Seattle Public Utilities Completes Emergency HDD Beneath Heavily Regulated Waters

摘要

Seattle Public Utilities (SPU) completed emergency design and construction of an 1160 foot-long 16-inch OD HDPE sewer beneath Shilshole Bay Waterway in the Ballard area of Seattle using horizontal directional drilling (HDD). Staheli Trenchless Consultants served as designer and provided specialty inspections during construction on the WWPS43 Emergency Force Main Replacement Project. This paper presents the case study of the project, lessons learned, and evaluation of the data collected during construction.In 2017, SPU discovered a severe leak in an existing cast iron force main discharging sewage into the Shilshole Bay Waterway, the canal leading from Puget Sound to Lake Washington and a USACE governed waterway. To mitigate the concern of leaking sewage, three months of 24-7 emergency response consisting of extensive pump and truck bypassing was undertaken to remove flow and leakage from the pipe. An above-ground temporary bypass was constructed, and given the risk of impending combined sewer overflows, the force main full-line replacement project was given emergency design and construction status. During pre-design efforts, HDD trenchless technology was determined as the least risk, most cost-effective method to install a replacement sewer line.The project required extensive coordination with many federal, state, and local permitting agencies, including: USACE, NMFS, USFWS, DAHP, DNR, WDFW and other organizations. The project encompassed sensitive communities on both sides of the waterway, one near a very popular biking and walking path - the Burke Gilman Trail. Furthermore, the design had to balance coordination with two active rail lines, adjacent construction projects, multiple property owners and construction fatigue in the surrounding neighborhoods.