The Second Narrows Water Supply Tunnel project will replace three existing water mains that were installed in 1948, 1954, and 1978. The 1.1-kilometer tunnel will cross under the Burrard inlet from north Vancouver to the City of Burnaby. The new water mains will be nearly 100 feet below the inlet and carry twice as much water as the existing mains that are seismically vulnerable and operating near full capacity.
Two tunnel shafts will be constructed. One is approximately 200 feet deep, located in Metro Vancouver’s yard in North Vancouver, immediately north of the inlet and east of the Seymour River. The other 360-foot-deep shaft is situated in a forested area of Second Narrows Park on the south side of the inlet in Burnaby.
The north shaft is 200 feet deep and 52 feet in diameter. Tremie concrete will be placed in the shaft, allowing for dewatering and the bottom 50 feet of the shaft lining concrete to be placed. The new Herrenknecht slurry tunnel boring machine (TBM) is being assembled simultaneously on the surface. Due to the small diameter of the shaft the TBM will be launched incrementally, initially with the shield in the shaft and trailing gear on the surface. As tunneling progresses, the trailing gear for the TBM will be lowered into the shaft in two stages until fully assembled. Tunneling is expected to take nine months.
Three steel water mains — two approximately eight feet in diameter and one five feet in diameter — will be installed in the tunnel, which will be 21 feet in diameter inside.
The new mains will be connected to the existing system in new valve chambers and restoration will take place at Second Narrows Park between fall of 2023 and spring of 2024.
Additional key technical challenges include tunneling through variable soft ground conditions including silt, cobbles and sand, and weak rock with a 2.1-foot-diameter mixed shield TBM at 6.5 bar, construction of a safe haven via ground freezing, hyperbaric cutterhead inventions via saturation diving at 6.5 bar, and wet excavation of the north valve chamber.
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