Going down to see what’s up – WAY down, in the Big Pipe Project

By David F. Ashton

The Bee, Jan 30, 2008, Updated Oct 30, 2009

David F. Ashton / THE BEE

When the Eastside Big Pipe goes into operation, this pipe will be carrying pressurized, raw sewage. We preferred to tour the pipe in its current pristine state!

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Portland’s “Big Pipe Project” to divert sewage out of the Willamette River isn’t news – the West Side part of the project has been completed, and is currently in operation. But the opportunity, before Christmas, to travel 160 feet underground to see this even-larger tunnel being dug on Portland’s East Side did catch our interest.

Our tour began at the contractor’s East Portland offices at the Portland Opera building, just south of the Oregon Museum of Science and Industry (OMSI) on S.E. Water Street, just north of the Ross Island Bridge.

“We are at the Opera Shaft location,” said Steve Marriott, director of Portland’s Bureau of Environmental Services, as he began our orientation.

“This shaft is where we inserted ‘Rosie’, the Tunnel Boring Machine (TBM). It’s driving north from here toward Swan Island.”

Marriott explained that Portland is among some 1,000 cities across the nation which were built with a “combined” sewer system. The raw sewage and stormwater are collected in the same system, with any overflow sent into the Willamette River.

“When the sewage treatment plant was built in the 1950s,” Marriott went on, “they also constructed interceptor pipes to convey the flow to the treatment plant on Columbia Boulevard in North Portland. But, they didn’t size these pipes big enough to handle the runoff from the rainstorms. At the time, they considered that having a clean river only in the summer was good enough; it provided a huge improvement in water quality.”

But in the 1990s, the city committed to the federal government to begin a 20-year program to address the chronic wintertime problem of combined sewer overflows into the river. And, Portland is ahead of most cities in solving this problem, Marriott said. “Many other cities have yet to address it.”

The underground tour begins

Our first stop on the tour was seeing the technology that supports the excavation of tons of sand, dirt, and gravel, and sealing a pipe – all 160 feet underground.

Our tour guide, Shane Yanagisawa, walked our group over to a three-story tall building on the project site, just east of the Opera Shaft.

“This is the separator,” Yanagisawa explained. “Everything that is cut by the mining machine is mixed with slurry made up of water and bentonite clay. It is pumped to the surface, where it is separated in into big chunks, small rocks, sand, and slurry. The slurry is pumped back down to the TMB.”

The leftover rocks and sand are barged from a conveyor belt extending over the riverbank just south of OMSI to load barges, which carry it to Ross Island, where it is used to fill in the lagoon, where once sand was dredged to make concrete.

After the TBM drilling unit pushes forward, cutting a section of tunnel, it is lined with a series of 25 identical, pre-cast concrete ring segments, and finished with one key segment to lock the ring in place. A special grout is then injected into the soil around each ring to help seal the pipe.

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