Brightwater Tunnel Construction Begins Trickiest Phase This Fall

The damaged BT-3 boring machine in shinier days. Photo: King County.

I was a little surprised to hear that the final phase of Brightwater tunneling is set for this fall. Back in mid-February, King County Executive Dow Constantine declared the project's status an "emergency," and I assumed that meant round-the-clock tunneling action. But really the emergency declaration was simply to allow Constantine to switch contractors, to Jay Dee Coluccio (JDC), and renegotiate terms with them. (JDC is also working on the University Link light rail project.)

After the tunneling that finished this June, the boring machines have been on hiatus while "reconditioning" is done. A fact sheet (pdf) explains:

JDC will move the tunneling machine that has mined from Point Wells into the 200-foot deep shaft at the Ballinger site. This will take about two weeks. Once the TBM is in the shaft, several sections will be lifted to the surface to be reconditioned. Remaining portions will be refurbished inside the shaft. Cranes will be used to lift the TBM sections to the surface and to lift equipment and crews in and out of the shaft.

Brightwater is a huge wastewater treatment system north of Seattle, with a 114-acre treatment plant, 13 miles of pipeline, and a marine outfall that, via twin pipes that shoot about a mile out into Puget Sound, diffuses treated water with the Sound. It's designed to treat 36 million gallons per day, and expansion planned by 2040 should increase that capacity to 54 million gallons.

Right now, eleven miles of pipeline are complete, but tunnel boring machine BT-3 is stuck below Lake Forest Park at 53rd Avenue NE near NE 197th Street--300 feet down, it's in a ground water zone that precludes easy removal. (The pipeline ranges in depth from 40 to 440 feet, and from ten to thirteen feet in diameter.)

The easier part of the plan is for BT-4 to tunnel from the Ballinger Way Portal to Lake Forest Park. That final two-mile section should take a year, finishing up in fall 2011. But that leaves not one but two boring machines over 300 feet below the surface, with no vertical portal to allow a crane to remove them. See, tricky. The plan for removal of the boring machines, says King County's Annie Kolb-Nelson, is being reviewed by the machines' manufacturer, project engineers, and the contractor JDC.