1. The Project scope includes construction of a 7-million gallon below grade combined sewer overflow (CSO) storage tank, a diversion structure, demolition and replacement of the Chelan regulator station, and installation of approximately 2,600 lineal feet of open-cut and trenchless (micro-tunneling and pipe ramming) influent 60- inch pipe. Submersible drain pumps will discharge wet weather flows from the storage tank to the effluent conveyance system. The effluent pipeline will discharge to the West Duwamish Interceptor for treatment at the West Point Treatment Plant. Construction of the influent conveyance is expected to include removing abandoned concrete and timber foundations. The Project area has been used for a variety of industrial purposes and multiple sites in the vicinity have some level of documented soil or groundwater contamination. 2. Construction sequencing of the storage tank assumes a top-down construction method, utilizing interior and exterior slurry walls and multiple phases of excavation. Additionally, dewatering is anticipated for open cut, shaft, and trenchless excavations and over-excavation of a portion of the storage tank, along with permanent ground anchors to mitigate buoyant uplift forces on the structure. The adjacent facility foundation preparation includes ground improvements to mitigate seismic ground deformation. Chelan Storage Tank Top-Down Construction 1. Install Interior and Exterior Slurry Walls 2. Install Dewatering Wells 3. Excavate to Permit Installation of Shoring for Top Slab Construction 4. Construct Top Slab 5. Excavate Inside Tank 6. Construct Permanent Ground Anchors 7. Construct Bottom Slab 8. Construct Liner Walls 9. Pour Back Construction Access Openings in Top Slab (if required)
