In response to growth in the region, King County built a new wastewater treatment system called the Brightwater Conveyance System to serve north King and south Snohomish counties. Brightwater construction included five sites: the treatment plant site in Snohomish County near Woodinville; and four portal sites in Bothell, Shoreline, Kenmore, and unincorporated Snohomish County. Construction started in early 2006 and was completed in September 2011. The system of pipes that transport wastewater to the treatment plant and treated wastewater to Puget Sound is called the conveyance system. To limit construction impacts above ground, the Brightwater conveyance pipelines were built almost entirely below ground in tunnels approximately 40 to 440 feet below the surface.
JWF worked as a subcontractor for the Vinci Construction Grands Projets / Parsons RCI / Frontier-Kemper joint venture on the Central Contract portion of the Brightwater Conveyance System. Our scope of work, the Swamp Creek Connector, consisted of the installation of two 60-inch diameter single-pass microtunnels of 750 lineal feet and 850 lineal feet in length.
Portions of the tunnels were accomplished at a depth that exceeded 85 feet with one of the tunnels installed under an environmentally sensitive wetlands area and the other was installed under a stream with approximately a pipe diameter of cover. Both tunnels were installed under the water table with artesian pressures.
The baseline soil conditions were mixed face conditions of varying degrees of fine-grained plastic soils; fine-grained non-plastic soils; fine to medium sand with varying amounts of gravel, silt and clay; and course sand and gravel with varying amounts of fine to medium sand, silt and clay.
Adding to the complexity of the project, baseline conditions predicted forty-six boulders two feet and larger in excess of 40,000 psi. When these boulders were encountered, our personnel made on-the-spot modifications to ensure ground adaptability through this major differing site condition.
The project also required a very small launch area and an 85 feet deep recovery of our microtunnel boring machine in conditions with artesian groundwater pressures.