Henderson Combined Sewer Overflow Reduction Project

CLIENT Seattle Public Utilities

Location Seattle, WA

The Henderson Combined Sewer Overflow (CSO) Reduction Project addresses CSO deficiencies in Henderson Basins 44 and 45.

Henderson Basins 44 and 45 are located west and southwest of Seward Park along the western shoreline of Lake Washington. As the geotechnical engineer-of-record, Shannon & Wilson provided geotechnical recommendations, as well as dewatering and shoring design services for the project.

The project addressed CSO deficiencies in Henderson Basins 44 and 45, west and southwest of Seward Park along the western shoreline of Lake Washington. Project components were developed at two sites to limit CSOs in these basins.  The first component consists of a 2.65-million gallon underground CSO offline storage tank in Seward Park, designated CSO Facility 8A.  Geotechnical challenges included a near surface groundwater table; weak, fractured Blakely Formation bedrock from the ground surface to below the excavation, and environmentally sensitive areas including a slope just west of the site.

To create a watertight excavation, and prevent movement of the adjacent slope and damage to abutting private properties, Shannon & Wilson recommended a secant pile wall and tieback shoring system, integrated with a pumped well dewatering system.  The piles were overlapped during drilling to provide a continuous wall, cutting off groundwater and supporting the fractured rock.  Two rows of about 40-foot-long tieback anchors were installed through the walls to stiffen the shoring system and provide additional support.  Shannon & Wilson recommended that the shoring piles be structurally connected to the permanent tank structure, providing uplift resistance with their dead weight and axial friction.  This eliminated the need for permanent costly tiedowns, and reduced the thickness of the storage tank slab.  A dewatering system consisting of 15 pumped wells was installed inside the excavation, rather than outside, to provide efficient groundwater drawdown, and guard against potential drawdown impacts to adjoining properties. The excavation was completed well ahead of schedule, and the storage facility is currently in operation.

The second component consists of below grade modifications to the existing sewer piping and a control structure to provide approximately 16,000 gallons of storage adjacent to an existing pump station.  These modifications consist of five new manholes, a new 96-inch-diameter storage pipe, and smaller diameter piping to connect the storage pipe to the system.  These modifications were installed ahead of schedule, and the facility is currently in service.


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Erik Scott
Senior Geologist

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