Second Narrows Water Supply Tunnel

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Project Update by Greg Emslie, PEng

The Second Narrows Water Supply Tunnel project broke ground in early 2019, and substantial completion is expected in late 2023. It replaces three of Metro Vancouver’s existing water mains between North Vancouver and Burnaby (“the Second Narrows Crossing”). These mains convey drinking water from the North Vancouver water sheds, across the Burrard Inlet, and into the City of Burnaby, where it is distributed to Metro Vancouver’s municipalities.

McMillen Jacobs is engineer of record for all underground structures and prime consultant for the design team, which includes AECOM and Golder. During construction, we have been providing design services and full-time construction inspection services.

Support of excavation (SOE) includes a 60-meter-deep, 16-meter-diameter (197 x 52 ft) TBM launch shaft in North Vancouver; a 5.8-meter-diameter (19 ft) segmentally lined tunnel under Burrard Inlet; and a 110-meter-deep, 10-meter-diameter (361 x 33 ft) TBM reception shaft in Burnaby. Permanent works include three welded steel pipes installed in tunnel and shafts; a reinforced concrete liner in the North Shaft; and a valve chamber on either side of the crossing.

Significant construction progress is being made, and several major project milestones have been met:

North (TBM Launch) Shaft Completed December 2019: Shaft SOE includes 22- to 80-meter-deep (72–262 ft) unreinforced concrete slurry panels, installed by hydromill in a challenging area of sandy soils with a high cobble content and high hydraulic conductivity. See a previous Second Narrows update for more details.

TBM Launch Completed September 2020: The 6.64-meter-diameter (21.78 ft) Herrenknecht mixshield (slurry) tunnel boring machine (“Lynn-Marie”) is the first mixshield TBM in Canada. The mixshield method is well suited for the expected pressures and geology. The machine is required to provide about 6.5 bar of support pressure but is designed for up to 8 bar. The bentonite slurry provides excavation support at a pressure controlled by mechanical means via a compressed air bubble. The slurry circuit is a closed system, and bentonite slurry is constantly cycled in and out of the excavation chamber while mining. Bentonite acts as a transport medium for excavation cuttings, which are piped out of the tunnel, separated at a treatment plant, and recycled back into the system.

The TBM was launched from a pressurized “launch lock” with no ground improvement outside the shaft. Therefore, the launch lock was required to seal against the full face support pressure (about 6.5 bar) prior to breaking through the shaft wall and to provide support for TBM thrust and rolling action.

Tunneling Progress:
The tunnel drive consists of three reaches: Reach 1, at about 525-meters (1,722 ft), is mostly sand and silt mixtures; Reach 2 is a 145-meter (476 ft) transition zone from soil to rock with possible nested cobbles and boulders; Reach 3 is a 415-meter (1,362 ft) drive through mostly weak sandstone. In July 2021, the TBM mined steadily through the Reach 2 transition zone, into the Reach 3 sandstone, and stopped for an intervention. Little wear on the cutterhead structure and tools was observed through the first 690 meters (2,264 ft) of tunneling, but most of the discs were replaced for the remainder of the drive. The TBM is expected to breakthrough at the South Shaft in early September 2021.

South Shaft Progress: Shaft lining and excavation are complete. The shaft was excavated through rock, using conventional methods, and its lining consists of synthetic fiber-reinforced shotcrete and steel ribs. The contractor recently installed the TBM break-in structure at the shaft base and performed pre-excavation grouting around the tunnel eye for TBM reception.