A century or two ago, the Black Canyon through which the Colorado River flows was fit for neither man nor beast. Whether by foot, horse, or covered wagon, its pronounced walls could hardly be traversed by the early settlers. The Black Canyon, named for black volcanic rock which formed the steep walls millions of years ago, was truly inhospitable.
But in the early 20th century, the West started to grow by leaps and bounds. The population of Los Angeles, less than 300 miles away, grew more than 10-fold from 50,000 in 1890 to 576,000 in 1920. And it would double to 1.2 million by 1930. The need to irrigate Western farmland and provide electricity to growing cities led to the construction of Hoover Dam at the Black Canyon from 1931 to 1935.
Fast forward seven decades and more growth forced further change. People had moved to Nevada and Arizona in great numbers, mostly from the Northeast and Midwest. Anyone driving to Las Vegas from Phoenix, Flagstaff, Kingman, or other Arizona cities was funneled onto the two-lane U.S. Route 93 directly onto the dam. The traffic, averaging 14,000 vehicles a day, caused tie-ups which sometimes lasted hours.
Officials planned a solution, which finally has come to fruition. The bypass, formally dedicated as the Mike O'Callaghan-Pat Tillman Memorial Bridge, opened Oct. 19, 2010, 75 years after President Franklin D. Roosevelt dedicated Hoover Dam, the other concrete icon just 1500 feet away. Those settlers' jaws would have dropped at the sight of the 726-foot-tall dam. So what would they think of the dam's concrete cousin right next to it, the 1060-foot-long gem 900 feet above the Colorado River?
The plan was on the drawing board long before the September 2001 terrorist attacks; it received the official go-ahead in March 2001. Keeping traffic off the bridge for security reasons is a secondary goal of the project. The original road is still used, but only by motorists visiting the dam. While the twin arch bridge gets the most attention, the bypass project also includes 3.5 miles of roads, smaller canyon crossing bridges, powerline tower relocations, and wildlife crossings in Nevada and Arizona.
A project management team consisting of several government agencies oversaw the project, with the Central Federal Lands Highway Division acting in the lead management role for all elements of procurement, design, and construction.
Located so near the dam, there was little doubt that the bridge would be concrete and not steel. The recommendation of a design advisory panel was nearly unanimous. “The state historical preservation officer in Arizona was quite assertive that we needed a design that complemented the dam,” says David Goodyear, chief engineer of record for T.Y. Linn International and designer of the bridge. “It was certainly destined to be concrete.”
While site excavation removed 60,000 cubic yards of earth, segments for the vertical columns were cast at a Casino Ready Mix plant about 10 miles away. These formed the vertical columns connecting the arch to the deck. Casino Ready Mix was eventually purchased by Silver State Materials and then acquired by CalPortland.
For the arch, the most intricate portion of the project, “cast-in-place was the only way to go,” Goodyear says. “We allowed a precast option for the arch and one bidder took it a pretty long way. But when the sections get as big as they are for this arch rib, handling the logistics of precast gets to be complex and expensive.”