Work is progressing on the challenging Port Mann Main Water Supply Tunnel, in Vancouver, BC, with the recent completion of the project’s South Shaft. Operations required Ballard Diving & Hyperbaric Services’ commercial divers to descend inside the shaft and perform difficult work in challenging conditions.
Project client, Metro Vancouver, awarded the CA$150 million contract in 2011 to McNally/Aecon JV (MAJV), a joint venture between McNally Construction and Aecon Constructors, a Division of Aecon Group. Designed to replace the existing subaqueous watermain crossing, and provide an earthquake resistant water supply for communities south of the Fraser River, the project involves the construction of a 1km (3,280ft) long tunnel driven at depth of up to 30m beneath the Fraser River between the cities of Coquitlam and Surrey. Groundwater conditions at the tunnel horizon are artesian, making this a challenging project, with expected earth pressures of up to 6 bar.
To facilitate the TBM drive and installation of a 2.1m (6.8ft) diameter welded steel pipe water main, two shafts are being excavated, one on each side of the Fraser River. MAJV started work on the 13m (43ft) diameter South Shaft in late 2011, with Texas based subcontractor Bencor installing 30 slurry wall panels to a depth of 68.8m (225ft) using a Bauer BC40/MC128 Hydro mill.
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Bencor then relocated to the North Shaft. Meanwhile, MAJV excavated the South Shaft using a heavy-duty clam bucket within the flooded shaft. Underwater dive crews have been working on setting the interlocking dowels for the connection of the base plug. The shafts will be finished with a heavily reinforced cast-in-place lining.
TBM excavation will be by completed using a new 3.5m (11.5ft) o.d. Caterpillar EPBM, which will erect a 2.8m (9ft) i.d. fibre reinforced precast segmental liner. The segments are being manufactured at Armtec’s nearby facility in Richmond.
On completion of the tunnel drive and demobilization of the machine from the North Shaft, the 2.1m (6.8ft) i.d. steel watermain will be installed and grouted into place within the tunnel. Finally, the steel watermain will be installed in the shafts and surface cast in place valve chambers constructed. The TBM and associated systems are being designed with the aim of eliminating interventions; however, preparations are being made for hyperbaric entries as a contingency.