Seattle, WA

Ship Canal Water Quality Tunnel Storage

Seattle Public Utilities (SPU) and King County


The combined sewer overflows (CSOs) in the Seattle neighborhoods of Ballard, Fremont, Queen Anne, and Wallingford must be limited to one overflow per year on average for each affected outfall. The current average is 130 overflows annually, equating to 80 million gallons (MG) (303 ML) of polluted water in the Seattle waterways. 

The 14,000-foot-long (4,267 m) Tunnel Storage project is the program’s central feature and consists of an internal diameter of 18.83 feet (5.74 m) and a storage capacity of 29.2 MG (110.5 ML). CSOs are intercepted and sent to the tunnel via a series of new diversion structures, which route them to one of five new drop structures. 

Connecting the drop structures to the new tunnel requires five shafts with diameters ranging from 11 feet to 86 feet (3.4 – 26.2 m) and depths up to 110 feet (33.5 m); an approximately 650-foot-long (198 m), 94-inch-internal-diameter (2,390 mm) curved microtunnel beneath the Lake Washington Ship Canal; and two conventionally mined adits, each having a finished internal diameter of 8 feet (2.4 m).

Fast Facts
  • 14,000-foot-long (4,267 m) storage tunnel
  • 29.2 MG (110.5 ML) stormwater capacity
  • 35- to 85-foot (10.7–26 m) depth
  • Five drop shafts

Scope of Work

As the prime consultant, Delve Underground provided preliminary and final design services while leading a team of 20+ consulting engineering firms across five sites. During design, the scope of work included segmental tunnel lining geotechnical and structural design; launch shaft geotechnical and structural design; final adit design; tunneling settlement analyses; geotechnical instrumentation design; civil site design; technical specifications; cost estimating; and overall project management. 

With the project now in construction, Delve Underground’s scope of work during construction includes reviewing contractor submittals and requests; coordinating changes requested by the owner; working with the contractor and owner to resolve challenges encountered in the field; reviewing geotechnical instrumentation; and performing geotechnical special inspections. 

Challenges & Innovations

Some of the challenges and innovations that Delve Underground managed or collaborated with the contractor and owner on include designing the slurry wall support of excavation for the tunnel boring machine (TBM) launch shaft to provide groundwater cutoff during construction, designing new conveyance facilities to keep existing facilities in operation, implementing a temporary pressurized steel bell system inside the TBM launch shaft as an alternative to ground improvement outside the shaft, mining through abandoned steel cable tieback anchors with negligible observed surface impacts, and utilizing a hydraulic joint design to facilitate construction of a curved microtunnel.