Second Narrows Water Supply Tunnel: Slurry Wall Construction

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Project Update by J. Andrew McGlenn, P.Eng., PE, SE

Construction is now well underway on this 1.1-kilometer (0.7 mi) tunnel project. The Second Narrows Water Supply Tunnel follows on the completion of the Port Mann Water Supply Tunnel in a series of seismic and marine upgrades being carried out by Metro Vancouver in the Lower Mainland of Vancouver, BC. McMillen Jacobs Associates served as Prime Designer for the shafts, tunnel, and pipeline design on a design team that includes Golder Associates and AECOM.Construction of the tunnel and its two shafts is being performed by a joint venture between Aecon Constructors and Traylor Infrastructure Canada. As Engineer of Record, McMillen Jacobs is providing Construction Engineering Services.

The 6.5-meter-diameter OD (21 ft) tunnel will house three water supply mains: the two 2.4-meter-diameter (8 ft) Seymour Mains and the 1.5-meter-diameter (5 ft) Capilano Main. The Second Narrows Water Supply Tunnel replaces the existing aging mains that convey drinking water under Burrard Inlet from North Vancouver to Burnaby, just east of the Ironworker’s Memorial Bridge.

The 16-meter-diameter (52 ft), 67-meter-deep (220 ft) North (launch) shaft—located within Metro Vancouver’s Beach Yard maintenance facility—is being constructed in alluvial and glacial soils. This challenging geology extends out underneath Burrard Inlet, where it transitions to sedimentary bedrock that continues to the 10-meter-diameter (33 ft), 110-meter-deep (360 ft) South (receiving) shaft in Second Narrows Park, Burnaby.

Slurry walls provide the initial support of excavation for the North shaft. During Final Design, this element was identified as one of the most challenging aspects of the project, due primarily to the coarse granular nature of the soil deposit, which constitutes the upper 35 meters (115 ft) of ground. Panel excavation began in April 2019. The general construction sequence involved excavating the upper portion with a mechanical grab (clam) down to about 35 meters, backfilling with lean mix, and then re-mining with a hydromill and continuing to the target depth with the second pass. The shaft interior is now being excavated and, if all goes well, the base slab will be poured “in the wet” before Christmas.

The tunnel boring machine (TBM) is currently being manufactured by Herrenknecht in Germany. It will be a mixshield (slurry) machine designed for a working pressure of 8 bar. TBM assembly in the shaft is scheduled to begin in early 2020 with anticipated launch in April 2020.