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Microtunnel project ends with wet retrieval

A combined team from Michels Canada and Michels Corporation completed a tunnel through hard granite from a shaft near the shore of Victoria, BC into the Pacific Ocean. The 2,121-mm (6.95 feet) diameter tunnel will be an outfall for an under-construction wastewater treatment plant that will serve the core area municipalities of Victoria, Esquimalt, Saanich, Oak Bay, View Royal, Langford and Colwood, and the Esquimalt and Songhees Nations on Vancouver Island.

The 117-metre (385-foot) tunnel was completed in a single drive from a 15-metre (50-foot) deep rock wall shaft out into a rock mattress retrieval pad. The ocean depth at the tunnel exit was 11 metres (36 feet). A Herrenknecht AVN 1800 microtunnel boring machine (MTBM) was used to complete the drive.

MTBMs are typically operated remotely from an above-ground control container; however, for this project – given the size of the shaft – much of the ancillary equipment, including the control container, was located in the upper and lower tiers of the shaft.

Because the drive ended below the ocean floor, a wet retrieval was conducted to lift the 96,000-pound MTBM to the surface. During the wet retrieval, divers filled the space between a bulkhead at the front of the tunnel and the MTBM with sea water to equalize internal and external water pressure. Next, hydraulic cylinders push the machine away from the bulkhead. Then, divers attach rigging from a crane on a barge to the MTBM, allowing it to be slowly lifted to surface and loaded onto a barge.

Did you know Michels is one of the most experienced contractors at successfully executing wet retrievals of an MTBM? Click here to watch the video

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