King County

Brightwater Conveyance System

King County, WA


The Brightwater Conveyance System project has significantly improved Seattle’s regional sewerage treatment system, and Delve Underground played a pivotal role in its completion over the nearly 10-year life of the project. Design and construction involved the use of tunnel boring machines (TBMs) with diameters ranging from 14 to 19.5 feet (4.3 to 6 m), launched and retrieved via deep shafts in glacial tills well below the water table. Approximately 13 miles (21 km) of soft-ground tunnels were constructed, ranging from 40 to 440 feet (12 m to 134 m) underground beneath urban development.

Fast Facts
  • 13 miles (21 km) of soft-ground tunnels
  • Four tunnels connecting 335 miles (539 km) of existing sewer pipes
  • 40 to 440 feet (12 m to 134 m) below ground
  • 36 million gallons per day (136 MLD) treatment plant

Scope of Work

In addition to preliminary engineering and final design of shafts and four tunnels, Delve Underground assisted the client in evaluating alternative contract delivery methods. As part of a joint venture, the firm served as managing partner that delivered final design, which included determining the final internal diameters of various system components. 

Challenges & Innovations

Delve Underground overcame several design challenges, including the high groundwater and compressible soils of one of the four tunnels, the 83-foot-deep (25.3 m) Influent Pump Station, and 74-foot-deep (22.6 m) influent structure shafts, which precluded dewatering. Instead of conventional internal-bracing methods, we designed a twin circular-cell shaft using slurry diaphragm walls and a structural invert slab constructed with tremie concrete methods.