The world’s longest tunnel, Switzerland’s 57-kilometer-long Gotthard Alpine rail crossing, broke through on Oct. 15. The Gotthard twin tunnels will be the longest of several being built through the Alps on the Milan, Italy, to Basel, Switzerland, corridor. The joint-venture Tunnel AlpTransit-Ticino (TAT) broke northward from its Faido section of the east drive into the awaiting Sedrun stretch, roughly halfway along the tunnel. The west tunnel’s breakthrough is scheduled for next spring. TAT used 8.8-meter-dia Herrenknecht Gripper tunnel-boring machines for most of the excavation. Because of difficult ground conditions on the south drives, including the Faido and Bodio sections, the
Arch dams, classically curved monoliths situated in narrow valleys, are not as common as other types of dams, such as embankment dams and gravity dams. ENR takes a look at some of the biggest arch dams in the world, both completed and under construction.