We've reported on a number of prominent sinkholes appearing in city streets recently. To find out more about what they are, how they occur, and what the city is doing about them, Hoodline talked to Jean Walsh, Communications Manager at the San Francisco Public Utilities Commission (SFPUC). We've put together an explainer to answer your questions.

What is a sinkhole?

A sinkhole is a section of street that slowly or spontaneously drops downwards into a cavity below. In San Francisco the underlying cause is typically a compromised underground sewer or water pipe. It's easy to tell the difference: a broken water pipe will spray water everywhere; those are less common.

Geologic sinkholes caused by, for example, underground limestone erosion--follow a related process but are not endemic to San Francisco. Your Corvette collection is safe here.

Recent sinkhole at Waller & Scott | Photo: Rose Garrett/Hoodline

What causes sinkholes?

Although it's easy to imagine a broken pipe suddenly collapsing and taking down the street with it, that's not what happens. The reality is more gradual.



Sewer pipes can become compromised in different ways over time. Small cracks or fractures can occur in old pipes, or misalignments ("offset joints") can occur at a connection point. These openings can be tiny but still allow small amounts of dirt to sift into the pipe. That dirt is carried harmlessly away with the wastewater, but over time enough soil from above the pipe is flushed away that a hollow space can form above the pipe and below the street surface.

Sinkhole formation: (1) A small fracture in a sewer pipe results in (2) a void forming in the soil above it leading to (3) sag or collapse of the surface; note that the sewer pipe is not always damaged in a collapse

Illustration: Ben Zotto/Hoodline

Streets are made up of a couple inches of asphalt atop several inches of concrete. Neither of these surface layers are structurally supportive; they rely on the soil below to hold their weight, as well as that of people and vehicles above.

If enough soil quietly sifts into the sewer, and a big enough void forms below the street's concrete, the surface may no longer be able to support its own weight, and collapses into that subterranean space. This can happen suddenly, or gradually.

Haven't rather a lot of them appeared recently?





It does seem that way, but Walsh says that SFPUC's "records don’t show an increase or pattern in the number of sinkholes in SF. Sometimes a large sinkhole in a busy area — like the recent one on Mission Street — will garner media attention... making it appear as if sinkholes are happening more frequently."



Workers repairing the damaged brick-and-mortar conduit on Mission St. after a sinkhole opened above it | Photo: Courtesy SFPUC

What do we do to fix them?

Depends on how serious the sinkhole is. When depressions of any kind form in the street, Public Works crews fill the holes with asphalt to make the surface safe for traffic. (If you notice one, let the city know.)



Walsh describes what happens next: "The SFPUC then conducts an inspection to assess the condition of the sewer pipe underneath to determine if the cause of the depression is sewer-related and if a repair is warranted. If failing sewer infrastructure is the cause, repairs are scheduled."

"If a very large sinkhole forms and there’s an urgent need to repair the sewer pipe immediately we mobilize crews, equipment, permits and other utilities to begin work ASAP."

Often the streets most sensitive to inconveniences will also be home to very old underground infrastructure. The biggest streets (think Market St or Mission St) developed earlier, and are likely today to be both busy and have additional infrastructure above (think Muni tracks), presenting a bonus challenge for the SFPUC when upgrades are needed.

Sinkhole tries to swallow truck on Waller Street in 2012 | Photo: Andrew Dudley/Hoodline

How can we prevent them?

By replacing compromised sewer lines. The SFPUC prioritizes which lines get attention by factoring in "the physical condition of the sewer, age, location, risk, public safety, the upcoming street paving schedule, and various other factors," says Walsh.



San Francisco runs over 1,000 miles of sewer infrastructure beneath the streets. The sewer system began life in the Gold Rush era, and even today four percent of the existing pipes date to the 1880s and earlier. (Fully 30 percent are over a century old.) They run the gamut from old brick-and-mortar conduits to the current vitrified or reinforced clay pipe.



Although occasional sinkhole appearances are in some sense an inevitable side-effect of having water and sewer lines underground, we can do more to keep surface disruptions to a minimum. That means continuing to improve the sewer infrastructure under our streets.



The SFPUC has ramped up visual inspection of sewer conditions — performed via video camera robot — to 150 miles of pipe a year, says Walsh. We are also well into the first phase of their ambitious Sewer System Improvement Program (SSIP). That's a multi-billion dollar upgrade program with a marquee goal of replacing the aging Southeast Treatment Plant in Bayview. But SSIP also includes resources for additional sewer pipe replacements on top of the current rate of 15 miles a year. This will contribute to reducing the occurrence and severity of street surface sinkholes.

Old illustration from 1892-1893 San Francisco Municipal Reports showing defects in brick conduit. | Photo: City of San Francisco

Bonus: are potholes just baby sinkholes?

No. Potholes are caused by intrusion of water and subsequent weakening of the street surface, and are characterized by cracked and missing asphalt. They are unrelated to underground water and sewer infrastructure.



If you spot a sinkhole, pothole, or any hazardous condition on a street or sidewalk, contact 311 by phone (dial "311" from within the city), by web, or use the handy 311 app.