Inglewood Sanitary Trunk—Phase 2 Bow River Siphon: A Case Study

The Inglewood Sanitary Trunk (IST) was designed to offload wastewater from the Inner City Trunk (ICT) and the West Memorial (WM) trunks, and convey these flows to the Bonnybrook Waste Water Treatment Plant (BBWWTP), servicing the west and northwest areas of the City of Calgary. This offload allows for the existing 15th Avenue trunks to provide additional sanitary capacity to the north portion of the City. Overall, the system provides service to a total equivalent population of 1.1 million residents and will support growth to the 2,076 design horizon. Phase 1 of the trunk was completed in 2020, intercepting the ICT south of the Bow River. Phase 2 has been tendered and is in the early phases of construction. It consists of a twin 1,500 mm by 400 m long siphon crossing of the Bow River, intercepting the West Memorial trunks on the north side of the Bow River. There were many unique design challenges and considerations on this phase of the project. Complex hydraulic considerations were reviewed in order to maintain serviceability of the siphon through its 50 plus year development life. Complicated flow control and bypass strategies were developed to ensure constructability. This included reviewing and developing connection phasing of intercepting two existing 1,200 mm and 1,950 mm trunks which are located under a major arterial roadway, adjacent to LRT, including detour design. Connection to the existing IST Phase 1 utilizing a purpose-built flow diversion and TBM retrieval shaft in a sensitive residential neighborhood. Workspaces needed to be developed in sensitive parklands. Construction utilizing microtunneling was selected to deal with the challenging geological conditions, which were located through an environmentally sensitive river environment. This paper will explore design challenges encountered in the completion of this important wastewater conveyance project.