The West Side CSO project involved final design for the 3.4-mile-long, 14-foot diameter soft ground tunnel which diverts, stores and transports overflows from the outfalls to the Swan Island Pump Station. The project includes 1,300 feet of new parallel force mains, and shafts to provide storage capacity, venting and equipment and personnel access. The 29,000-foot-long, 22-foot diameter East Side tunnel reduced the frequency and volume of CSO into the river from thirteen outfalls.
Challenges of the projects were numerous. The East Side CSO tunnel runs through mixed soil conditions at depths of up to 160 feet and at least 100 feet below the groundwater table. WSP chose reinforced concrete slurry walls as the initial ground support system to provide an effective groundwater cut-off during shaft excavation.
Another major challenge involved threading the tunnel through the foundations of six major bridges and elevated highway structures, the main north-south West Coast rail link and more than 100 buildings that stand within the underground construction zone of influence. WSP evaluated the structures to ensure they would not be affected by ground movements during construction and was successful in siting seven large diameter (up to 62 foot) shafts in highly populated areas and constructing them within small work areas while minimizing community impacts.
WSP served as lead designer form both the East Side and West Side tunnels. Key elements of our project work included design for large tunnel alignment and diversions and shafts; soft ground tunneling with slurry tunnel boring machine; geotechnical baseline report; hydraulic modeling; vortex drop structures; constructability review and value engineering; risk management; environmental documentation; public and third party outreach; and permitting.
