San Francisco’s sinking sidewalks: Is local weather change responsible?

By Jessica Wolfrom

Examiner employee author

The city’s sidewalks have started to buckle, crack, and slope in one of the newest neighborhoods in San Francisco, creating stumbling blocks for pedestrians and frustrating local residents forced to navigate the city’s uneven sidewalk.

Sidewalks in some blocks of the Mission Bay neighborhood have sunk 10 to 15 centimeters and in some places even as much as 30 centimeters deep, KPIX first reported.

But not only the sunken sidewalks of the quarter are shifting in this densely built-up area. Almost three kilometers away in SOMA, the tilting Millennium Tower also struggles to balance on the sinking floor that bears the building’s multi-story weight.

A possible culprit could be climate change. As a historic drought continues to dry out the state, it can also worsen the subsidence or subsidence of the soil surface as the city’s groundwater, which usually flows from higher elevations like Twin Peaks or Bernal Heights into the sediments below the lower districts, becomes a trickle slowed down.

“When there is a lot of rainfall, the water goes into the clay. It’s slow, but it builds up the water table, ”said Lawrence Karp, a geotechnical engineer from the Bay Area. “In periods of drought, as we have had for a long time, the water table sinks … and the clay thickens.”

As soft clay solidifies from the loss of groundwater, its weight increases, causing it to carry away structures on top, including the sidewalks and streets of Mission Bay.

The redeveloped waterfront neighborhood was built over an old railway station and reclaimed landfill that historically served as a landfill for industrial waste, including the burned remains of the 1906 earthquake.

Today it is home to a sprawling, state-of-the-art medical campus, high-rise apartment buildings, a hotel, school, and the recently completed Chase Center.

But the sidewalks here have become a dangerous headache for residents and local businesses alike. The gap in front of Cafe Réveille on Long Bridge Street is so steep that two ramps, bright yellow paint and whimsical signs point the way into the cafe and warn the guests: “Please watch your step!”

A man walks past a “Watch your Step” sign on Friday, September 24, 2021, alerting customers to a step from a sinking sidewalk outside Cafe Reveille in Mission Bay. (Kevin N. Hume / The Examiner)

It wasn’t always that bad. “There’s been a big change in the last couple of months,” says Aaron Nelson, barista at Cafe Réveille.

Others say the sinking sidewalks are old news. “Since we’ve been here, the streets have been sinking,” said long-time resident Peggy Fahnestock, who moved to Mission Bay in 2009.

A few years ago, a member of her hiking group stumbled in front of Café Réveille and broke his arm. “It’s not been that bad for a long time,” said Fahnestock, board member of the neighborhood association. “I tripped and fell out there … you have to be careful where you go.”

While more research is needed to draw concrete conclusions about the links between the drought and the sloping sidewalks, scientists don’t think this is surprising given the conditions.

“From a scientific point of view, we don’t have any evidence yet, but we do know that (this) process happened elsewhere,” said Manoochehr Shirzaei, a professor of geophysics and remote sensing at Virginia Tech University who did the subsidence in the Bay Area examined in detail. “When we change something in any part of the hydrological system, the chain of events begins,” he said, calling the city’s groundwater a “connected system”.

And at the moment this system is under increasing stress from the persistent drought.

“The drought is affecting groundwater by temporarily reducing its natural replenishment from rain and stormwater runoff,” said Will Reisman, spokesman for the San Francisco Public Utilities Commission. However, he said, “the total amount of groundwater stored in the aquifer remains large and will be replenished in the wet years that follow.”

Still, water is clear in the minds of many in the town hall. To further protect the city’s water supply, the board of directors unanimously adopted a measure last week that more than doubled the amount of water that new large buildings have to collect and reuse on site.

“This summer of intense drought and terrible forest fires reminds us that the climate crisis is now and will not go away by itself,” said Supervisor Rafael Mandelman. “Although we have zero carbon emissions targets for our city and our world, the reuse and recycling of water is becoming increasingly important to our survival.”

Mission Bay isn’t the only neighborhood suffering from settlement, but it’s one of the newest, which is what makes the crisp infrastructure noteworthy. Other areas built on landfills, like San Francisco International Airport (SFO) and Treasure Island, have also declined as the landfill slowly compacts, according to research, making these areas more prone to flooding and sea level rise .

A cyclist rides elevated pavement slabs outside an apartment building along Mission Bay Boulevard North in Mission Bay on Friday, September 24, 2021. (Kevin N. Hume / The Examiner)

Subsidence alone would not cause a building to collapse or a sidewalk collapse, Shirzaei said. What matters, he said, is what is called differential motion, or the uneven speed of subsurface subsidence.

“Differential movements that can seriously damage any infrastructure,” he said. “In fact, there is no man-made structure that can withstand such a load. So, pipelines, concrete pillars, they would all break and eventually lose their integrity. “

So far, however, the buildings in Mission Bay have remained stable because they are built on stilts, Karp said. But for local residents like Fahnestock, the question remains who will pay to repair the damaged sidewalks.

The city said that such corrections fall on local residents. “The maintenance of the walkways is the responsibility of the adjacent property owners as per state and city regulations,” said Rachel Gordon, Public Works spokeswoman. “Public Works inspects the sidewalks and notifies property owners if a problem needs to be addressed.”

The goal, Gordon said, is not a punishment, but a safe path for pedestrians.

jwolfrom@sfexaminer.com

A pedestrian walks across raised pavement slabs outside an apartment building along Mission Bay Boulevard North in Mission Bay on Friday, September 24, 2021. (Kevin N. Hume / The Examiner)