Geologic Characterization, Colorado River Bridge Foundations, Hoover Dam Bypass

The Colorado River Bridge is a key element of the 5.6 km (3.5-mile) U.S. Highway 93 Hoover Dam Bypass Project from Clark County, Nevada, to Mojave County, Arizona. The Central Federal Lands Highway Division of FHWA is the managing partner responsible for project delivery. The planned 579 m (1,900-foot) long Colorado River Bridge will cross a rugged bedrock setting in Black Canyon, 460 m (1,500 feet) downstream of the historic Hoover Dam and 274 m (900 feet) above river level, with a 332 m (1,090-foot) main arch and seven approach spans. Various alternative bridge types were evaluated for the site based on aesthetic, technical suitability and cost criteria; the selected design includes steel box girders, composite concrete deck, and reinforced concrete arch rib and pier columns, foundations and abutments. Geotechnical characterization of the bridge site and particularly the skewback foundations required a specialized approach in the rugged bedrock environment, including 3-D laser scanning of the canyon walls, geologic mapping by multiple methods, core drilling with specialized rigs/access equipment, optical televiewer borehole logging of rock fracture data, in-situ borehole jack testing and down-hole seismic surveys, and laboratory testing of rock strength. Geology of the bridge site consists of Tertiary-age volcanic and sedimentary rocks, with the key skewback foundation areas composed of variable quality but generally low-strength massive tuff and strong but highly irregular basalt dikes.