designed to live 60-100+ years. If the
issue is not groundwater related (which seems unlikely with surrounding
structures largely OK) then expensive remedial work (underpinning / grouting /
similar) is going to be required in the immediate future before this causes
serious issues.

Admittedly I don't work in an seismically active region but I design for max
settlements of ~50mm over a building's _lifetime_ , and the Millennium Tower
is experiencing that every year.

~~~
guelo
> designed to live 60-100+ years

Really? That's what they design them for? That seems extremely short-sighted.
What is supposed to happen in 60 years? Why were medieval structures able to
last hundreds o f years but we can't do it now? Are new York's hundred year
old sky scrapers due for being torn down now? I don't understand why anybody
would think that's a good idea besides the fact that it'll be your
grandchildren's problem and you won't have to deal with it.

~~~
roel_v
Nobody lives in those 100's years old buildings; those that live in buildings
say 200 years old do so at great cost, because they love the history.
Functions change, tech changes, living patterns change, and retrofitting is
much more expensive than rebuilding after some point. Especially for
commercial buildings and downtown areas, it makes little sense to use
buildings beyond 100 years, save for some of historical significance.

~~~
stevekemp
As a European this is just .. shocking. I was born in a city that has been
settled for almost 2000 years, which has the oldest parts surrounded by walls
that were constructed between the 12th and 14th century.

Even when I moved away from there to a new city the flat I bought was in a
building almost 200 years old.

I know that America is "new", and that modern buildings are, by definition
modern, but the idea that you don't plan for the long-term, and that buildings
aren't old is very surprising.

~~~
roel_v
I'm not sure what you're saying, but I'm very likely to live in a city older
than yours. Buildings hundreds of years old are castles, churches or a limited
number of inner city monuments. They are very hard to live in, let alone live
comfortably in; they are very expensive to retrofit and in general nice to
look at but a pain to live or work in.

Buildings 200 to 100 years old are more common, but still suffer from many
defects; unless renovated in a way that is essentially 'build a new house
inside the old one'. Which is more expensive than knocking the old one down.

Anything less than 100 years but older than say 20 can be made to 'modern'
standards, for some values of 'modern'. It won't be nearly as comfortable as a
new house though, again unless renovated the expensive way.

Housing needs change. If you think we Europeans build for 100+ year use, you
obviously know nothing about real estate or construction, and the history
thereof. If you think the fact that we have a bunch of old buildings is
because they were designed that way, you are very mistaken.

~~~
stevekemp
I'm trying to say that while uses change that building is "effort" and if
you're going to do it you should do it with the assumption that it will last.

Re-purposing old buildings is very common in the UK, and whilst sometimes that
might mean essentially ripping it all out and rebuilding form the inside-out
that's an extreme case. When it comes to "public buildings" and "houses" that
people live in mostly the changes aren't so drastic.

I'm sure that people putting up buildings in the 1600s didn't imagine they
would still be in use, but the fact that they are is a good thing. What I'm
really trying to say is that planning to build something with the assumption
it'll be dead/useless/retired in a hundred years seems wasteful and short-
sighted. Europe is used as an example of how things have turned out otherwise.
Though I appreciate there aren't any skyscrapers/tower-blocks of that age in
the world. Unless you think of tenement buildings from the 17/18/1900s.

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packetized
Interesting that the lead photo[0] also shows Salesforce East (formerly
Salesforce Plaza, 350 Mission) also sinking at a similar rate. I'm wondering
what all of this will end up meaning for Salesforce Tower (the large hole in
the ground to the left of the Millennium Tower), since it's close to topping
out and will be one of the two tallest buildings west of the Mississippi
River.

[0]:
[http://www.esa.int/spaceinimages/Images/2016/11/Millennium_T...](http://www.esa.int/spaceinimages/Images/2016/11/Millennium_Tower_sinking)

~~~
packetized
Also of note: you can actually see the buckling of the sidewalk and curbing
immediately adjacent to the Millennium Tower. I'll see if I can get a photo to
share today. There's certainly quite a bit of heavy construction equipment
traveling on Fremont/Mission, but I can't see how it would cause buckling
sidewalks.

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laurencei
"The Sentinel-1 satellites have shown that the Millennium Tower skyscraper in
the centre of San Francisco is sinking by a few centimetres a year"

I'm no engineer - but to me "a few cm a year" seems very significant, and
would be easily detectable by current surveying technology? You would probably
even visually be able to see it at the entrance to the building after 1-2
years?

