Contractors on the centerpiece of San Francisco’s $4.6-billion upgrade of the Hetch Hetchy water system, a $215.3-million tunnel under San Francisco Bay that will replace the 1920s-era pipeline structure, were told to start work on April 1. Officials say the seismically enhanced structure and others planned will better protect the city water supply in an earthquake, such as the Southern California temblor on April 4. Photo: San Francisco Pulbic Utilities Commission Image shows underwater pipeline and tunnel, which will have new seismic protections. The five-mile-long tunnel will replace old structures with a 9-ft-dia welded steel pipeline bored as deep as
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.