Annacis Island Wastewater Treatment Plant Tunneled Outfall System

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Project Update by Fred Marquis, P.Eng., PE

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.

McMillen Jacobs Associates was retained by Metro Vancouver to provide engineering services for the detailed design of the on-land shafts’ initial and final lining structures, as well as seismic analysis and electrical design of the project.

Annacis Island Treatment Plant

Reinforcing ongoing work on shaft final lining (Source: Metro Vancouver)

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 approximately 20 meters (65 ft) below the river’s bottom. The shafts will be connected by a 4.2-meter-inside-diameter (13.8 ft) bored tunnel measuring approximately 800 meters long (2,625 ft).

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.

In 2019, Pomerleau Bessac General Partnership (PBGP) was awarded a CA$184 million contract to construct the new outfall pipeline; construction of the shafts started shortly after. Slurry trenching by hydromill to depths up to 58.5 meters (192 ft) required tight control to achieve the specified verticality tolerance. Specialty subcontractor Keller Foundations started construction of the on-land shafts in January 2020 and completed diaphragm wall installation in approximately 4 months, achieving production rates of up to 10 m/hr (33 ft/hr). Excavation and base slab placement were executed in the dry. Construction of the outfall shaft final lining presented several challenges. For example, more than 1,000 meters (3,281 ft) of cooling pipes were required inside the 3.75-meter-thick (12.3 ft) base slab to mitigate the risk of delayed ettringite formation and thermal cracking of the concrete. The outfall shaft also 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.

During the construction phase, McMillen Jacobs has so far reviewed and responded to requests for information, requests for variance, and submittals from the contractor. We have also provided regular on-site underground inspections and monitoring. The effluent and outfall shafts have been successfully constructed, setting the stage for the tunneling effort. The slurry pressure balance tunnel boring machine arrived from France in late 2020, and launch is expected in February 2021.

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