Terwilliger Boulevard sewer project heads for the finish line
On one side of the row of temporary barriers that block off part of Highway 43, the remaining roadway still looks pretty much the same as it always has. But just a few feet away on the other side, workers have spent most of the past month digging — and then refilling — a massive, 35-foot-deep hole to reach the Tryon Interceptor sewer pipe buried under the street.
It's one of the final phases in a $3.44 million project undertaken by the Portland Bureau of Environmental Services to install a new 8-inch sewer line under Terwilliger Boulevard. The project has kept the southernmost mile of Terwilliger closed since December, which officials say is necessary due to the narrow road surface and a lack of space to park vehicles and staging equipment nearby.
The partial closure of Highway 43 — restriped lanes now carry two-way traffic — began earlier this month in order to give crews room to install two new manholes and 80 feet of pipe to connect the new Terwilliger line to the Tryon Interceptor. That means digging two large pits and excavating a trench, and PBES crews have been able to get most of the work done in just a few weeks.
The Terwilliger pipe will eliminate the need for an aging pumping station on Terwilliger that currently pushes sewage uphill to reach the wastewater system in Dunthorpe. The station was costly to maintain and the destination system is already taxed to its limits, officials say. The Tryon Creek treatment plant has more capacity, and the sewage won't need any help to reach it.
"The more sustainable option is to let it go downhill and let gravity do the work," says Ashley Tjaden, PBES's public outreach officer for the project.
During the initial public outreach period, residents were told that Terwilliger Boulevard would remain closed for up to a year. That estimate was shortened to eight months before the closure began, but any hopes of shaving further time off the project were dashed when crews ran into what Tjaden describes as "soupy soils" under the road — more than anticipated, and made worse by winter conditions.
"It was just all water," Tjaden says. "We had to dig through water. It would be like digging through chili."
Despite being softer, watery soil doesn't allow for the kind of work the crews needed to do. About 1,000 feet of the Terwilliger pipe was installed by excavating trenches, but the other 6,000-foot portion was installed using a remote drill that tunneled under the road surface.
The drill needs a minimum soil density in order to bite into the surrounding earth, so the crews paradoxically found that in order to dig material out of the ground, they'd have to put more in first. The project was delayed by almost a month while crews worked to drill new holes and inject grout and cement to stabilize the soil.
"We just didn't expect all that water," Tjaden says. "We haven't seen that on different projects."
Tjaden says a sewer pipe is normally installed in an uphill direction, but in this case one crew worked to start installing the pipe at the base of the hill while a second crew worked to stabilize the soil at the top. Having two teams sped up the work, but Tjaden says any gains from the extra staff were canceled out by the time needed to deal with the soupy conditions.
Compared to the Terwilliger adventure, Tjaden says, the Highway 43 portion of the project is going quite well — in fact, progress is faster than anticipated, with crews hitting almost no large rocks or other unexpected obstacles that would slow down the digging.
"We're ready to finish early," she says. "We hit a sweet spot."
Tjaden and PBES spokesman Don Poletski say that's because the area under the highway used to be a gully and was later filled in when the state road was constructed. (The adjacent train tracks used to be on a trestle.) The gully was filled in with regular dirt rather than any kind of artificial filling material, but it still means there were no major rocks or other obstacles.
With a clear path, workers were able to quickly dig the first 10-foot-wide hole in just a few days, instead of the week-long timeframe that Poletski says is closer to the average. In total, crews extracted roughly 500 cubic yards of dirt to reach the Interceptor pipe.
Even without natural obstacles, the crews have to stay mindful of other pipes and utilities under the road — some of which may be old enough that they're not well-documented, or have incorrectly listed locations.
"This is a really sensitive area," Tjaden says. "When we get closer to utilities, we just have to slow down how we dig."
Once the hole reached the Interceptor line, a new manhole shaft was built in the center of the hole to connect to the top of the pipe. The area around the shaft was then filled back up, this time using crushed rock to maintain the highway grade.
"We have to put back material we can compact to a certain stiffness," Puletski explains.
A smaller, second hole still is needed for another manhole at the point where the Terwilliger line changes direction to align with the Tryon Interceptor. A short length of new pipe, roughly 8 feet underground, will carry sewage from the Terwilliger pipe to the first manhole, where it will travel down the shaft to reach the main pipe. All of that construction work was expected to be completed by the end of this week.
Tjaden says the highway will likely be reopened to its full width in June — or even May, if things remain ahead of schedule. It's partially just down to the weather; the repaving and restriping of the road requires a stretch of consistent dry days, which at this time of year isn't always readily available.
But it will likely be a longer wait until Terwilliger is open again for traffic. Tjaden says the second crew is still working on installing pipe near Powers Court and Northgate Avenue; if that work progresses smoothly, she says, the road could reopen as early as June.
Still, PBES doesn't want to commit to that date, she says, because underground sewer construction is unpredictable. In addition, the final phase in the project — to repave the mile and a half of Terwilliger above the new pipe — will come at a time of year when Portland's usual paving contractors will be in high demand.
"We get in line with all of the construction projects that need paving," Tjaden says.
The only official deadline, she says: "No later than August."