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.)