When the Hoover Dam was completed in 1935 it was rightly hailed as an engineering marvel. The massive gravity-arch dam – then known as Boulder Dam – became the world's largest electric power producing facility and the world's largest concrete structure.
Less noticed at the time was the fact the two-lane roadway atop the dam finally completed a direct road link between Las Vegas, Nevada and Phoenix, Arizona – U.S. Highway 93.
Within a decade, the Grand Coulee Dam in Washington State eclipsed the Hoover Dam in its record-breaking categories but the importance of the highway continued to grow as more people began moving to the American Southwest.
Today, more than seven decades later, the highway is in dire need of upgrade and another historic structure is in the making to achieve that. Just 460m downstream from the dam a massive steel and concrete single arch-span bridge is being built.
When completed in 2010, the structure 270m above the Colorado River will support a four-lane highway and become the longest concrete span in North America.
The project is being built under the direction of The Federal Highway Administration (FHA). The primary contractors for the work on the bridge are California based firms Obayashi and PSM Construction whose joint venture won the job with a £57M ($114M) bid in 2004.
The bypass project was originally slated to be completed this year at a cost of £120M but delays including an accident involving the crane system needed to build the massive arches altered the deadline and have claimed the full £3M contingency budget on the bridge section.
The formal name of the 580m span is The Mike O'Callaghan-Pat Tillman Memorial Bridge. O'Callaghan was a former governor of Nevada and Pat Tillman was a player for the Phoenix's NFL football team who was killed in Afghanistan.
The type of bridge was selected by a design team lead by HDR Engineering of Omaha, Nebraska. Of eight possible alternatives, the team, which also included Jacobs Engineering unit Sverdrup Civil and T.Y. Lin International, chose the arch bridge.
"The arch has a number of advantages," explains FHA project manager David Zanetell.
"It's truly the classic solution for a canyon face type crossing, it has significantly lower long term operation and maintenance costs and it has a very low vulnerability to threats - environmental and malicious."
Keeping the look of the bridge harmonious with the historic dam was a key priority, he adds. The shape of the arch echoes the curvature of the dam and the shared building material, concrete, help achieve that. That said, the bridge also had to be contemporary as well.
"And the arch does, in many ways, provide a link from the past to the present," Zanetell says.
Almost all of the elements of the bridge except for the arch itself are precast off-site. The individual arch segments will be poured in place. Each of the arches will be constructed of 53 cast-in-place concrete segments – 12 base segments, 40 cable supported segments and a final connecting segment.
The initial six segments on each side of each arch are being post-tensioned and extend 46m out from the canyon wall. Internal guy wires pull the sections back to the cliff face which enables them to be self supporting. Work on these segments is expected to be complete by summer, Zanetell says.
The remaining sections will be supported by external guy wires held aloft by two temporary stay towers on each side of the bridge. As the section is laid, it will be strapped to the precast concrete tower and held in place so the next section can be set – essentially creating a cable-stayed bridge. A final segment – or a closure pour – will connect the two sides of the arches.
According to Zanetell, the precast towers will be put in place by June so the work on the arches can begin.
The high line crane system will be used to deliver and erect the pre-cast spandrel segments. The columns which have been precast will be brought in from overhead using the crane system, set and then stacked on top of each other.
The span itself will require 15,200m3 of unusually high-strength C70/85 concrete. To allow better control over the construction, a high energy or 'pan' concrete mixer was installed on site.
"It is a real advantage in terms of quality in terms of the actually produced material," Zanetell said. "We also get at the reliability of the delivery itself and the ability to then make revisions live time and not have any material en-route."
To do the extensive work above the canyon, the contractors opted to use a pulley-type, high-line crane system. The system consists of two sets of 100m tall towers secured by staybacks and concrete foundations.
The set of 760m long cableways would then have the ability to carry loads up to 45t.
The contractors purchased a 42 year old system from American Bridge Company for the bypass bridge job. The system had been built to erect West Virginia's New River Gorge Bridge through 1977 and was taken out of service a decade later. Refurbishing the cranes and producing a second set for the bypass job cost £5M.
High line cable cranes have a number of very specific drawbacks including limitations in how they deliver material, slower speeds and availability of the hook. But when you factor in the demands of the location, they are a solid choice, Zanetell says."The advantage is they are providing you continuous access from above your whole jobsite," he says.
On 15 September 2006, the crane system collapsed. Although no-one was injured traffic on the highway was detoured through Laughlin for three days.
According to the agency, one of the reconditioned cranes failed first and caused the entire system of support cables to fail bringing the rest of the towers down. Winds through the canyon at the time of the accident were at approximately 90km per hour – not unusually strong for the location.
The cause of the incident remains under investigation. Preliminary assessments of the accident indicate one tower on the Nevada side collapsed causing a failure in the support cable system and leading to the collapse of the other towers.
The contractors are expected to absorb the cost of the tower collapse and the costs associated with the delay to the project will be absorbed by the contingency fund, Zanetell says.
The main work on the arch itself was completely halted but to keep working on other aspects of the bridge near the canyon walls two large derrick cranes were brought in to the job – a Manitowac 2250 and an S70 Derrick. As a result, all of the column work on the canyon walls has been completed.
A new crane system has been designed, fabricated and constructed for the job and is now completing final testing before being put into operation.
Within the next few weeks, Zanetell says he expects to complete testing of the new system and begin the difficult work on the arches out over the canyon. As frustrating as the delays to the project have been, the one advantage is they have allowed all of the preliminary work to be completed and finished.
"So now this job is ready to rock and roll," he says. "Everything is done and we are ready to go."Key Statistics
Height of the arch crown above the Colorado river
Length of bridge main span
Total length of bridge
THE HISTORY OF THE HIGHWAY
At the time the Hoover Dam was built, Las Vegas was only a quarter-century old and had just 7,500 residents. Phoenix, by contrast, was celebrating 50 years since being incorporated as a city and boasted more than 48,000 residents.
A lot has changed since then. Today, Phoenix and Las Vegas are two of the fastest growing cities in the United States. As of 2006, there were about 4M people living in the Phoenix metropolitan area and about 1.7M reside in the Las Vegas metro region. And as the urban centres have expanded, so has the traffic between them. Currently, the road handles more than 16,000 vehicles daily.
Worse, every day the two-lane road is beset by hundreds of visitors whose numbers swell to the thousands during peak periods. Speed over the dam itself is capped at 15mph but often is slower than that as visitors wait for parking spaces.
"The accident rate within the limits of the bypass itself is three times the accident rate on the rest of the highway," Zanetell explains.
The search for a solution to the highway problem dates back to the 1960s when the U.S. Bureau of Reclamation began working with officials from Arizona and Nevada to examine the problem. Their solution was for a crossing 1.6km south of the dam but the project languished.
In 1997, officials from Nevada and Arizona appealed to the US Department of Transportation for funding to restart the project. The FHWA took over as lead agency for the project. But the price tag allied with the unavailability of funds hampered progress.
The initial cost estimates for the project jumped substantially after a geotechnical survey, conducted by AMEC and Environmental Inc., found the canyon walls required substantial excavation.
There was a lengthy delay getting approval by congress. But in 2004, Arizona and Nevada each pledged Ł24.6M in bonds to allow the bypass work to move forward.