Vanir provided extensive preconstruction services, full construction management and inspection for this design-build marine outfall project, which won eight industry awards. Vanir’s quality assurance and quality control support covered onshore and offshore work, plus off-site locations for pipe fabrication. The project was completed two years ahead of the contracted completion date and within budget.
The outfall begins as an 84-inch inside diameter (ID) steel pipe connecting onshore to conveyance tunnels that extend back 12 miles to the treatment plant. Shortly after leaving the shoreline, the 84-inch ID pipe branches at a steel wye to two, 5,000-linear-foot, 63-inch outside diameter high-density polyethylene (HDPE) lines. The HDPE pipe terminates at a water depth of 600 feet in Puget Sound, with the last 250 feet of each pipe being diffuser pipe.
The onshore and near shore pipe was installed in an open-cut trench, which was sheeted up to a water depth of 30 feet and unsheeted thereafter to -80 Mean Lower-Low Water. From 80 feet deep to the end of the line, the HDPE pipe was bottom laid. Over a 72-hour period, the two preassembled HDPE lines were floated into place and installed using controlled submergence by flooding the pipeline while it was under 60 tons of tension.