San Francisco has a problem.
One of the city’s glittering new skyscrapers is leaning over.
“San Francisco’s sinking and leaning Millennium Tower”
“There’s now a revised plan to stop the sinking and leaning Millennium Tower”
The 197-metre Millennium Tower has, at the time of this video publishing, sunk more than
45 centimetres, causing it to lean 60 centimetres to the west and 20 centimetres to north.
Despite efforts to correct the issue, it’s continuing to tilt about 7 centimetres north
and 2 centimeters west every year.
If nothing is done, the tower could reach a lean of more than a metre.
At which point, according to the building's own chief engineer, the elevators and plumbing
will no longer be able to function.
The race is now on to right the skyscraper.
Engineers are installing new piles underneath the foundations of the 311,000-tonne building
in an attempt to stop it from forever becoming the leaning tower of San Francisco.
But there’s a problem there too …
Millennium Tower was always expected to sink, just a little.
Most skyscrapers are expected to settle once construction has finished, sinking a few centimetres
under the enormous weight of the glass and steel above them.
Normally the sinking is so miniscule and takes so long that nobody notices.
Engineers estimated Millennium Tower would sink no more than 10 centimetres over the
course of its life.
When the Millennium Tower first opened back in 2009 everything seemed fine.
The building even won awards for its structural integrity, including the American Concrete
Institution Award for Construction and the Structural Engineering Project of the Year.
Demand for its luxury condos was so great that owners were reselling their units at
two to three times the price they originally paid for them, with the penthouse selling
for $9.8M.
“It’s a wonderful location.”
“Eight years ago, Jerry and Pat Dodson paid $2.1 million for a two-bedroom and planned
to live out their retirement, enjoying the sweeping view from the 42nd floor.”
But it soon became clear that something was wrong.
In 2016 it was revealed that the tower was sinking much faster and further than expected.
Worse, the sinking was uneven, giving the building a pronounced tilt to the northwest.
“So Montana is now suing the developers of San Francisco’s sinking Millennium Tower.”
Cue a media storm as developers, engineers, homeowners, and the city of San Francisco
all rushed to blame — and sue — each other, especially when it became clear that the cost
to fix the lean could exceed $100M.
So what went wrong?
Ron Hamburger, the engineer leading the Millennium Tower retrofit, blames construction work on
a nearby site.
While working on the Salesforce Transit Center, crews dug an 18-metre hole and pumped millions
of litres of water out of the ground.
Hamburger claims that clearing the water from the soil created an effect called “soil
compaction” where the soil started compressing at a far faster rate than predicted.
But well before the Transit Center got going, during the construction of Millennium Tower’s
basement, contractors had to pump out massive amounts of water just to keep the area dry.
No matter who or what is to blame — the lawyers will have fun figuring that out — the
tower’s titling has had consequences
“Growing cracks in the sidewalk, and most shocking, what the couple discovered when
they visited the lowest part of the garage.”
“Water appeared to be seeping through the concrete walls and huge chunks of walls seemed
to have crumbled away, even though the building was only seven years old.”
In 2017, the building was declared still safe to inhabit, despite cracked windows, walls
and sidewalks.
But something had to be done.
Hamburger came up with a plan to correct the tilted skyscraper. 52 additional piles would
be put underneath the building, placed through the sidewalk and under the building’s main
foundation.
These piles would extend beyond the original foundation, reaching 76 metres down to the
layer of bedrock below.
This is a process called “underpinning”, and it’s fairly common.
These new piles were intended to even out 50 percent of the building’s tilt over the
following 10 years, leveling the rest of it over the next half a century.
But in 2021, stabilization efforts were halted.
It was discovered that the installation of these new piles might actually be making the
skyscraper’s tilt even worse.
The tower sunk an additional three centimetres, contributing to a further 12 centimetres of
lean at the top.
The new piles were designed to stop the sinking when they were driven into the bedrock and
attached to the skyscraper, an operation scheduled for later in 2022.
However, there are fears the holes that had been dug for the new piles were causing even
more soil movement.
It was later revealed that construction teams had also left a one to four day gap between
removing the soil and putting in grout meant to reduce the risk of soil collapse.
This delay went against regulations and could have caused further damage.
“The building does continue to settle at a rate of about one half-inch per year, and
to tilt at a rate of about three inches per year.”
“The building remains safe, but although the building remains safe, we believe the
project needs to resume construction and complete this construction quickly to stop further
settlement and titling, and to allow some recovery of the tilt, as was originally planned.”
“I believe that the building remains safe, and in fact can tilt more without becoming
a safety issue or a serviceability issue.”
At a hearing in January 2022 Hamburger pitched a new plan to save the building.
“Engineer Ron Hamburger, who’s in charge of the fix, is only calling for a total of
18 piles.”
“Humburger adds seeking only 18 piles won’t provide as much uniform support as 52 piles
would have, so they can expect less tilt recovery.”
Instead of installing 52 piles they would only install 18.
These 18 piles would go further than 76 metres and because there would be fewer of them there
would be less disturbance to the soil underneath the building.
The new plan is currently under review and will be decided on by March 2022, it will
likely go ahead but further balloon the cost of the fix from US$100M to an estimated US$500M.
With millions upon millions of dollars at stake in lawsuits, this story seems very dramatic,
but is the Millennium Tower’s tilt as bad as it sounds?
Many tall buildings all across the world tilt.
The Elizabeth Tower in London — which holds Big Ben — tilts 0.26 degrees.
At its worst, the Leaning Tower of Pisa tilted five degrees, although it was later “stabilized”
to four.
If Millennium Tower was leaning that badly it would be tilting a very visible 13 metres
at its top.
As of right now, Millennium Tower’s tilt is unnoticeable to the naked eye.
Its current lean is less than 0.2 degrees.
The building’s engineers have continued to insist the skyscraper is safe to live in
and not in any immediate danger.
Hamburger has claimed that the Millennium Tower would need to lean more than 180cm for
it to be unable to withstand an earthquake, a far cry from its current 60cm tilt.
That said, the building would be rendered uninhabitable if the plumbing and elevators
stop working, and that could occur when the lean gets to 100cm.
So for the engineers it's a race against time to stabilise the site, one with millions of
dollars at stake.
But this is more than a case study in what can go wrong — it's also a reminder of just
how robust the engineering of modern megaprojects really is.
Massive buildings with low fault tolerances are built on compactable soil in places all
around the world, and virtually none of them suffer from the problems we've seen here in
San Francisco.
And the fact that Hamburger and his team of engineers can even try to un-tilt a 197m skyscraper
is a testament to just how far the construction industry has come.
Let's just hope that for the Millennium Tower, the third time’s the charm.
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