Photo: Luetta Callaway Southern Nevada’s Water Authority recently unveiled its newest megamachine: a $25-million custom-made hybrid tunneling-boring machine that operates in both the open and closed positions, meaning the drill face is pressurized for more efficient ground and water control. It took Schwanau, Germany-based Herrenknecht AG 17 months to design and manufacture the 1,500-ton, 600-ft-long TBM, which is being used as part of the third raw-water intake tunnel project at Lake Mead. The additional straw is needed since lake levels have dipped 110 ft since 2000, leaving it at half capacity. In March 2008, SNWA awarded a $447-million design-build contract
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.