Encountering Caverns in France: TBM Tunneling in Karst Conditions

France’s recently completed Galerie des Janots featured some unexpected challenges: Karst cavities large and small were found throughout the bore as well as two large, uncharted caverns. During excavation with a Main Beam TBM, crews encountered an 8,000 cubic meter cavern studded with stalagmites and a 4,500 cubic meter cavity directly below the bore path. How did the crew overcome the conditions and continue past the voids?