Annacis Island Wastewater Treatment Plant is the third largest plant of its kind in Canada, serving a population of 1.2 million people from 18 local municipalities. Located in Delta, British Columbia, the existing plant discharges treated effluent water into the Fraser River via an outfall built in the early 1970s. As the region prepares to welcome a million more people by 2040, the facility is currently undergoing a major expansion to accommodate that growth as well as projected population increases over the next 100 years.
The underground portion of the project consists of two 40-meter-deep (131 ft) shafts adjacent to the treatment plant and a riser shaft conveying the plant effluent up from the outfall tunnel to the diffuser manifold installed at approximately 20 meters (65 ft) below the river’s bottom. The shafts are connected by a 4.2‑meter (13.8 ft) inside diameter bored tunnel measuring approximately 800 meters (2,625 ft) long. The effluent shaft has a finished inside diameter of 7 meters (23 ft) and conveys plant effluent from the surface to the new tunnel about 30 meters (98 ft) below grade. The outfall shaft has a finished inside diameter of 16 meters (52 ft) and collects flows from the effluent tunnel for discharge to the outfall tunnel.
As a subconsultant to CDM Smith, Delve Underground was retained by Metro Vancouver to provide engineering services for the shafts and tunnels, including the overall seismic design for the project. Detailed soil-structure interaction analyses were carried out to evaluate the permanent displacements under the design ground motions using the 2D and 3D finite-difference software FLAC. During the construction phase, Delve Underground monitored the installation of the shaft’s support of excavation, jet grouting, underground cast-in-place concrete, steel pipe installation and welding, and the TBM launching/receiving works.
Delve Underground contributed with Pomerleau-Bessac General Partnership to the implementation of avant-garde construction methodologies such as slip-forming for the permanent shafts lining. The outfall shaft includes three 5.2-meter-diameter (17 ft) tunnel eyes and a 1.5-meter-thick (5 ft) dividing wall. Their installation required close coordination between the engineer, prime contractor, and subcontractors.