The City of Seattle's Public Utilities (SPU), looking for a cost-effective means to reduce combined sewer overflows (CSO's) in the downtown central business district, developed a plan to use a proposed electrical power transmission and distribution tunnel and construction
shaft for combined sewage conveyance and storage. The power transmission tunnel initially was conceived to meet Seattle City Light's (SCL) need to replace and relocate major power transmission lines within Seattle's congested central business district. A deep bored tunnel would be
constructed to avoid major traffic impacts, utility congestion, and poor soils. The tunnel would have an 11-foot inside diameter, be 4,000 feet long, and 65 to 85 feet deep. A tunnel boring machine (TBM) launch shaft would be a minimum of 60 feet in diameter. The receiving shaft would be no
smaller than 20 feet square and 65 feet deep. Both shafts would be furnished with stairs and house emergency life safety equipment. Power transmission cables would be brought in and out of the tunnel through the shafts and hung on the tunnel walls Building on knowledge accumulated from
operational history of CSO storage facilities in the Pacific Northwest, a conceptual design was developed incorporating combined sewage conveyance and storage into the tunnel project that resulted in beneficial cost sharing of the facilities. From this design, engineers and utility managers
have learned how flexible, creative engineering can provide cost-effective solutions. The proposed combined sewer modifications include the following major components: High flow diversion structure including coarse screens and a grit chamber Drop
structure and pigging station using space within the tunnel north access shaft Two 20-inch-diameter conveyance pipes using space below the tunnel floor 1.4 million gallons of combined sewage storage (by converting
the TBM launch shaft into a storage tank) Odor control Pumped discharge from the storage tank to the collection system using removable submersible pumps Design challenges that were
solved include: SCL required that SPU's sewer maintenance personnel have no potential for access to the electrical transmission cables. Sewer maintenance personnel required assurance that the tunnel
CSO pipes would not plug. The pipes entered the bottom of the storage tank and would flow under available gravity pressure; as a result, flushing velocities could not be achieved during most CSO flow conditions. The
team needed to develop a method to easily maintain and clean the deep storage tank after a CSO flow event. SPU's sewer maintenance personnel are not currently trained in maintenance of a deep structure. These significant challenges, and others, were solved.
The conceptual design resulted in a practical and feasible solution for both SCL and SPU.
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