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This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: March/April 2003|
Issue No: Vol. 66 No. 5
Date: March/April 2003
Building a bridge that will overlook Hoover Dam—and enhance it—is a once-in-a-lifetime engineering challenge.
Millions of people visit Hoover Dam every year to marvel at what is often called one of the engineering wonders of the world. They drive there on a highway that crosses the Colorado River on the crest of the dam. Just downstream from this national historic landmark, only 458 meters (1,500 feet) to the south, construction has begun on a new road and bridge over the river gorge. The new route will provide a bypass for through-traffic from Arizona to Nevada and relieve concerns about congestion, safety, and security on the existing highway.
As this aerial rendering shows, construction of the bridge will involve the challenge of designing a complement to the magnificent Hoover Dam.
The type study for the new bridge paints a striking word picture of the setting for the bypass: "The Black Canyon below Hoover Dam is a 244-meter (800-feet)-deep gorge carved by the Colorado River through a rugged, hard rock landscape forged by eons of geologic transformations."
Tourists walking on the crest of Hoover Dam and boaters upstream on Lake Mead will see the new bridge soaring across this dramatic canyon. Visitors downstream strolling across the new bridge's pedestrian walkway will have an equally breathtaking view of the dam.
Since its construction 70 years ago, Hoover Dam has been recognized internationally as the standard of excellence in dam design. As the type study states, "Hoover Dam is a [magnet] for millions of visitors in large part because the designers and builders of the Dam expressed an art for engineering that went beyond the austere and functional."
No pressure here for the new bridge's designers and builders—all they have to do is create a span that will frame Hoover Dam for future generations with the same grandeur, elegance, and grace.
A Partnership for Safety, Mobility, and The Environment
The Federal Highway Administration's (FHWA) Central Federal Lands Highway Division is the lead agency for the bypass project in a multiagency cooperative effort by the Arizona and Nevada departments of transportation (DOTs), the National Park Service, Western Area Power Administration, and the Bureau of Reclamation. Nearby communities, Native American tribes, and individual citizens are taking active roles as well. The diverse range of groups makes the bypass project truly a multiple stakeholder partnership.
Construction of this historic project is now underway. At the bridge's groundbreaking ceremony on October 21, 2002, FHWA Administrator Mary E. Peters spoke from the observation deck of the Hoover Dam Visitors Center. After mentioning that she worked on the Hoover Dam Bypass project for many years as director of the Arizona DOT, well before she served as FHWA administrator, she continued, "At FHWA, we have three key priorities [saving lives, reducing congestion, and protecting the environment], and all three are represented in this project."
To address these vital few priorities, the new project will meet current roadway design criteria for safety, improve travel time for through-traffic, and reduce air pollution caused by the current congestion.
The 5.6-kilometer (3.5-mile) section of U.S. 93 on either side of Hoover Dam has a crash rate three times that of the rest of U.S. 93. The two-lane highway is narrow, steep, and winding with switchbacks and hairpin curves. The roadway widths are inadequate, the sight distances poor, and the shoulders too narrow for traffic volumes that are double those of 15 years ago.
What's more, once the highway reaches the dam, pedestrians crossing the road to view the facility are at risk. The daily mix includes more than 3,500 pedestrians competing with 10,000 cars and 1,700 trucks for use of U.S. 93. As Dave Zanetell, FHWA project manager for the Hoover Dam Bypass, says, "The safety concerns are exacerbated by the fact that this facility is trying to serve two purposes, so that an unsafe blend of pedestrians and through-traffic exists. An estimated six million visitors a year walk on the deck of the dam, so you have a melee of people and traffic."
Fixing a Bottleneck
In her speech at the visitors center, Administrator Peters pointed out that traffic on the road sometimes backs up for more than 24 kilometers (15 miles), with motorists delayed by as much as 5 hours. U.S. 93 is a major commercial route carrying traffic from Arizona to Nevada and Utah, connecting Phoenix and Las Vegas (our Nation's two fastest-growing cities over the past 10 years). In addition to interstate commerce, U.S. 93 is a designated North American Free Trade Agreement (NAFTA) route—a critical link carrying international commerce on the north-south CANAMEX corridor between Canada and Mexico.
"Hoover Dam itself was never intended to handle this type of traffic and traffic volume," says Zanetell. "Its use as a transportation facility simply evolved out of its original functions." As early as the mid-1960s, the Nevada DOT and Bureau of Reclamation began pursuing efforts to bypass the dam.
