The Sinking of San Francisco’s Millennium Tower

Emily Ma, Journalist/Editor

Opened in 2009, San Francisco’s Millennium Tower, the tallest residential building in the city, has since sunk 18 inches into the soil and tilted a total of 26 inches northwest, and is still slumping. Nicknamed “The Leaning Tower of San Francisco”, the issue continues to stump engineers as they attempt to fix the building before it’s too late.
So what’s causing this?
Project engineers predicted beforehand that the building would end up settling a total of 4 to 6 inches (10-15 cm) because of the bad foundation and support offered by the yielding soil. The Millennium Tower, like many other high-rise buildings in San Francisco, has a concrete foundation with friction piles extending down into the dirt to stabilize the building, and distribute the pressure evenly through the ground. Friction piles are long cylindrical stabilizers made of concrete. They are commonly used to support large buildings such as stadiums, bridges, and skyscrapers.
The dirt found beneath these piles, colloquially called “Old Bay Clay”, seems to be a major source of trouble. When soil is put under immense pressure for long periods of time, it will compress and squeeze out the water in a process called consolidation. The reaction of the soil is similar to a sponge: as water is pressed out, the sponge scrunches up. The dewatering of land near the building also had a negative effect on the tower, as it accelerated the consolidation process of the “Old Bay Clay”.
While the building began to sink and tilt into the soil before its construction was finished, the issue was first revealed to the public in 2016. By this time the building had sunk 16 inches (40 cm) into the ground. This tilt, while dramatic, was able to slip by unnoticed due to its relative slightness when compared to the 645-foot-tall building. The slight slant and decreased height went unseen by all but the most observant residents. A much more noticeable sign, however, came in the form of cracks in the ground surrounding the tower. Though this had no effect on the building’s safety and ability to function, it caused residents of the tower significant unease and worry. Several lawsuits were filed claiming that the faulty construction was devaluing their property. After years of legal battling, it was concluded that owners would be compensated for their losses, and the building would be fixed.
However, attempts to fix the situation, including a $100 million project conducted last year, have all proved unsuccessful. New inspections have shown that if the tower continues to tilt at its current rate of 3 inches (7.5 cm) per year, the building could be rendered unable to operate properly in the next few years. Key components including elevators and the plumbing system will also be malfunctioning.
The clock continues to tick as engineers rush to find a way to save San Francisco’s Millennium Tower…’s%20Millennium%20Tower%2C%20one,between%2017%20and%2018%20inches