West Seattle sinkhole followup: Almost 3 stories underground!

Craig Young has been keeping an eye on the emergency sewer-main repair at Fairmount/Forest in the Admiral District – which started with a once-small sinkhole – and sent us a new photo this morning: Repair workers almost 30 feet under the street! So we called Seattle Public Utilities to get more details about this “big dig” – and a conversation with Frank McDonald revealed a lot about what goes on underground, here and elsewhere:

McDonald says the collapsed main that’s being replaced is a century-old, one-foot-wide clay “sanitary sewer” pipe. He says clay is actually a “resilient” material for pipes like this, so the age and material aren’t necessarily the cause of failure. Though the pipe is almost 30 feet – which means a contractor is doing the repairs, as SPU workers stop at about 18 feet – McDonald says it was only five feet under when it was installed in 1910, and most pipes are only about 10 feet down. This street has been graded and straightened over the years, and so the pipes just kept going further down.

We say “pipes” because McDonald says there’s another one under there that may have something to do with this one’s failure – an 18-inch-wide storm-drain pipe installed in 1974, when the “combined” sewer system was split in this area as part of the Forward Thrust program. That pipe crosses over the sewer main in a few spots, he says, and the interplay between the pipes and groundwater may have led to the settlement that revealed the growing problem underground. It was confirmed with use of a special camera, McDonald explains, equipment that’s carried by five SPU trucks. They don’t just respond to problems – they also use data analysis to figure out what types and ages of pipes around the city might be most at risk of trouble, so they are out proactively checking on pipelines too. (With 43,000 pipelines around the city, McDonald notes dryly, they have to prioritize.)

In this case, he says, after they dug down to the “original problem,” the crews started checking further down the line – literally – and “are finding some other potential problem areas,” so they’re going to keep going until they get to a section that seems to be OK. He still expects the work will be done by the end of this week – unless they find some major additional problem.

The digging has included the removal of the intersection’s traffic circle – here’s our photo from February 18th, before the repair work began:

McDonald says it’ll be up to SDOT to come in and replace the road surface, as well as features like the traffic circle. (And then, he points out, SPU will get an interdepartmental bill.)

12 Replies to "West Seattle sinkhole followup: Almost 3 stories underground!"

  • coffee March 7, 2011 (3:49 pm)

    I find this to be very interesting, and fascinating! What I am wondering, why do city workers stop at 18 feet down?

  • Stephen Coughlin March 7, 2011 (3:49 pm)

    Thankfully we have citizens like Craig Young that keep us up-to-date with what our utility companies are doing around town. Without people like him something like this would have never been brought to our attention.

    • WSB March 7, 2011 (3:53 pm)

      Stephen – Couple other folks also tipped us to the original ‘sinkhole’ but Craig’s been diligent about updates on this so we are quite appreciative! And this photo was extra impetus for us to finally call SPU for more details on the project.
      Coffee – I should have mentioned that but it seemed overkill in the parenthetical way I mentioned the depth … After 18 feet, the shoring issues call for extra help, according to SPU. (And as you can see in Craig’s photo, the shoring is no small job …) – TR

  • Wessy Attle March 7, 2011 (4:20 pm)

    Can someone let me know if they find Jimmy Hoffa? He still owes me $5. I’m thinking I’m probably not going to see that any time soon (along with the $3 that DB Cooper owes me).

  • Eric B March 7, 2011 (5:14 pm)

    I love learning about those hidden parts of our infrastructure that (almost) always make life better. Thanks WSB!

  • genesee hill March 7, 2011 (5:25 pm)

    Wow! In answer to coffee, my guess is the City does not have the equipment, nor expertise, to go deeper than 18 feet. How often would they have to? Sort of like having a fleet of 600 snowplows! Not necessary, usually!

  • Genesee Hill March 7, 2011 (7:01 pm)

    I do give these contractors all the credit in the world. Not too sure that I would like to be at the bottom of that pit. I would need a lot of faith in the engineers and my fellow workers..not the type of faith you buy at Mars Hill Church.

  • JB March 7, 2011 (9:18 pm)

    That’s some serious engineering! I’ve done a fair bit of masonry work in excavation sites, and when you’re in a pit that’s over head-high the psychological nature of the job changes from ‘work’ to “Oh boy, if something goes wrong, I’m a dead man.” Kudos for the top notch reporting. Love this part “This street has been graded and straightened over the years, and so the pipes just kept going further down.” Really brings home how radically we change our environment. I’m wondering what’s under some of the houses in that neighborhood.

  • Gina March 8, 2011 (1:43 am)

    So, not a job for the Pothole Rangers, eh?

  • redblack March 8, 2011 (7:57 am)

    those “walls” are actually iron shoring boxes that get successively smaller as they descend. the excavators drop them in with a backhoe. chances of cave-in are pretty slim.
    nonetheless, it’s a double-whammy for me: i don’t like tall ladders or tight spaces. that job has both.
    throw in the potential for drowning, and i’m thinking those guys deserve hazard pay.

  • Walt March 8, 2011 (11:39 am)

    Great work Craig. I’ve known Craig for a long time since back in our military days. We are following this event from Davenport, IA.

  • Cecelia March 8, 2011 (5:59 pm)

    Another hazard in working in a pit like this is noxious gases.

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