General Constructions work started almost 2 miles offshore where an outfall tunnel is connected to the seabed by a 9 ft. diameter riser structure. The 10 ft. diameter seabed outfall trunk line originates at the riser and proceeds west for 4,500 ft., to a 320-ton wye structure. From the wye structure, two diffuser lines extend 2,000 ft. to the north and south along the seabed. This project required dredging 210,000 cubic yards of excavation, and placement of 350,000 tons of rock to bed, ballast and armor the 1.6 miles of outfall pipe. All work was performed 2 to 4 miles offshore, and at an average depth of 100 ft.
The first 1,200 ft of pipe was installed on the beach with conventional land equipment from behind the Kingston Marina out to the zero tide mark. The next 500 ft was laid and backfilled one joint at a time during high tides from a derrick barge offshore. The next 2,200 ft was installed and backfilled in a pre-excavated 6-9 ft ditch in 96 ft sections. The ditch was backfilled first with gravel and quarry spalls, then native material previously removed. The final 1,400 ft was laid directly on the bottom out to a set of diffusers at -170 MLLW. All underwater connections were made with divers. At the outer end of the outfall a four-diver crew was required, with each diver only having 14 minutes of bottom time due to the depth.
This project required construction of a steel intake pier 450 ft into an active drinking water reservoir. Our work included 500-tons of fabricated steel supported by 22 piles in four piers. The 175 ton, 50 x 100 ft pump platform was assembled in place before lowering 20 ft and driving piles. In completed form it has steel walls and floor, slide gates, a jib crane and 8 pumps manifold to twin 6 ft pipes back to the beach. The work required a trestle, sheet cell, sectional barges with a 165-ton crane and extensive silt curtains.
he initial 285 ft of pipe was installed through a jacked and bored casing installed by a subcontractor. This runs from the Des Moines Beach Park, under a hill, and out the upper portion of the beach. The next 600 ft were installed at low tides with an excavator in a sheet pile trench. All work was performed on non-floatable mats to protect the beach. The next 400 ft of pipe going offshore was installed by divers and a derrick barge in an excavated trench with sheet pile to protect the eel grass. Past the sheet pile, the pipe continues in an excavated trench for 200 ft then daylights and was set on the bottom out to the station 21+00. Precast anchors were set over the unburied section of pipe.
The project began with the relocation of approximately 1,000 sq ft of eelgrass. Then a 250ft long by 50 ft wide trench was dug. Serving as a foundation for the diffuser, three, 4 ft by 12 ft articulated concrete ballast mats were placed at in 55 ft of water in Puget Sound. The 620 ft. long, 54 inch HDPE outfall pipe was sealed at each end and pulled from the beach into the water. While it was floating, 23 concrete weights were attached. Each weight was over 15,000 pounds. After attaching the onshore end to its tie-in point, we used controlled submergence to sink the pipe into the trench and onto the mats. Finally the trench was backfilled and the beach was restored to its previous appearance.
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33455 6th Avenue South
Federal Way, WA 98003
Tel: (253) 943-4200
Fax: (253) 943-4021