The International Information Center for Structural Engineers

Thursday, 17 May 2018 01:00cat

Millennium Tower in San Francisco is leaning and sinking

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Huge cracks spreading in the building's basement force its residents to sell their high-priced apartments at a loss

The Millennium Tower, located at 301 Mission Street in San Francisco, is a 58-story luxury high-rise that opened in 2009, and at that time it was acclaimed to be the tallest residential building west of the Mississippi. The problem is that the concrete tower is sinking -17 inches so far- and has also started tilting (14 inches to the northwest), while it is estimated that it will continue sinking an additional inch per year. The $550 million construction, as it was advertised, had attracted various buyers, from tech barons to San Francisco royalty and former football quarterback Joe Montana, that had paid millions to buy condos there. Now, the value of the units has dropped like a rock. Various cracks on the basement’s columns rising from the foundation indicate the reason. After the suggestion of structural and geotechnical engineers, some of the building’s residents make daily rounds there to check on the stress gauges. "There's enough of them, a spiderweb of cracks, that you have to be concerned about what's going on underneath," said Jerry Dodson, a resident who lives on the 42 floor, in an interview.

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CBS / ’60 minutes’

The phenomenon is being surveilled by various engineers from all over the world. One of them is Petar Marinkovic, who works with the European Space Agency to track earthquakes. Marinkovic, using signals from a satellite 500 miles above the earth, measures ground movements around fault lines; in 2016, while studying the Bay Area, he observed that the zone around the Millennium Tower is sinking, in fact between 1.5 to 2 inches a year. The fact was also confirmed by satellites in late 2016, when Norwegian scientists, working with the European Space Agency (ESA), combined multiple radar scans from the Copernicus Sentinel-1 twin satellites of the same area to detect subtle surface changes – down to millimetres. According to the experts, there's nothing to suggest the sinking and tilting are slowing down, much less stopping. However, last summer, the city of San Francisco and its engineers asserted the building is safe, even in the event of an earthquake.

Who’s to blame

The first indications show that the problem lies in the fact that the building’s foundation doesn’t reach bedrock. The soil under the Millennium Tower consists of layers of sand, mud, clay and landfill from the time of the gold rush, and you need to go 200 feet (61 meters) below the surface in order to reach solid rock. However, the 900 existing pillars of concrete piles only reach down to 91 feet below surface (28 metres).

The developers of Millennium Tower, Millennium Partners, blame the Transbay Joint Powers Authority, the agency responsible for the new transit center built right next to the tower. They claim that during the transportation hub’s construction, builders pumped too much water from the ground, causing the water levels under the Millennium Tower to drop, resulting to the sand’s compression and the building’s sinking. The opposite part however, blames an "inadequate foundation." Others blame the city of San Francisco for allowing Millennium Partners to anchor the building 80 feet into packed sand rather than 200 feet down to bedrock.

Proposed solutions

There are some solutions being proposed by engineers (including the removal of the top 20 floors to reduce the tower’s weight), with the most dominant being to bore 275 to 300 “micro-piles” to anchor the building to bedrock. But before that, it must be stabilized, meaning that the piles first have to be placed on the sinking side of the tower and wait for the other sides of the tower to “catch up” or sink to the same level as the lowest side is today. At that point, the additional piers could be drilled, while the entire process could “take from two to five years.”, with an estimated cost of between $200 and 500 million. The general contractor and engineers are working on the approvals and permits needed to start the retrofit.

The legal battle

There are various pending Millennium Tower lawsuits which typically involve at least 20 parties; lawyers for Millennium Partners, for the Transbay Terminal next door, for the tower's structural engineers, and geotechnical engineers, for the architect and the builder, for the homeowners association and for the city. It takes the court 30 minutes just to take attendance during legal proceedings! And while the lawyers are doing their best to clear who will foot the bill, "everybody is afraid to tell the truth. Because if we get to the bottom of this, they are worried that it is going to, in some ways, slow down the building boom that is happening in San Francisco", says Aaron Peskin, the city supervisor. 

Source: CBS News

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Marinkovic's chart of ground movements

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