Why was a satellitie needed here to confirm it?

~~~
Dylan16807
A cheap GPS device can give you millimeter-level measurements every few hours,
so using an active satellite does seem like overkill.

~~~
surveyor
I don't know what your exact definitions is of "cheap", "millimeter-level",
and "every few hours", but your comment does sound a bit misinformed.

What you are possibly referring to is called dimensional control in geomatics
and civil engineering, and there are various measurement techniques that can
be applied, including GNSS-based.

In a way, the hardest part of it is the planning phase, where you have to
decide which technique is most appropriate to meet precision, accuracy,
timeliness, and cost constraints. All those factors kind of fight against each
other. Finding the sweet spot is non-trivial.

~~~
Dylan16807
Okay, tell me where I'm wrong: AFAIK you can buy a few hundred to few thousand
dollar receiver that tracks the carrier phase, let it sit around for a few
hours, and then with aggressive post-processing you should be able to figure
out the right phase and know your location within a few mm.

Edit: And worst-case, you should be able to do an exact survey once, and then
you can remember that and know which phase to use in the future. (Barring
sudden multi-cm shifts, but in that case you have bigger problems.) You still
have atmospheric fluctuations to deal with, but that's why you're measuring
all day.

~~~
7952
Vertical accuracy is usually lower than horiztonal accuracy. It is more
difficult to calculate elevation than lat/long.

~~~
Dylan16807
Not by a huge amount. Should be less than 2x, at least at some point during
the day.

~~~
jofer
Much worse that 2x. Closer to an order of magnitude worse.

~~~
mturmon
Indeed. Just to anchor this with current state-of-the-art numbers, I took a
look at the CSV data file linked to by the page referenced in my above
comment:

[https://news.ycombinator.com/item?id=13040765](https://news.ycombinator.com/item?id=13040765)

The quoted horizontal standard errors of the daily position measurements are
~1.5mm. The vertical standard errors are ~5-7mm. (Not as bad as a factor of
10, but certainly worse than 2x.)

The vertical position measurement responds well to averaging of daily errors
to beat down the RF propagation effects that cause them.

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deegles
Somewhat unrelated, but for me this is another argument in favor of renting
vs. buying. Sure, most people won't buy a super expensive condo in a super
expensive city that happens to be sinking into the ground, but if you taken
that $1.5m (they go for much more[0]) and put it into index funds instead, you
could pay ~$5k/mo in rent forever and just leave when problems arise.

[http://sf.curbed.com/2016/8/5/12390756/on-sale-millennium-
to...](http://sf.curbed.com/2016/8/5/12390756/on-sale-millennium-tower)

~~~
bjacokes
Someone buying a $1.5m condo only has to put up $300k for the mortgage, so
it's not correct to say that the alternative is to just put $1.5m into the
stock market: the buyer often doesn't have that much.

Also, it's more precise to use the S&P dividend yield (2%) or 10-year Treasury
yield (2.4%) to gauge how much someone with $1.5m could spend in rent,
assuming they put all their interest into rent. They'd be renting something at
more like $3k/month.

Now take the fact that home prices are, at the higher end, about 25x the
yearly rent of the same place. So this person with $1.5m is essentially
renting a place worth $900k, whereas the buyer (who perhaps only has $500k) is
living in a nicer place that cost $1.5m. And that's not considering the fact
that rents could rise further.

There are taxes and maintenance to consider, the risk of putting a lot of
money into a volatile asset, and being tied to one place. Financial leverage
in general is risky. But it's not a clear-cut decision like most people try to
make it.

~~~
Retric
That Treasury is vastly safer than a condo, which fluctuate significantly more
than a standalone structure and often has dramatic increase in fees over time.
Further renters can move to a new building every few years for minimal costs.

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cobbzilla
it boggles my mind that the developers were not required to sink the
foundation piles all the way into bedrock. how unusual is this for tall
buildings in the city by the bay? there is so much wealth in SF, this is a
luxury tower, it's all a bit strange.

~~~
scarmig
There are bedrock piles, and there are friction piles. The latter rely on
friction of the material they're plunged into to stay put. They are effective
and commonly used.

Despite how comforting the word bedrock is to our ears, neither one is
"superior" to the other.