"The volume of traffic is such that, for nearly 3 hours out of every day," says Zanetell, "we can anticipate a minimum of a 30-minute or longer delay for motorists crossing the project site."
Given this inadequate capacity, if the bypass were not built, traffic volumes would increase the delay's time frame to 10 hours a day by the year 2027. "We are in a frequent state of absolute gridlock," says Zanetell. "As a major commerce corridor, this is inefficient and costly to society."
Traffic backs up on the existing road over Hoover Dam, and police help pedestrians cross the road.
The delays not only represent increased costs to trucking companies and the firms that ship and receive goods and materials, but congestion also affects air quality and noise levels. Pollution from gridlock lessens the experience of the national historic landmark for motoring tourists, boaters on Lake Mead, and rafters on the Colorado River.
The bypass project had extensive environmental review. Overcoming a number of fiscal hurdles and environmental restrictions required a cooperative effort by all stakeholders. The project team worked with the U.S. Fish and Wildlife Service to assess the potential impacts on wildlife species of concern, such as the Mojave desert tortoise, a federally listed threatened species.
The Fish and Wildlife Service found that the bypass is not likely to jeopardize the tortoise but stipulated several mitigation measures just in case. Qualified biologists will conduct preconstruction population surveys and will relocate individual tortoises from the construction zone. To preserve tortoise habitat, construction workers will minimize soil compaction, erosion, and vegetation destruction. Disturbed land will be revegetated.
To minimize impacts on desert bighorn sheep, construction workers will place fencing along the bypass corridor to guide the sheep to wildlife crossings. Before, during, and after construction, biologists will monitor peregrine falcon breeding pairs and will protect bald eagle perch sites, but the route selected is expected to have no impact.
Desert washes are being protected, and use of best management practices will safeguard water quality from hazardous spills and roadway runoff. Proactive measures will minimize rock-fall during the bridge's difficult foundation excavation.
"The project is unique in that it will pass through an environmentally sensitive area within Lake Mead National Recreation Area and in direct proximity to a national historic landmark," says Zanetell. "So we created a design advisory panel that includes representatives of a number of agencies and specialties [architects, engineers, historic preservationists, landscape architects, and a Native American representative] to help us develop a facility that will be distinctive yet complement Hoover Dam."
Citizens at the Hoover Dam Visitors Center viewed an exhibit on the new bypass and voted on alternatives.
From the earliest days of the bypass project, protection of Hoover Dam has been a real concern. Prior to September 11, 2001, freight-carrying vehicles routinely crossed the dam carrying a variety of materials. A major catastrophe could involve innocent bystanders, result in millions of dollars in property damage, contaminate the Colorado River and the stored waters of Lake Mead, and interrupt the Southwest's power supply.
Although the project already was on an aggressive schedule, the tragedy at the World Trade Center created a further sense of urgency. The road over the dam was completely closed to traffic immediately, and it still maintains some vehicle restrictions. After September 11, the already-expedited schedule was reevaluated to further advance it.
Administrator Peters noted during her speech: "With the new bridge, the potential for a catastrophic accident at the dam will be lessened."
The new bridge will be a composite concrete deck arch, as pictured in this computer rendering.
Selection of the Route
Completion of the final environmental impact statement in 2001 gave the green light to move forward on selection of the best alternative route. Through a public involvement process, the public favored the preferred alternative by a 3-to-1 margin. Given the public's preference and weighing each alternative against a set of evaluation criteria, FHWA chose the route that is best suited to minimize impacts on wildlife, noise, public safety, air quality, and traffic circulation.
The bypass project will include construction of a new high-speed four-lane highway, canyon crossings, grade separation structures, and a signature long-span 580-meter (1,900-foot) bridge over the Colorado River. The bridge will be the longest concrete arch span in the United States.
The project design includes a parking lot off the Hoover Dam access road, a trail to the new bridge, a pedestrian plaza, and a sidewalk on the upstream side of the bridge. The existing road will continue to provide direct access to the dam for tourists and Bureau of Reclamation workers, but no through-traffic.
