Shani Wallis, Editor

End of the drive and raise pilot hole connection

After a long hard go of it, the vintage TBM has completed its 4.4km long water diversion tunnel for the Ashlu Creek hydropower project in the mountains behind Vancouver, British Columbia. The 4.08m diameter TBM intersected with the pilot hole of the upstream intake shaft in late February and has since been retrieved from its dead-end heading. This week (early April 2009), excavation of the 3.8m diameter x 120m deep raisebore shaft began and is expected to take about two months to finish.

“Completing the TBM drive was a great milestone to achieve,” said Serge Moalli, Project Manager for contractor Frontier-Kemper. “Conditions for the many-times refurbished TBM were tough with rock strengths of up to 250-300MPa encountered in different reaches and broken rock conditions in the six predicted fault zones on the alignment, three of them major. There were also mechanical issues that caused regular maintenance downtimes. In additon our progress was not as high as the 20m/day we had planned. It was about 6-10m a day on average, but it did get through.”

Grand Old Lady retrieved

The tunnel is the headrace for a 50MW run-of-the river hydro installation for the Innergex Renewable Energy Inc company which operates other wind and hydro power stations in British Columbia.

On being awarded the contract to drive the tunnel, Frontier Kemper brought out of retirement a WIRTH TBM of 1986 vintage that it refurbished and shipped to the site in early 2007. The 4.08m diameter cutterhead of the TBM had 30 x 17in cutters and an installed power of 1,200kW to attack the strong granite and its highly abrasive quartz content.

The Vancouver office of Hatch Mott MacDonald is design engineer for Frontier Kemper on the design-build contract. With no subsurface site investigation allowed for the maximum 600m cover in the natural environment, the contract’s GBR was based on design studies and surface mapping. Competent granite with compressive strengths in excess of 225MPa predicted a drive with little immediate support except in the identified fault zones, and an unlined finish. Rockbolts, mesh, timber lagging and steel sets were used to pass through the first faults. After being installed, the invert section of the steel sets was cut out to allow the trailing gear to come through and tracks for the rail-bound service trains.

“There was little water ingress,” said Moalli, “with a total 70-80 gal/min measured at the portal at the end of the drive. Pattern rockbolting was used to control pressure-release slabbing beneath the 600m of cover.” Wire mesh, lagging, channels and steel sets with the inverts subsequently cut out, supported conditions in the fault zones. “The tunnel will remain largely unlined,” said Moalli. Shotcrete will be applied to line the tunnel across the total 100m or so of the fault zones and to cover the bolt heads.

With the raisebore anticipated to take about two months, Moalli is expecting a clean up and contract sign off in about June. From Ashlu Creek, Moalli and his tunneling team will go over to the site of the Seymour Capilano water tunnels project where Frontier Kemper in joint venture with J.F. Shea and Aecon has been awarded the contract to pick up and continue the twin TBM drives after the original contract was terminated in January 2008. Successful completion of the Ashlu Creek drive sets up the tunneling team with valuable experience to finish off the Seymour Capilano project. The other common factor on both projects is Hatch Mott MacDonald, which designed both tunnels projects.

Reference

Twin tunnel restart comes at a high price – TunnelTalk, Apr 2009

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