When the building was designed, the project geotechnical engineers predicted that it would settle 4 to 6 inches (10 to 15 centimeters) over the structure’s entire lifetime, mainly from this layer of Old Bay Clay below the bottom of the piles. Below the Colma Formation is a thick layer of Ice Age mud locally known as the Old Bay Clay. However, it’s not the dense sands causing problems for the Millennium Tower, but what’s underneath. This is a fairly common design in San Francisco, with more than a dozen tall buildings in the downtown area utilizing a similar foundation system, including some nearly as large as this one. The piles also allow the foundation to bear on stronger soils than those at the surface.ĭriving the piles so deep allowed the building to not sit on the surface layer of artificial fill, or even the soft underlying layer of mud but rather on the dense sandy soil of the Colma Formation below. Friction piles spread out the load of the building vertically, allowing much more of the underlying soils to be used to support the structure without becoming overwhelmed. The Millennium Tower’s foundation consists of a 10-foot or 3-meter-thick concrete slab supported by 950 concrete friction piles driven into the subsurface to a depth of about 80 feet or 24 meters. That tremendous pressure is why most tall buildings use deep foundation systems. It would be impossible for just the ground surface to bear that much weight, especially in this case where the ground surface is a weak layer of mud and rubble placed during the City’s infancy to reclaim land from the bay. With its concrete skeleton, the Millenium Tower was designed to impose a load of 11,000 pounds per square foot or 530 kilopascals to its foundation (about 100 times more than an average house). That’s about 5 kilopascals, the pressure at the bottom of a knee-deep pool of water. An average single-story residential home is designed to apply a pressure to the subsurface of maybe 100 pounds per square foot of building footprint. That might seem self-evident, but it can’t be overstated in a story like this. Today, we’re talking about the Millennium Tower in San Francisco. I’m Grady, and this is Practical Engineering. How do engineers predict how soils will behave under extreme loading conditions, and what do you do when a skyscraper’s foundation doesn’t perform the way it was designed? Let’s find out. The tower opened to residents in 2009, but even before construction was finished, engineers could tell that the building was slowly sinking into the ground and tilting to one side. The Millennium Tower is the tallest residential building in San Francisco, with 58 stories above the ground and 419 luxury condominium units.
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