Lessons Learned Designing Large Diameter and Deep Fiberglass Pipes

The City of Modesto, California, is currently constructing a realignment of its River Trunk wastewater interceptor system due to the fact that the existing interceptor is significantly corroded from high levels of hydrogen sulfide gas. This project includes 2.5 mi (13,200 ft) of 48-in. to 60-in. gravity sanitary sewer pipeline at depths from 20 ft to 30 ft below ground surface. Several pipeline materials were evaluated during preliminary design, and the city selected fiberglass pipe with a vinyl ester lining to mitigate potential corrosion impacts from the very high levels of hydrogen sulfide gas. The project alignment is through a narrow, two-lane residential neighborhood. Trenchless construction and open-cut construction were evaluated with several metrics, and open-cut construction was ultimately selected by the city. During bidding, the contractor was required to identify the fiberglass pipeline manufacturer. However, once construction started, the pipeline manufacturer was not able to supply the pipe material due to supply chain shortages. The contractor requested an “or equal” manufacturer, and the pipeline was supplied by Future Pipe Industries manufactured in Abu Dhabi, United Arab Emirates. The city allowed the pipeline contingent on factory inspection by the Engineer of Record prior to shipment to the United States. Additional challenges included approving an engineered shoring system for the deep trench, preventing pipeline floatation with a slurry bedding, and compaction immediately above the pipeline. This paper summarizes the design analysis and decisions, pipeline material approval and inspection, construction challenges, and related recommendations for future projects.