Do ADU / Garage Conversions Need A Geotechnical Soil Report?
San Francisco’s fourth-tallest skyscraper has a big, expensive problem: it’s sinking. Not only that, but it’s sinking twice as fast as originally feared. How does something like this happen? What can be done?
The Curious Case of The Sinking Tower
Usually, you might think that a massive, 58-story skyscraper in one of America’s wealthiest cities seems pretty immovable, right? After all, it’s a giant, monolithic building… can it really move?
Actually, buildings can and do move — but usually by design in order to withstand high winds and earthquakes. The Millennium Tower in San Francisco is experiencing a different kind of movement: it’s sinking. Constructed in 2008, the Millennium Tower is the 4th largest building in all of San Francisco, and it’s a residential haven boasting multi-million dollar condos with breathtaking views of the Bay. But for all that money its inhabitants spent, they’re getting a little more than they bargained for, as the skyscraper has sunk over 18 inches into the ground and begun tilting laterally toward the Salesforce Tower under construction across the street.
Although city officials have determined that the building is still safe to occupy for now, you can imagine residents’ distress at being in a building that is sinking and leaning so rapidly after construction. Owners of condos in the Millennium Tower are fleeing like rats escaping a sinking ship — many selling their units at a loss to the tune of $300,000 or more. Many residents are also filing huge lawsuits alone or as part of the homeowners’ association, contesting that the building is uninhabitable and unsafe; particularly in the event of a catastrophic earthquake.
Can The Millenium Tower Be Saved?
From a geotechnical engineering standpoint, there are several quirks about the Millennium Tower that need to be considered when we think about ways to fix its multiple problems. For starters, the tower was built on a foundation of packed sand about 80 feet below street level. In order to build the tower on bedrock, construction crews would have needed to anchor the building a jaw-dropping 200 feet below street level — which obviously would have tremendously increased the cost and timeframe of the project. It’s a tradeoff that’s not particularly unheard of in San Francisco, as many other (non-leaning or sinking) towers were also built on this packed sand foundation.
However, construction projects adjacent to the Millennium Tower might have made its sandy situation a bit worse, as excessive groundwater pumping and a massive underground transit tunnel continue to destabilize the foundation of that piece of land. Since the tower is built on sand prone to liquefaction, engineers estimate the tower will continue sinking at a rate of 2 inches per year.
A fix is planned, however; a $150 million project seeks to add some pillars straight down through the basement floor into the bedrock an additional 120 feet below. While this isn’t quite as stabilizing as if the tower had been anchored to bedrock to begin with, engineers are hopeful that this will stop the sinking or at least slow it to a negligible pace.
The case of the Millennium Tower in San Francisco is just another eye-opening example of how important soil engineering is to the construction of any project, whether it’s a simple single-family unit or a massive 58-story skyscraper. The earth doesn’t care how expensive your project is: it will do what it wants.
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