As a part of West Edmonton Sanitary Sewer (WESS), the Rat Creek to McNally inverted syphon project (Stage W12) is being implemented by the City of Edmonton (COE) to direct wet weather flows to the Gold Bar Wastewater Treatment Plant (GBWWTP) and to reduce a significant amount of combined sewer overflows (CSO) to the North Saskatchewan River (NSR). The WESS Stage W12 project includes a 3600 ft (1100m) long, 96 in (2.4m) ID syphon tunnel buried at a depth ranging from 66 ft (20m) to 230 ft (70m) with a design flow rate of 530 ft~3/s (15m3/s), a real time control facility (RTC#3) and monitoring facilities both upstream and downstream. Since the syphon tunnel was built under the NSR, it was constructed with two liners to prevent exfiltration from the sewer and contamination of the NSR. The preliminary liner, a rib and lagging liner with an ID of 118 in (3m) provides the support for installing the secondary liner, which was designed to achieve zero leakage. During the design stage of this project, several options for secondary liners were evaluated, including cast-in place reinforced concrete liner, steel pipe, pre-cast concrete pipe with inside coating and fiberglass pipe. The centrifugally cast, fiberglass-reinforced, polymer mortar pipe (HOBAS pipe) was selected as the secondary liner for the syphon tunnel due to various merits of this product, and the annular space between preliminary liner and HOBAS pipe was decided to be filled with grout materials. The 20 ft (6m) long HOBAS pipes were installed from the working shaft, which was built between the syphon inlet and outlet. To install the 101 in (2.57m) OD HOBAS pipe in the 118 in (3m) ID preliminary tunnel liner, the COE designed and built its own pipe carrier. The COE began the installation of HOBAS pipe in June 2010, and the productivity of installing HOBAS pipe and grouting annular space was improved as more experience accumulated. The syphon tunnel was successfully completed in June 2011.
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