Transitioning Away from Collection System Concrete Structures

This paper discusses how the designer and owner addressed operational and maintenance challenges of a sewer system built over 32 years ago. The existing 16,000 linear feet (LF) of 27-in. reinforced concrete pipe (RCP) interceptor sewer has experienced collapses due to hydrogen sulfide corrosion. Preliminary engineering included condition assessment, hydraulic analysis, and cost–benefit alternative analysis. Replacement of the interceptor includes a segment of the pipeline that crosses a tributary to the Red Oak Creek. The existing crossing at this location is an encased aerial system with supporting piers on both ends. Design of a new tributary aerial crossing revealed that an encased 48-in. carrier pipe would backup stormwater flow in the tributary channel because of a new lower flow line, which would require a new, long easement for grading upstream. An alternative to the aerial crossing would be a siphon design that traditionally would use concrete structures on either end. Due to the owner’s operational and maintenance challenges associated with concrete siphon structures, an alternate solution to a traditional siphon design was developed in the form of a siphon with FRP wye manifolds.