DJC TopProjects: Lower Tualatin Pump Station

FIRST PLACE PUBLIC WORKS/INFRASTRUCTURE/TRANSPORTATION $5.1M - $15M

Bikers and joggers in Tualatin may have noticed the construction of the Lower Tualatin Pump Station, but they didn’t complain about it. The pump is located in the middle of the heavily used Tualatin Community Park, so Natt McDougall Company elected to construct permanent concrete pathways at the onset of the project so that normal activity in the park would not be disrupted.

The Pump Station is a new sewage pumping plant that sends 30 million gallons to the Clean Water Services Durham Treatment Plant every day. The project was unique in that it included sinking a caisson as the key element, which Michael McDougall, project manager for NMC, said is a rare and risky undertaking.

The concrete caisson was sunk 60 feet below grade in extremely fine and wet soils that were not considered dewaterable. NMC proposed a scheme to firm sub-strata around the caisson and then perform partial dewatering, which allowed the team to sink the cast-in-place cylinder.

"We were undermining something that weighed two million pounds and trying to control where it went," McDougall said. "There's a person down inside the structure, so part of the risk and sensitivity is keeping the thing plumb and level as you're going down." Despite unfavorable conditions, the team sunk the caisson within half an inch of the targeted line and grade.

Atop the well is a rectangular, three-level station that contains pumps, piping, and mechanical and electrical equipment, but looks like a rail stop station. The team added architectural finish features so that the Pump Station would fit in with the Community Park.

Further challenges included the structure's proximity to the new light-rail line and the active P&W Railroad trestle at the Community Park. Both tracks were active, which aroused concern because the deep excavations adjacent and under them could have led to dangerous settlement issues. Both sites were under a strict monitoring program during construction, but no settlement was found at either location and the project was completed under budget and on schedule.