Design of Utility Pipelines through an Active, Urban Landslide: The City of Astoria's 11th Street Combined Sewer Overflow Separation Project

Much of Astoria, including the 30-block project area, is constructed over slow-moving earth flows that result in ongoing ground deformations. Within project limits, a new stormwater conveyance system bypasses the sanitary interceptor, eliminating untreated combined storm and sanitary overflows into the Columbia River. The project included 12,600 ft of storm, 1,500 ft of sanitary, and 4,900 ft of water pipelines constructed entirely within an urban residential area and routed across two large, active landslides. Other challenges included extreme slopes, shallow groundwater, and groundwater springs surfacing through pavement cracks. Comprehensive risk assessment and management of these challenges was achieved through careful geotechnical and civil planning and design. Included were geologic research and mapping, strategic selection and placement of pipeline materials like HDPE pipe and flexible fittings, and a permanent groundwater collection system. The final design accommodates ground movement and controls groundwater, providing the City with a longer useful service life for the pipelines.