Geologic Characterization for Bridge Foundations, Colorado River Bridge, Hoover Dam Bypass Project

摘要

Increasing congestion on US 93 (a NAFTA route) at Hoover Dam resulted in the 3.5-mile Hoover Dam Bypass Project from Clark County, Nevada, to Mojave County, Arizona. CFLHD-FHWA, the managing partner responsible for project delivery, awarded the design to the Hoover Support Team, led by HDR Engineering with T. Y. Lin International, Jacobs Civil and subconsultants, including AMEC for geological and geotechnical engineering services. The 1,900-foot long Colorado River Bridge is planned in a rugged bedrock setting in Black Canyon, 1,500 feet downstream of the dam and 900 feet above river level, with 1,090-foot main arch and seven approach spans. Bridge components are to include steel box girders, composite concrete deck, and reinforced concrete arch rib and pier columns, foundations and abutments. Geology of the bridge site consists of Tertiary-age volcanic and sedimentary rocks. Foundation areas feature basalt flows, local breccia, massive tuff and highly irregular basalt dikes. Geotechnical investigation of the bridge site required a highly diverse and specialized approach to data collection in the rugged bedrock environment. Included were 3D laser scanning for topographic mapping of the canyon walls, geologic mapping by multiple methods (including with mountaineering techniques), core drilling with specialized rigs/access equipment, optical televiewer borehole logging to acquire fracture data, NX borehole jack testing and down-hole seismic surveys, and laboratory rock strength testing. Analysis of distribution of rock types and capacities in the various foundation areas, and excavation slope designs and stabilization designs, were performed using the collected surface mapping, rock quality and fracture information, and televiewer, in situ strength and seismic data.