Brightwater Conveyance System | 2006 - 2010
King County conceived Brightwater as a major improvement to the regional sewerage treatment system in response to rapid population growth in greater Seattle. The project also will bring important environmental benefits, such as controlling untreated overflows into Lake Washington and Puget Sound, and recycling sewage into irrigation water.
The conveyance system portion of this project consists of 13 miles of new bored soft-ground tunnels. The tunnels extend from Woodinville to Puget Sound. These tunnels will connect about 335 miles of existing sewage pipes to a new 36-million-gallons-per-day treatment plant. From the plant, treated wastewater will flow to a new outfall in Puget Sound. Tunnel construction, currently underway, employs four 14- to 20-foot diameter tunnel boring machines, launched and retrieved via deep shafts in glacial tills well below the water table.Topographic conditions along the tunnel alignment range from hills to intervening valleys, resulting in external groundwater pressures that vary greatly, up to a maximum of 7.3 bars. This geologic factor significantly influenced the design of the tunnel lining system—bolted and gasketed precast concrete segments—and type of tunnel boring machines used for excavation—earth pressure balance (EPB) and slurry shield. In fact, the Brightwater Conveyance System marks the first specified use of slurry shield technology in the United States on a competitively-bid contract.
AUS provides diving services for the East Tunnel Contract and performed a sonar survey to determine configuration and dimensions of shaft. Diving services include, but are not limited to, installation of shear key dowels and rebar mat in the IS and IPS shafts