Building on Our Mistakes: Sinking Sidewalks in Mission Bay

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Mission Bay sidewalks continue to sink, a challenge that could point to greater problems than twisted ankles and scraped knees. 

“People fall a lot,” said Brian L. who has worked at Cafe Reveille for the past three years. The café, located at 610 Long Bridge Street, is moated by a separated sidewalk edged with a yellow hazard line warning. “It’s always been a problem, but it’s gotten a lot worse in the past year.” 

Spanning 300 acres, Mission Bay is built on piles of compacted debris, dirt and silt, making it prone to subsidence, or settling. New buildings are anchored to bedrock deep below; sidewalks aren’t. Since they were first installed barely fifteen years ago, paths along Fourth Street have sunk visibly, with gaps from four inches to almost a foot forming between sidewalks and buildings. Separations have been repaired with caulk, ramps, steps, and not at all. 

The issue isn’t unique to Mission Bay, nor new to San Francisco, with the Marina, South-of-Market, and other neighborhoods experiencing intermittent sinking. The American Society of Civil Engineers published Subsidence and the Foundation Problem in San Francisco in 1932.   

“Realty has more value than reality,” said Gray Brechin, a historical geographer and author who is a visiting scholar in the Geography Department at the University of California, Berkeley. “Building on fill areas is stupid, but it doesn’t stop because there is money to be made.” 

“Bayfill is not a great place to build for multiple reasons: subsidence, climate change, and sea level rise,” said San Francisco natural history educator and cartographer Joel Pomerantz. “But people are trying to make as much money as possible.” 

Both Brechin and Pomerantz pointed out the dangers buildings located on fill face during earthquakes, when it’s unclear how structures and what’s underneath them will behave. The 1906 Earthquake triggered a four-foot displacement in SoMa and other neighborhoods, with streets “becoming like the sea, rising and falling,” said Brechin. 

During the 1989 Lomo Prieta earthquake parking lots pulled away from buildings at Fort Mason because of subsidence, according to Pomerantz. 

The most casualties caused by the 1906 earthquake in a single incident — approximately 100 out of 498 total quake deaths — happened because of subsidence, when the Valencia Hotel on Valencia and 18th streets sank into the ground, its windows filling with mud. A water main break outside the building drowned residents waiting to be rescued from the upper floors, Pomerantz explained. 

The building and sidewalk at Strata, on 1201 Fourth Street, separated so much that the entryway was closed for almost a year for repairs, according to Chris Chang, who operates the Happy Lemon tea shop across the street. 

Slip sidewalking away, in Mission Bay. Photo: Julie Zigoris

“They learned their mistakes from that building,” Chang said, claiming the lack of issues with his sidewalk as proof.

Chang hasn’t had any problems with his sidewalk so far, other than a crack that’s formed in the sloped stone up to his front windows. He encourages skateboarders not to ride over it lest they fall. 

It’s unclear whether other Fourth Street structures haven’t experienced the issue because they’re built better, or because they’re newer and so less settling has occurred. 

“The City’s priority is that the sidewalks are kept accessible and safe,” said Rachel Gordon, San Francisco Public Works Director of Communications and Policy. “Sidewalks are not the City’s responsibility according to state and local code,” emphasizing that it falls to owners to maintain paths. 

Still, the City has made some repairs, installing ramps and railings to smooth connections between buildings and sidewalks. 

“We don’t own the sidewalks, that’s the City,” countered Brian L. at Cafe Reveille. Fixing them is “up to the City, not us.” 

According to the barista, the City installed an accessibility ramp along the Fourth Street entrance to the cafe and added the yellow reflective stripe to the sidewalk gap. 

Gordon said the issue is particularly noticeable in Mission Bay because the new development is built on bayfill, while other neighborhoods have had more time to settle and be repaired. 

According to Pomerantz, areas that’ve been filled over the past two centuries were “extremely important” to the Bay’s ecology, “doing an important job for humanity and the species.” State legislation largely halted further filling. 

Pomerantz is concerned that poor development choices will continue to be made in San Francisco. “Things are not likely to change when land values are so high,” he said. 

“The City is becoming more dangerous all the time,” Brechin said. “It’s amazing how we don’t learn from history.”