Robbins TBM Overcomes Multiple Caverns to Make Breakthrough
The obstacles overcome at the recent breakthrough are a significant achievement, said Marc Dhiersat, Project Director of the Galerie des Janots tunnel for contractor Eiffage Civil Construction. “We are proud to have led a motivated and conscientious team to the end of the tunnel who worked well without accidents despite the many technical difficulties encountered.”
The water tunnel, located below the community of Cassis, France, is an area of limestone known for its groundwater, karstic cavities and voids. The limestone, combined with powdery clays, made for difficult excavation after the machine’s March 2017 launch. At the 1,035-m (3,395-ft) mark, the crew hit a cavern on the TBM’s left side.
The cavern, studded with stalactites and stalagmites, was grazed by the TBM shield. The crew had to erect a 4-m (13-ft) high wall of concrete so the TBM would have something to grip against. The TBM was then started up and was able to successfully navigate out of the cavern in eight strokes without significant downtime to the operation — the process took about two weeks. Despite the challenges, Dhiersat thought positively of the TBM throughout the ordeal: “This has been the best machine for the job due to all the geological difficulties.”
Crews probed in front of the cutterhead and began work to stabilize and secure the cavity with foam and concrete, as well as excavate a bypass gallery. “After filling much of the cavity (1,500 cubic m/53,000 cubic ft), our biggest difficulty was to ensure the gripping of the machine: We needed six bypass galleries and four months of work to reach the end of this challenge,” said Dhiersat. For the last 600 m (2,000 ft) of tunneling, “we were finally in good rock,” he emphasized. Overall rates for the project averaged 18 m (59 ft) per day in two shifts, and topped out at 25 m (82 ft) in one day.
Galerie des Janots is one of 14 operations designed to save water and protect resources, which are being carried out by the Aix-Marseille-Provence metropolis, the water agency Rhône Mediterranean Corsica, and the State Government. The Janots gallery, once online, will replace existing pipelines currently located in a railway tunnel — these original pipes have significant deficiencies with estimated water losses of 500,000 cubic m (132 million gal) per year. The new tunnel will increase capacity to 440 liters (116 gal) per second.
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