The geotechnical conundrums of Millennium Tower's sinking and tilting problems are complicated even for engineers. So, maybe it's not hard to understand why, this many years later, we still haven't gotten a particularly great, layman's explanation for the whole thing.

Enter Practical Engineering, a YouTube channel with 2.4 million subscribers, which has now done an excellent 13-minute video explaining the history of Millennium Tower's troubles; the mud, clay, and bedrock layers that lie underneath this part of downtown San Francisco; and how, in some ways, it's not easy to cast blame on a single entity or engineer when it comes to why this residential tower can't stand up straight.

First off, our narrator Grady Hillhouse diagrams the starting conditions when Millennium Tower was first being planned and constructed during the mid-aughts. As we've heard before, many buildings in downtown that came before Millennium Tower had gotten away with drilling foundation piles into the layer of what's called Old Bay Clay, without their buildings settling too dramatically. But, at 58 stories, this was the tallest residential building to be built here, and with its concrete frame it is a rather heavy one. And between the high weight of the building and the potential impacts of de-watering the upper soil layers at the adjacent Transbay Transit Center site, developers saw extreme settlement occurring before Millennium Tower had even completed construction in 2009.

A diagram of how Millennium's foundation was designed, with a 10-foot thick concrete slab over piles. Via Practical Engineering
Diagram showing how deep Millennium's piles went, versus bedrock, via Practical Engineering

All this stuff about the building tilting over a foot to the northwest only came out in mid-2016, and by that point, the building had sunk about sixteen inches. As of 2021, when work finally began on the fix for the tower's foundation, the tower had sunk 18 inches, with an extra inch that occurred just in the short time between May and August of this year, when work was halted. Also, the tilt at the top of the tower accelerated in that time from 17 inches to 22 inches.


The fix involves concrete piles drilled down to Franciscan bedrock under the sidewalk along two sides of the building, which Hillhouse explains will ultimately act as hydraulic jacks to even out the tilt.

It's fascinating stuff, even for nerds far away from San Francisco who get into this kind of thing. And Hillhouse kind of points the finger at the media (ahem) for the flurry of negative press that has driven down the condo prices in the building, which remains perfectly safe from an engineering standpoint.

"The story of the Millennium Tower is a fascinating case study of geotechnical engineering," Hillhouse says. "Our ability to predict how soils will behave under new and extreme conditions isn't perfect, especially when those soils are far below the surface."

Related: Millennium Tower's Accelerated Sinking Halts $100 Million Effort to Stop It

Photo: Justin Sullivan/Getty Images