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French water tunnel construction reveals unexpected caverns

Eiffage Civil Engineering’s work on the Galerie des Janots project in La Ciotat, France has now restarted after the bore encountered an unexpected cavern in the limestone.

The tunnel alignment clipped the edge of the 8,000m3 cavern on the water resources scheme that is being funded by the Aix-Marseille-Provence metropolis, the water agency Rhône Mediterranean Corsica and the state government.

“We hit the corner of it. To cross it, we had to erect a 4m high wall of concrete so the tunnel boring machine (TBM) would have something to grip against,” said Eiffage project director Marc Dhiersat. A small door allowed access inside the cavity, which formed naturally at a point 60m below the surface.

The 3.5m diameter Robbins Main Beam TBM has now successfully navigated out of the cavern and work on the 2.8km long tunnel is now continuing with another 1.8km left to complete.

The project team has said there could be further caverns though.

“It is possible there could be more unknown caverns,” said Dhiersat. “We have a geotechnical Bore-tunnelling Electrical Ahead Monitoring (Beam) system on the machine, which is a ground prediction technique using focused electricity-induced polarisation to detect anomalies ahead of the TBM. We are also conducting probe drilling, shotcreting, and maintenance in a separate shift.”

The cavern is not the first geotechnical challenge encountered on the project as the limestone has weak areas and lenses of powdery clay. These ground conditions have called for the use of resin-anchored rock bolts and concrete rings in bad ground with thicker layers of up to 150mm of shotcrete.

Eiffage has said that it is on track to complete the tunnel within five months and the cavern discovery has not resulted in a significant delay.


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