Following completion of the environmental process, the project team selected a design team led by HDR Engineering with Jacobs Civil and T.Y. Lin International. The design work began in July 2001 and is expected to be complete for all elements of the project by fall of 2003. Construction is currently ongoing on the Arizona approach portion of the project, with the Nevada approach slated to begin in June and construction of the Colorado River Bridge scheduled to begin in the fall of 2003. Final completion of the project is expected by 2007.
Innovative Bridge Design
The project team screened five distinct structure types for further evaluation: deck arch, cable-stayed structure, suspension, cantilever girders, and trusses. The team selected the deck arch configuration because it best meets the technical challenges of the project, complies with the environmental impact statement commitments to minimize the view impacts, and is fiscally advantageous. A deck arch bridge also provides opportunities for using different options for materials.
Given the long span and deep gorge, the bridge will have to be constructed from above using high lines and cranes. The team selected a combination of a twin rib concrete arch and steel superstructure. This concrete composite design blends the economy of a concrete arch with the light weight and speedy erection of a steel deck structure. The blend of steel and concrete allows the arch construction to begin while the steel for the superstructure is being fabricated and delivered to the site. In addition to the advantage of an accelerated schedule, the span-to-rise ratio is ideal for concrete, using it to its best advantage in compression, while steel is used to its best advantage in bending.
This blend of materials adds greater design flexibility to control costs. The construction estimates show that the concrete composite is projected to be the least costly alternative, at a total of less than $85 million for the arch, spandrel and approach columns, and deck structure, and a total estimated project cost of $234 million. With this investment, the construction will provide numerous jobs and other major economic benefits, just as Hoover Dam did during its original construction.
The bypass project is completely within the Federal lands of the Lake Mead National Recreation Area and the Bureau of Reclamation's Hoover Dam Reservation. Therefore, funding for the project is a team effort. Federal funds through FHWA are the primary source, with significant contributions also being made by each State. After completion of construction of the bypass facility, FHWA will transfer ownership to Arizona and Nevada for operation and maintenance.
The roadway portions of the project are challenging as well. The site has significant access restrictions and logistical challenges. "Enormous volumes of material have to be moved in an expedient manner to achieve our schedule," says Zanetell. "We are fortunate to have a highly capable technical community and highly cost-effective construction industry to help us meet these challenges."
The new bridge will be a composite concrete deck arch, as pictured in this computer rendering.
Because of the cultural, religious, and historical significance of two areas affected by the bypass—Sugarloaf Mountain in Arizona and Goldstrike Canyon in Nevada—the National Park Service and FHWA initiated a government-to-government consultation with 13 interested Native American tribes.
Richard Arnold, executive director of the Las Vegas Indian Center and chairperson of the Pahrump-Paiute tribe, served as spokesperson for the tribes. "We don't want to see the area disturbed because so much has been lost," says Arnold, "but we recognize that the project is necessary and a high priority. We could have either taken a position of not becoming involved or decided to work collaboratively with all parties. The tribes chose to work collaboratively and be a part of the process."
Arnold served on the design advisory panel for the project. "We've been able to lessen the impacts of the bypass by moving the route from the original point proposed. And this was a milestone in that the Indian voice was listened to in the process. We look forward to a long relationship with the Federal agencies because the area is still used today for religious ceremonies and will continue to be used for all tomorrows to come."
Zanetell adds, "We are extremely proud of our relationship and teamwork with the Native American tribes. They have been strong contributors to the project in helping us understand their cultural and historical perspectives."
As the type study points out, the bypass project pairs off one of the most famous civil engineering landmarks in the world with a new bridge in such close proximity that the two will henceforth be tied together as one. Given the millions of eyes who will view the bridge as they enter Black Canyon to visit Hoover Dam, the focus on the aesthetics and form of the new bridge will be similar in intensity to what happens with a major urban bridge. In this case, though, the bridge will need to be cost-effective; aesthetically pleasing; environmentally, culturally, and historically sensitive; functional; and, most of all, a source of pride to the Nation.
The project team used three criteria to evaluate the proposed visual quality: vividness, intactness (integrity), and unity with its site. The type study notes: "Just as the Dam is so perfectly seated into the canyon walls that it seems to "grow" from the earth, so too must the new bridge spring from the canyon as though it belongs to the site."
No small challenge, indeed.
Norah Davis is a contract writer for FHWA and editor of Public Roads magazine.
For more information, contact Dave Zanetell at 303-716-2157 or firstname.lastname@example.org, or see www.hooverdambypass.org.