The International Information Center for Geotechnical Engineers

Friday, 13 July 2018 01:00

Unexpected cavern in tunnel construction in France

Unexpected cavern in tunnel construction in France Unexpected cavern in tunnel construction in France.

A Tunnel Boring Machine successfully passed through an unexpected cavern during its drive on the Galerie des Janots water tunnel project in La Ciotat, France.

The Galerie des Janots is part of a project to save water and protect resources being carried out jointly by the Aix-Marseille-Provence metropolis, the water agency Rhône Mediterranean Corsica, and the State Government. The TBM, launched in 2017 by contractor Eiffage Civil Engineering, was 1km into the drive when the cavern, studded with stalactites and stalagmites and measuring 8,000m3 was discovered to the left of the planned route. Eiffage project director Marc Dhiersat states: "We hit the corner of it. To cross it, we had to erect a 4m high wall of concrete so the tunnel boring machine would have something to grip against". A small door allowed access inside the cavity, which formed naturally at a point 60m below the surface. The TBM was able to successfully navigate out of the cavern in eight strokes and without significant downtime to the operation. "Limestone is a rock easy to excavate but there is the possibility of being confronted with the phenomenon of karst features,'' explains Loïc Thévenot, Director of Underground Works for Eiffage. To cope with the risk, the TBM is equipped with a probe drill and equipment to backfill small karst features. 

Karst* cavities were a known risk during the bore, but the cavern was not shown in vertical borehole reports conducted from the surface along the alignment. With 1.8km still remaining to excavate before the tunnel is complete, the possibility of new caverns encountering is not negligible. Therefore, the machine was fitted with a geotechnical BEAM (bore-tunneling electrical ahead monitoring) system to predict ground conditions using focused electricity induced polarization.

The TBM will bore primarily through limestone to excavate the Janots gallery in order to improve access to water in the Cassis, Roquefort-la-Bédoule, La Ciotat and Ceyreste communities east of the Aix-Marseille-Provence metropolis. The 2.7km long drive will pass under Le Parc National des Calanques and beneath a cover of between 15m and 180m.

* Karst is a topography formed from the dissolution of soluble rocks (such as limestone, dolomite, and gypsum). It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant rocks, such as quartzite, given the right conditions.


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