The Millennium Tower may be the most recognizable sinking building in the city, but one researcher says earth-based and space-based observations confirm the entire downtown area around it is sinking as well.
“I looked at every building in the Bay Area, so just under a million buildings,” said U.S Geological Survey research geophysicist Tom Parsons, who estimates that over the last century, 3.5 trillion pounds of development and human activity – including the subsidence tied to loss of groundwater -- have led to an estimated settlement of three inches across the entire Bay Area.
“Clearly, the most density and the tallest buildings are centered in that downtown San Francisco area, and that's where we see the most calculated cumulative settlement from all of those buildings together," he said.
Turns out that at an estimated 686 million pounds, the Millennium Tower is the third heaviest building in the city. The top nine all weigh more than 300-million pounds, but the only one that’s leaning significantly is the Millennium. Groundwater loss from adjacent construction has been blamed for the problem by the tower’s developers, while geotechnical experts say the key is that its foundation is not rooted in bedrock.
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“The Millennium Tower’s an unusual example of tilt, but generally they go down vertically,” said Parsons, who has long studied the stresses that build up on the earth’s crust.
He says all the buildings downtown likely weigh enough to exert sufficient downward force to influence an earthquake fault. Fortunately, the San Andreas fault runs offshore before it reaches the city. So, he says, the billions of extra pounds will not likely have seismic implications.
Still, the mass of sinking buildings shows up clearly in yellow in enhanced images captured by the European Space Agency’s Sentinel 1 satellite.
“If you have a series of buildings, fairly heavy buildings all clustered together, they're going to influence each other,” said Harry Poulos, an internationally recognized expert on tall building foundations. He says while there may be no seismic concern for San Francisco, there’s been little research about the collective impact of entire corridors of high rises on the earth below.
“I've been doing foundation design for, I guess, nearly 30 years now, and it's not something that we've actually ever even thought about. Not on this sort of scale that you're talking about,” Poulos said.
Parsons says he’s now studying Manhattan, because it is also sinking and faces the same threat as San Francisco from climate change. Parsons points out that ground level and sea level heading in opposite directions is a dangerous scenario for coastal cities.
“If you have all of this going on when you're right near the waterline, in some cases in San Francisco, then you have to worry about big storms that sea level comes up and inundation more frequently.”
San Francisco building officials say the city has long-term plans to shore up the seawall that protects the Embarcadero. A tall building task force separately concluded that high-rise buildings -- with properly designed foundations -- should not sink more than four inches. Still, the task force urged that all new buildings be monitored for at least a decade after construction.