The 13-mile long Brightwater tunnel is the main component of a $1.8 billion project to increase sewage treatment capacity in King County. The tunnel was split into four excavation projects. Section 4 required construction of 21,100 ft. of 13 ft. diameter precast concrete segment tunnel. The tunnel was excavated utilizing a 15 ft. diameter earth pressure balance tunnel boring machine (EPBTBM). The tunnel was constructed from a single heading commencing from the Owner’s Port Wells Sampling Facility Site and terminating into the Ballinger Way Access Shaft.
The tunnel lies within glacial and non-glacial deposits containing boulders and high permeability sands and gravels. A unique challenge to the project was that all excavated spoil from the shaft and tunnel operations could not be removed from the site via the local road system. A conveyor system was designed and utilized to transport and load the spoil onto barges. The barges then transported the spoil to the disposal site.