Obviously something is going wrong here, but it's not a case of "oh the
developers cheaped out and decided to just build a giant skyscraper with no
foundation!"

~~~
ChoGGi
I'd say it's less about piles going to bedrock and more about the ground
itself.

Building on sand, trash, and whatever other gunk comprises San Francisco
ground...

~~~
anigbrowl
You could say whatever you like, but none of the other tall buildings in SF
are sinking noticeably, which suggests your theory may itself lack a solid
foundation.

~~~
ChoGGi
Are the pilings for those other tall buildings in the bedrock?

~~~
anigbrowl
Depends. Some are but but in recent years it's been more common to use steel
structures which (being lighter) don't require that.

Noise might have something to do with too. The last building that I recall
having bedrock-depth pilings involved loud shuddering bangs through the
financial district for nearly a month. Like you could feel it through the
ground 3 blocks away every time they dropped the hammer..

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woliveirajr
I find it amazing how _civil_ satellite can be kept in orbit, be it
geostationary or not, and monitor "millions of points" spread to some great km
and be able to detect (or provide image with enough resolution) so that a move
of few cm can be detected.

~~~
dogma1138
Most countries launch EOS's that are either civilian grade or considerably
lesser than that.

Only the US has really the ability to fund the crazy bus sized EOS platforms.

~~~
adrianN
Himawari is a Japanese weather satellite that is 8m long and weighs 3.5 tons.

~~~
dogma1138
Block III and newer K-11 platforms are 20 meters in length and weight 20,000
KG.

Also the dry mass of Himawari is 1300 kg, it's based on the MELCO DS-2000 bus
which is meant for communication satellites, it doesn't have an 8M telescope
assembly.

~~~
adrianN
Okay, that's really big. Thanks for the info!

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joeblau
I would be infuriated if I purchased a condo there. It seems like every month
since last year, there are articles talking about the project. I haven't heard
any solutions to the problem and I don't know anything about construction. Is
this the type of problem that can be fixed, or will they have to destroy the
building and rebuild it?

~~~
jaclaz
The main issue here is understanding what is the actual cause of the sinking:

1) extensive dewatering of the soil under the building and around it

2) some mistake in the calculations of the project and/or some foundation work
made in difformity from the project

3) a combination of the two

IF it is only #1 (this is what the builders say) then stopping the water
drainage or even (in certain case) re-watering appropriately the soil would
stop the sinking (at a relatively low cost).

IF it is #2 or #3 it is certainly possible to "better" the soil underneath or
"add" some (underground) supporting structures (at a much, much higher cost).

And of course someone will have to pay the bill.

Techically, IF the cause is #1, it is called "subsidence":

[https://en.wikipedia.org/wiki/Subsidence](https://en.wikipedia.org/wiki/Subsidence)

more specifically:

[https://en.wikipedia.org/wiki/Groundwater-
related_subsidence](https://en.wikipedia.org/wiki/Groundwater-
related_subsidence)

Edit: P.S.: Seemingly the SF Millennium got just cited here also:
[https://en.wikipedia.org/wiki/List_of_leaning_towers](https://en.wikipedia.org/wiki/List_of_leaning_towers)

(maybe exaggerating a little bit, look for Santos, Brazil, strangely not
listed in the wikipedia page)

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sulam
It's not discussed in the article but I'm very curious about the brightest
spot in the SF Bay Area in the survey image. It's not Millenium Tower (which I
work next to so that's pretty interesting too). It's actually the land just
west of Oakland airport, the part of Alameda built up in the 80s called Bay
Farm. As I understand it the area was marsh up until then -- that and a
garbage dump that has since been covered.

Does anyone know if the land there really is sinking so fast? It's not on a
fault so I'm fairly sure it's not moving much laterally.

~~~
dspig
Maybe a garbage dump that has been covered would be expected to sink?

~~~
sulam
Sure, but it's the entire development.

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mannykannot
This is from memory, and I have not been able to confirm it with a cursory
search, but FWIW: In most of central London, the bedrock is far below (which
allows for deep Tube tunnels), so high-rises do not rest on bedrock, and are
at least partially supported by the buoyancy of large rigid basements.

Of course, London does not have the seismic risk of San Francisco. I would
guess that buoyancy might be an asset with regard to support, but possibly a
liability with regard to stability, in the face of liquefaction.

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debt
could there possibly be an enormous sinkhole beneath or nearby the pilons?

