Metro Vancouver is constructing the Second Narrows Tunnel which will act as a new water supply under Burrard Inlet, east of the Ironworkers Memorial Bridge in British Columbia. This project is one of five new regional water supply tunnels that are being designed to meet current seismic standards to ensure the reliable delivery of drinking water in the region in the event of a major earthquake. When complete, the tunnel will also increase the capacity of the existing system to meet the long-term needs of the growing population.

The second narrows tunnel is 3,609 ft. long tunnel with a diameter of 22 ft. excavated deep underground using a tunnel boring machine. The tunnel boring machine will launch from a vertical entry shaft on the North Vancouver side and progress south under Burrard Inlet to the vertical exit shaft on the Burnaby side. The 50 ft diameter entry shaft (North Shaft) will be 275 ft. deep and had to be constructed through very challenging alluvial deposits that contain high percentage of cobbles and boulders with groundwater level being right at ground level. Due to very tight verticality tolerances of only 0.4%, the design of the support of excavation (SOE) was based on a 48 in. wide un-reinforced diaphragm wall ring. We employed state-of-the-art hydro cutter equipment in combination with real time verticality monitoring systems to meet these strict requirements. The difficulties the subsurface conditions posed onto the team were overcome by quickly adopting means and methods to stabilize such loose geotechnical formations.

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Ihab Allam –