Galerie des Janots: TBM hits two uncharted Caverns

摘要

In April 2019, a Robbins 3.5 m diameter Main Beam TBM broke through into open space, completing the 2.8 km long tunnel Galerie des Janots in France. It was not the first time the machine had encountered open space: twice during tunneling, the machine hit uncharted caverns, the largest of which measured a staggering 8000 m~3 in size. The water tunnel is located below the community of Cassis - 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 1035 m mark, the crew hit a cavern on the TBM's left side, studded with stalactites and stalagmites. The crew had to erect a 4 m high wall of concrete so the TBM would have something to grip against. The first cavern, while the largest, was not the most difficult void encountered. The machine was averaging 20 to 22 m advance per day in two shifts after clearing the first cavity, with a dedicated night shift for maintenance. While excavating, a combination of probe drilling and geotech-nical BEAM investigation - a type of electricity-induced polarization to detect anomalies ahead of the TBM - were used. At the 2157 m mark the machine grazed the top of an unknown cavity that extended deep below the tunnel path. The structure measured 22 m long, 15 m wide, and 14 m deep (about 4500 m~3) of open space.