Hoover Dam Bypass - Colorado River Bridge geotechnical characterization

摘要

A new transportation facility, the Hoover Dam Bypass, has been constructed to greatly enhance mobility in the vicinity of the historic Hoover Dam. The bypass carries through-traffic off the dam and onto a new four-lane roadway with a new bridge over the Colorado River, located approximately 1/4-mile downstream of Hoover Dam. The new Colorado River Bridge also known as the Mike O'Callaghan-Pat Tillman Memorial Bridge spans the Black Canyon, an 800-foot-deep steep-sided gorge carved by the Colorado River. The completed structure is a 1,900 foot long concrete composite bridge with a main concrete arch spanning 1,090 feet across the central section of the canyon. The structure includes five approach spans on steep slopes approaching the vertical walled canyon on the Nevada side and two on the Arizona side, which range in length from 100 to 120 feet. Arch skewbacks were constructed on rock benches carved into the canyon walls at an elevation of 1200 feet. The Hoover Dam Bypass includes five bridges on the 2.2 mile long Nevada approach and two bridges on the 1.1 mile long Arizona approach. Leadership for the project design and delivery team was provided by Dave Zanetell, Program Manager for FHWA-CFLHD and he was supported by HDR Engineering, Inc., Sverdrup Civil, Inc. and T.Y. Lin International designers. The geotechnical design was provided by AMEC Earth & Environmental, Inc., and supported by Crux Subsurface, Inc. for all the exploratory drilling. The site lies in a very rugged mountainous terrain with vertical canyon walls in the lower portion of Black Canyon. This terrain, coupled with overhead transmissions lines that parallel the proposed alignment, limited conventional access to the site. Geotechnical investigation of the bridge site required a highly diverse and specialized approach to data collection in the rugged bedrock environment. The approach included 3D laser scanning for topographic mapping of the canyon walls, geologic mapping by multiple methods, 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. The multiple methods of performing geologic mapping were crucial in understanding the scale of the site geometry and evaluating the distribution of rock units. The techniques included conventional ground reconnaissance, aerial inspection with the use of a helicopter, 3D point cloud reviews and cliff wall inspections in the steep canyon walls using mountaineering rappelling techniques.