Designing for the Future—A Seismically Resilient Transmission Pipeline Crossing in Earthquake Country

Portland, Oregon, is located in the heart of the Cascadia Subduction Zone and is expected to experience devastating impacts after an earthquake along this fault. In an effort to mitigate the impacts and improve system resiliency, the Portland Water Bureau needs a seismically resilient water pipeline across the Willamette River, as all of the six existing crossing are expected to be severely damaged due to liquefaction and strength loss of the loose riverbank soils. The proposed crossing is located in the Portland downtown core, which is a complex place with a wide array of existing utilities, multiple property owners, transportation corridors, and highly variable geology. This complex location requires design and construction methods to accommodate all the project constraints. Key to managing these constraints is using an appropriate trenchless construction technology. This Design–Build project evaluated multiple trenchless technologies, including horizontal directional drilling, microtunneling, and direct steerable pipe thrusting to install the more than 4,000 ft of 48-in. diameter welded steel pipeline needed for the project. Several unique shaft and pit configurations were needed to accommodate these various trenchless installations in a complicated urban area. This paper focuses on several design and construction method challenges unique to this project and the trenchless industry. Key among these challenges was determining how to accommodate up to 20 ft of lateral spreading of the riverbank ground surface, where a 36-ft diameter shaft will be sunk to accommodate a connection between two trenchless alignments. This paper discusses the comprehensive geotechnical design and analysis efforts to study the anticipated soil loading to the pipe and resulting pipe strains modeled using FLAC and ABAQUS computer software. These analyses were used to demonstrate pipeline performance at varying depths using different pipe wall thicknesses, steel pipe yield strengths, and cased/uncased pipe sections.