SCHREYER AWARD & AWARD OF EXCELLENCE – WATER RESOURCES West Edmonton Sanitary Sewer W12 Syphon
“This was a difficult project to execute in a small space and it has resulted in a 100% reduction in combined sewer overflow events in its first year of operation.”
The North Saskatchewan River that flows through the City of Edmonton supplies drinking water for the city and dozens of downstream communities. Unfortunately, like most pre-1940 municipalities, Edmonton has sewer systems that also serve as stormwater systems, and during heavy rains these combined sewers occasionally overflow into the river.
A combined sewer outfall at Rat Creek located in the river valley near downtown was responsible for 60-80% of the city’s combined sewer overflow. The outfall discharged up to 2 billion litres annually into the river, affecting water quality and fish habitat, and raising safety concerns.
As part of Edmonton’s West Edmonton Sanitary Sewer (WESS) project, the city decided to remedy the situation at the Rat Creek outfall by building the WESS W12 syphon. The syphon, completed in 2011, connects the Rat Creek combined trunk sewer to the South Highlands Interceptor. This then conveys the overflow safely to the Gold Bar Wastewater Treatment Plant on the south side of the river.
The work done by SMA Consulting and Associated Engineering on the W12 syphon has been highly successful. The project was expected to reduce combined sewer overflows by 80%. Yet between November 2011 and July 2012 overflow events at the Rat Creek location were reduced 100%. The project has improved the water quality for Edmonton and municipalities downstream and provides much-needed capacity to accommodate Edmonton’s booming growth.
For this complex and high-risk project, the City of Edmonton’s Drainage Design and Construction Branch brought in Associated Engineering (AE) to design the tunnel and SMA Consulting (SMA) to optimize the project’s delivery.
AE was the lead design consultant for the W12 tunnel and additional projects that link it with the city’s existing drainage network. AE’s role included the design of control structures and tunnel connections; construction costing; hydraulic modeling of the system and syphon; computational fluid dynamics analysis of the inlet, control structures and drop shaft; odour control design; environmental impact mitigation; and public consultation.
SMA facilitated the decision-making process during the preliminary and detailed design phases using sophisticated decision-making support tools. For example, SMA’s structured risk analysis process was used to help the team make several key decisions, along with other techniques such as construction simulation, value analysis, failure mode and effects analysis (FMEA), and the analytic hierarchy process (AHP). These tools were used to help determine the best construction sequence, to provide input on design decisions, and to determine the reliability of parts of the system. As well, SMA provided 3D and 4D visualizations that made it easy for decision-makers to discuss solutions in workshops involving several city branches and experts.
SMA also conducted project controls during construction, preparing daily site visit reports, tracking the budget and schedule, and using analysis modelling and forecasting techniques to predict performance and manage change orders.
In the footprint of
abandoned coal mines
The design and construction of W12 was extremely challenging. Almost the entire project is situated at an extreme depth — 70 metres below downtown Edmonton — and much of it is within the footprint of abandoned coal mines. Five deep shafts and a 1.2-km tunnel had to be constructed in ground laden with coal seams, water pockets, and voids. Methane gas under pressure was detected in several locations.
Access was also severely limited: the northern construction site was in Edmonton’s downtown, and most of the tunnel alignment was under the Riverdale Golf Course and the river itself, running between two coal seams.
Real time control structure
A real-time control structure (RTC) was installed for the Rat Creek outfall. Operated from ground level, it has precisely configured gates for controlling flow. The design required tying into an old, brick 3200-mm pipe while it continued in service with live flows. Multiple rounds of modeling helped simplify the design to ensure its constructability.
The complex RTC structure has to function flawlessly to avoid flooding basements upstream or releasing unnecessary overflows into the river. At the north end, in a residential area, special odour scrubbers were installed.
Thanks to the W12 project’s success, Edmonton’s river is now cleaner and safer. The project also protected the environmentally sensitive river valley during construction, such as by locating the working shaft for the tunnel boring machine in a previously disturbed area. This area was then later re-landscaped as parkland. cce
Project name: West Edmonton Sanitary Sewer (WESS) Stage W12 Syphon
Award-winning firms/prime consultant, project controls, decision support: SMA Consulting (Hussien Al-Battaineh, P.Eng.; Mohammed Al-Bataineh, P.Eng.; Ethan (Yang) Zhang, P.Eng.).
Prime consultant, design: Associated
Engineering (Herb Kuehne, P.Eng.;
Jason Lueke, P.Eng.)
Owner: City of Edmonton
Other key players: Thurber Engineering (geotechnical); Maple Reinders
(constructor); BPR-CSO (gate control
strategy); gh3 (architect RTC#3 facility); City of Edmonton Design & Construction, Drainage Services (design)