Why San Francisco Has Had a Major Sinkhole Every Year Since 2011

You didn’t think 2015 would end without our annual San Francisco sinkhole, did you?

We were seriously in danger of missing the deadline this time, but a 10-foot cavern (with a modest 2 by 4 surface opening) appeared beneath 24th and Church Streets in the wee hours of the morning today and kept our record secure. (It also snarled service on the J-Church Muni line.)

We’ve averaged a little over one large sinkhole per year since 2011, in case you haven’t kept score. See, for old time's sake, the craters that opened in 2011, 2012, 2013, and 2014

This time the culprit is almost certainly a burst sewer line.

[jump] “When it comes to sinkholes, if you see water geysering into the street, then it’s a water main, and if you don’t then it’s the sewer,” says Public Utilities Commission spokesman Tyrone Jue.

The line was possibly stressed from the recent rains (the same lines that carry sewage also channel storm runoff), although it’s equally possible that it’s just an old pipe whose number finally came up. The street collapse is still delaying the J-Church. It’s a pain, but we get off lucky on these things; big sinkholes in other places — I'm talking about you, Florida and Mississippi — sometimes swallow whole streets or entire buildings. 

Most of the worst are natural sinkholes, when water flowing underground gradually eats away at stone and loose soil. A pocket forms, and sooner or later the materials closer to the surface — which, unfortunately, may include man-made structures filled with unsuspecting people — collapse into that gap. To us, it looks as if the earth spontaneously opened and devoured everything, but a sinkhole may take hundreds of years to develop.

A natural sinkhole can happen almost anywhere, but we’re not in any particular danger here.

“Most sinkholes in the US occur in areas with a lot of limestone,” says Leslie Gordon, a geologist with the U.S. Geological Survey in Menlo Park. “As soluble limestone dissolves, it undermines the ground’s surface. But there’s none of that in San Francisco.”

Yosemite is Northern California’s only significant limestone country.

So, there’s minimal danger of your house being swallowed. But our streets seem to have a sinkhole habit they just can’t kick. In fact, it’s worse than you think: Every year the city has to repair many small sinkholes the public never notices. The potholes in your street may even be percolating sinkholes.

The problem is our patchy plumbing. A man-made sinkhole forms almost exactly like a natural one, albeit hundreds of times faster: Burst or leaky pipes eat away at the earth beneath the street.

“A broken sewer line can create an hourglass effect, with soil slowly falling into it and being washed away,” says Jue. “Dirt has to stay really compacted to support the street’s weight.”

The line break may be too small to create a service problem that anyone notices right away, giving it time to open that subterranean pocket. When that gets big enough, the street’s slab collapses under its own weight.

In October, SF Weekly did a deep-dive into the city’s flood-prone hotspots, including Cayuga Avenue and 17th and Folsom streets. Plans to fix those waterlogged areas run anywhere between an esimated $7 million and $250 million.

City inspectors are always on the lookout for problems, but there are more than 1,000 miles of sewer nested beneath the 7×7 grid of San Francisco, much of it aging and infirm. Depending on where you live, your sewer might be so new the pipes are still shining, or it might predate the Great Earthquake. In fact, a few city blocks are still serviced by brick and mortar sewer ducts dating to the 1850's.

(Ironically, these lines have never been replaced because in many cases they’re more reliable than the newer stuff. That’s Gold Rush craftsmanship for you.)

“It’s really important that the city invests in inspection and retrofitting,” Jue says, sounding a bit like someone who has nagged without effect longer than he’d like.

Of course, every city department always says it needs more funding. But the idea of a yawning gulf opening up underneath your daily J-Church certainly makes you pay a little more attention to the pitch.

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