In response to rapid population growth in greater Seattle, King County created Brightwater as a major improvement to the regional sewerage treatment system. The project also brought important environmental benefits, such as controlling untreated overflows into Lake Washington and Puget Sound, and recycling sewage into irrigation water.
AUS provided diving services for the East Tunnel Contract and performed a sonar survey to determine configuration and dimensions of shaft. Some of the diving services included the installation of shear key dowels and rebar mat in the IS and IPS shafts. The conveyance system portion of the project consisted of 13 miles of new bored soft-ground tunnels, extending from Woodinville to Puget Sound. These tunnels connect 335 miles of existing sewage pipes to a new 36-million-gallons-per-day treatment plant. From the plant, treated wastewater flows to a new outfall in Puget Sound. The tunnel construction used four 14 foot to 20 foot diameter tunnel boring machines. They were launched and retrieved via deep shafts in glacial tills well below the water table. Topographic conditions along the tunnel alignment ranged from hills to intervening valleys, resulting in greatly varying external groundwater pressures, 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—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.