![]() ![]() The media had a field day reporting on the leaning tower of San Francisco, and accusations started flying about who was to blame and whether the City had covered up details about the building’s movement. Instead, the northwest corner had sunk a little lower than the rest of the foundation, causing the tower to tilt several inches in that direction. Unfortunately, that settlement wasn’t happening evenly. The result was that, by 2016 (when the public first learned about the issue), the building had already sunk more than 16 inches or 41 centimeters, triple the movement that was anticipated for its entire lifetime. All these factors added up to a lot more settlement than was initially anticipated by the project’s geotechnical engineers. Then other construction projects nearby began, including the adjacent Transbay Transit Center, which required their own deep excavations and groundwater drawdowns. This deliberate and continuous dewatering of foundation soils accelerated the settlement. This is often done using vertical wells similar to the ones used for drinking water but usually not as deep. However, construction of the tower’s basement also required the contractor to pump water out of the subsurface to keep the site dry. The Millenium Tower was already designed to put more stress on the underlying Old Bay Clay than any other building in the area. But some things can accelerate the process, even if they’re not intentional. Settlement is usually a gradual process because it takes time for the water to find a path out from the soil matrix. An increased load will slowly squeeze the water out, allowing the grains to compress into the empty space. Essentially, the soil is like a sponge filled with water. ![]() The ground below your feet may seem firm and stable, but when subjected to increased loading - and especially when the load is extreme like that of a concrete skyscraper - soil can compress in a process called consolidation. But even before construction was complete, the building had already settled more than that. 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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