Abstract
The King County Department of Metropolitan Services (formerly called Metro), faced with the need to transfer wastewater from the West Seattle drainage basin to a secondary treatment plant on the other side of a ridge, opted to construct a tunnel that could also store stormwater. The 10,200 ft project, conveying up to 19 mgd, lies 30 to 385 ft below the ridge. The design team determined that the hydraulics called for 48 in. conduit in a 6 ft i.d. tunnel. As geologic studies and engineering design progressed, and as future peak flows were predicted as high as 96 mgd, coupled with project cost estimates, the optimum size rose to 9.5 ft i.d. tunnel. Construction will start in the spring of 1995 with an earth pressure balance tunneling machine equipped with flood doors in the cutter head shield and access through the cutter head for removing boulders and servicing the equipment. The tunnel liner will be bolted and gasketed precast concrete segments.
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