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기사입력 : 2022.05.20 18:38
Millennium Tower is a residential building located in 301 Mission Street, San Francisco, California, USA. Construction began in 2005 and completed in 2009. It has a total of 58 stories and a height of 197m. As of 2021, it turned out to be the 4th tallest and most expensive residential building in San Francisco. Nowadays, it is called by ‘Modern Tower of Pisa. Because this buildingis now tilting and sinking. This problem of the building triggered many other problems too. The unstable footing has occurred issues including cracks in the basement, decreasing house values and numerous legal battles. Residents complained of bubbling floors, cracked walls and large cracks in the window of 36th floor.
After constructors said the building was sinking and tilting in 2015, they first informed the problem to public in 2016. A 2016 survey revealed that the building sank about 41cm and tilted 15cm. The building is now tilted to the northwest. Furthermore, cracks have formed in the basement of the building and the pavement around the tower. In 2018, the settlement increased to 46cm, and the tilting was 36cm. In 2022, the top of the Millennium Tower is said to have tilted 66cm to the northwest.In fact, the problem of settlement is the expected result from 2005 when the groundbreaking was reached. At this time, it was expected that the building would sink by 10cm to 15cm. It is normal for skyscrapers to sink a little after completion. The design is made under this consideration, but the settlement of Millennium Tower was much more serious than expected. After the completion of the building in 2009, the foundation of the building had fallen about 25cm. When an unexpected settlement occurred, the constructor ‘Millennium Partners’ claimed that it was because a large transportation complex terminal right next the building started to excavate. However, the terminal countered that the foundation of the Millennium Tower had already fallen 25cm before the construction for terminal began.
Several factors have combined to sink the Millennium Tower. Millennium Tower is in the downtown area of San Francisco which is the area of infirm soil. Many building in downtown San Francisco (built before Millennium Tower came out) had gotten away with drilling foundation piles into the layer of Old Bay Clay, without settling too dramatically. By the way, Millennium Tower is a tall building with 58 stories build at the same area and with its heavy concrete frame.The average single-story residential home is designed to exert pressure on the subsurface about 4kg per square meter of the building footprint. This building is a concrete framework designed to impose a load of 465kg per square meter to its foundation. It’s about 100 times more than the average house. Particularly, in the case of a weak mud layer on the ground, it would be impossible to bear that much weight only with the ground surface.The pile was driven so deep that the building did not sit on the surface layer of artificial fill but rather on the dense sandy ground of Colma formation. This is quite common design in San Francisco. By the way, it’s not the dense sand that causes problems for the Millennium Tower, but what is below it. Under the Colma formation is a thick layer of Old Bay Clay. When the building was designed, project geologists predicted that it would settle for the entire life of the building about 10cm to 15cm, mainly in the Old Bay Clay layer below the bottom of the piles.Millennium Tower was designed to put more stress on the underlying Old Bay Clay than other building in the area. In construction of the tower’s basement, it required to pump water out of the surface to keep the site dry. Continuous dewatering of foundation accelerated the settlement too.
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