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Prime consultant: Shannon & Wilson
Client/owner: City of Portland Water Bureau

For years, two large-diameter water conduits carried water from the Bull Run Watershed to Portland via a 114-year-old steel bridge spanning the Sandy River. The arrangement left the conduits vulnerable to a range of possible disasters from earthquakes to terrorist attacks.

The city of Portland decided the conduits, and the water they contained, would be safest if located underground. But doing so required a construction design that would have minimal impacts on the environment, an adjacent park and people living in the area.

The city compiled a design-build team that was led by Kiewit Pacific Co./Parsons Corp. and included Shannon & Wilson as prime consultant. While other teams seeking the conduit project contract had proposed microtunneling designs, the Kiewit/Parsons team had proposed a conventional tunneling alternative that offered improved security, flexibility, smaller construction impacts and less risk of contaminating the rivers.

As part of the project team, Shannon & Wilson handled slope stability evaluations for excavations, and soil and rock characterization and design parameters for tunnels and shafts for the project.

Tim Collins, project manager for the Portland Water Bureau, gave Shannon & Wilson high marks for developing clear, accurate documentation of geologic conditions throughout the tunneling process and for categorizing subsurface conditions.

Once the conduits were installed beneath the Sandy River, they were encased in thick concrete, which is expected to help protect the city of Portland’s water supply for several hundred years.