Combined Sewer Overflow (CSO), consisting of approximately 22,710 million cu meters of combined sewage are discharged annually into Willamette River and Columbia Slough through the City of Portland's combined sewer system. The CSOs are a significant source of water pollution and their clean up is a major concern for the City, State and Federal environmental protection agencies. The Columbia Slough Consolidation Conduit (CSCC) is an important element of a mandated CSO abatement project for the City of Portland (COP), Bureau of Environmental Services (BES). It includes 2,548 m of 3.7 m. internal diameter underground conveyance and storage tunnel. The CSCC alignment traverses railroad, Columbia Boulevard (a major industrial thoroughfare for trucks going to and from the Port of Portland), and it also parallels in close proximity to an old concrete sewer. Geologic conditions include river alluvium and fine to coarse-grained flood deposits containing cobbles and large boulders. The existing site condition and geologic subsurface conditions presented construction challenges. This paper describes design and construction of a tunnel in soft-ground, with shallow cover and close proximity to an old concrete sewer.
The COP combined sewers were constructed before the 1940s, when treatment of wastewater was not provided and the primary objective was sanitation and disease prevention. During heavy precipitation, the storm water exceeds the capacity of the existing combination sewers and the excess flows go directly into the receiving waters at various outfalls. The Oregon Department of Environmental Quality Amended Stipulation and Final Order (ASFO) requires the control and reduction of CSOs to the Willamette River and Columbia Slough. Meeting the ASFO goal requires the elimination of all CSOs from storm events equal to or less than a 5-year winter storm or a 10-year summer storm. The CSCC is a key element in meeting the ASFO requirements.