San Francisco sinking under tall building construction weight, but scientists don’t foresee seismic risk

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By Michaelgimbel - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=83009455

A researcher says earth-based and space-based observations confirm that the entire San Francisco downtown area is sinking under the weight of heavy buildings. The most obvious example is the tilting Millennium Tower, but this isn’t the only one that is sinking, says U.S. Geological Survey research geophysicist Tom Parsons.

“I looked at every building in the Bay Area, so just under a million buildings,” Parsons told NBC-TV  for a report broadcast on Nov. 23. Parsons 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.

At an estimated 686 million pounds, the Millennium Tower is the city’s third heaviest building. 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, the broadcaster reported.

“The Millennium Tower’s an unusual example of tilt, but generally they go down vertically,” said Parsons.

All the buildings downtown likely weigh enough to exert sufficient downward force to influence an earthquake fault, he said. Fortunately, the there aren’t seismic implications because the  San Andreas fault runs offshore before it reaches the city. “Still, the mass of sinking buildings shows up clearly in yellow in enhanced images captured by the European Space Agency’s Sentinel 1 satellite,” NBC reported.

“If you have a series of buildings, fairly heavy buildings all clustered together, they’re going to influence each other,” said Harry Poulos, a tall building foundations expert. 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,” he said.

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,” NBC reported

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