Combined Sewer Overflows (CSO), estimated at approximately 22,710 million cubic meters, are discharged annually into rivers in the City of Portland (COP). CSOs are a major source of pollution for these water bodies. The COP under an Amended Stipulation and Final Order (AFSO) from the Oregon Department of Environmental Quality (OR-DEQ) has undertaken a major clean up task. To accomplish this goal, the City is constructing a series of projects consisting of collection, transport, short-term storage, and pump and treatment elements, known as the Columbia Slough Consolidation Conduit (CSCC). The Influent Pump Station (IPS) is a key element of the CSCC program. The Influent Pump Station (IPS) is a facility to handle 397.4 thousand cubic meters per day of CSO. Its construction required a large excavation adjacent to existing buildings and underground utilities. The excavation extended to a depth of 18 m below grade and approximately 9 m below the water. The underground construction for the IPS presented several challenges related to the existing site conditions and subsurface conditions. This paper describes how these challenges were mitigated. Conventional cut off type shoring system with internal dewatering was not acceptable and a sealed excavation shoring system with sealed base slab was selected.
The Influent Pump Station (IPS) facility is required to lift and convey Combined Sewerage to the Columbia Boulevard Wastewater Treatment Plant (CBWTP). It is located at the lower terminus of a 3.7 m diameter combination sewerage conveyance and storage pipeline constructed between the Columbia Boulevard Wastewater Treatment Facility and the Interstate Freeway 5 (I-5). The IPS consists of High Flow and Low Flow pumping structures, and Electrical Instrumentation & Controls Building as shown in Figure 1. It also includes construction of pipes of varying sizes and materials connecting structures and existing sewerage facilities.