2020 CANADIAN CONSULTING ENGINEERING AWARDS
WATER RESOURCES
AWARD OF EXCELLENCE
McLoughlin Point WWTP HDPE Outfall AECOM
A ‘float and sink’ installation was used for the offshore section.
“This record-setting project was notable for minimizing ecological damage and saving time and money.” –Jury 42
www.canadianconsultingengineer.com
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Before completion of the new McLoughlin Point wastewater treatment plant (WWTP) in Victoria, the Capital Region District (CRD) was discharging raw sewage into the ocean from two outfalls. After decades of public controversy over unsustainable waste management, construction of a new outfall for the WWTP would instead facilitate discharge of treated effluent. Early concepts included reusing existing outfalls, which would have required twinning conveyance pipelines to pump stations, an additional 1.2 to 4.2 km of underground infrastructure and lift station pumps to overcome limiting hydraulics. By installing a new outfall at the plant site instead, the additional infrastructure would not be needed, reducing initial conveyance system and ongoing operating costs. For this purpose, AECOM and joint venture (JV) partner Graham Construction designed and built North America’s largest solid-wall high-density polyethylene (HDPE) outfall pipe, 1.92 km long, in two distinct segments. Inshore segment The inshore segment’s section of steel pipe was microtunnelled through bedrock. A ‘wet exit’ technique avoided the use of expensive cofferdams and limited disruption to the sensitive intertidal marine environment. To prevent deterioration from corrosive seawater, the pipe was treated with an abrasion-resistant epoxy overlay, the tunnel annulus was filled with a calcium nitrite grout. Also, the interior of the pipe was treated with a polyurethane (PU) coating to prevent internal corrosion. The tunnel was installed between a shore-based launch shaft, blasted into the bedrock to a depth of approximately 8 m below sea level, and an
exit trench, blasted offshore approximately 10 m below sea level. A tunnel boring machine (TBM) was driven offshore into the exit trench and into the ocean. A specially designed bulkhead door was then sealed shut. A blind flange was installed in the launch shaft and the pipe was flooded. The TBM then disconnected from the pipe string and floated to the surface. Offshore segment The offshore segment consisted of solid-wall HDPE pipe, designed for a ‘float and sink’ installation. To resist the effects of ocean currents, wave-induced energy and buoyancy, a total of 350 reinforced concrete weights, each approximately 11,000 kg, were installed along the pipe at 4 to 6 m apart. Permanent fasteners were constructed of corrosion-resistant materials, such as superduplex stainless steel, silicon bronze and titanium. Designed for installation Design work included determining ocean-induced forces, using 100 years of wind data, as well as current velocities at a nearby monitoring mooring. Stability analysis determined the mass required to keep the pipe stable on the ocean floor. Installation force analysis was also undertaken, as the highest stress the pipe would undergo would be from bending during installation. The analysis determined a safe bend radius during the float and sink operation and determined a pipe ‘end pull’ of up to 100 metric tonnes was required. A multi-port diffuser was designed to achieve an initial dilution of the effluent of 100:1 for low current speeds during unstratified winter con-
October/November 2020
2020-10-05 9:16 AM
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