A quarter-mile downstream of the Hoover Dam, a 1060-foot bridge hovers 900 feet above the Colorado River. The seemingly weightless structure is North America's longest twin-rib concrete arch.
It's also the centerpiece of a much larger project to connect Nevada and Arizona on U.S. 93, one of the Southwest's most congested stretches of highway. Opened in 1935, shortly after the dam's construction, the single-lane road has since been the major commercial route between Arizona, Nevada, and Utah, carrying 14,000 vehicles daily—double the volume 15 years ago.
Designing around terrain
Long-span bridges are unique, expensive assets that require innovative design approaches for the factors affecting performance: cost, aesthetics, construction, and operations. Designers of the bypass project had almost no reference to other projects with similar requirements, making the project a unique challenge.
The terrain on the Nevada rim varies, with rock outcroppings and fault lines traversing the canyon walls. The unique topography funnels wind at gusts up to 60 mph through the bridge site along the alignment of the gorge, as well as through the rugged terrain on either rim. Foundation excavation required rock-blasting of approximately 60,000 cubic yards in the vicinity of an in use roadway and operating dam facilities. Construction of the new bridge had to take place nearly 900 feet above the canyon floor and 800 feet out from the canyon rim.
The first design challenge of the new bridge was to select the right structural system. The process was guided through two focus groups. The technical issues were presented to a structural management group (SMG) composed of the state and federal bridge engineers, as well as peer review consultants. The aesthetic issues were reviewed by a design advisory panel (DAP) made up of state historic preservation officers, the National Park Service, Bureau of Reclamation, Native American representatives, and architectural consultants.
A comprehensive review of potential structural systems was evaluated by the SMG. The unique character of the Hoover Dam site allowed designers to focus on the two most logical options—to traverse the canyon with a suspension bridge or an arch. With the Hoover Dam defining the character of the site, both focus groups recommended a deck arch.
Although the bridge site has experienced little recent seismic activity, the size of the project led designers to consider site-specific criteria for seismic design. The work predated the new American Association of State Highway and Transportation Officials (AASHTO) Guide Specification for Seismic Design of Bridges, but the site-specific process completed in 2002 resulted in a project criterion very similar to the AASHTO criteria approved in 2007. The team also used a similar approach of developing site-specific criteria for wind design.
The arch's framing system was tailored to meet demands of wind, earthquake, and construction logistics. Twin arch ribs would allow designers the option of providing ductile links between ribs for enhanced seismic performance. Twin ribs also lessen the size of arch elements during construction and provide greater flexibility for geometry control as the arch was cast.
Wind demands could be lessened with a chamfered box section. As the design evolved through the various stages of development, the twin-rib arch section was selected to provide the best balance of characteristics that met the needs of the site and structural demands.
Concrete in 100° F
In early 2005, the Obayashi/PSM JV construction team began assembling onsite. One of the chief topics of discussion was how 5000 workers built the 726-foot Hoover Dam in temperatures that can exceed 130° F. The team also faced the same logistical puzzle their general contractor counterpart on the Hoover Dam—Six Companies Inc.—had solved back in 1931: transporting crews, equipment, and materials to the site.
But a single challenge dwarfed all others: the concrete arches. Building them required a new approach to mix design, thermal control, concrete delivery and placement, consolidation, and quality control.
Work began two years before the first arch segment was cast. The design and ownership team required 10,000 psi in 56 days, aggregate selection for durability, and thermal control requirements to minimize cracking. The construction team added several more requirements to overcome delivery and placement challenges: pumpability—the concrete needed to be pumped through 600 feet of slickline up 275 feet; flowability issues to allow for a 7- to 8-inch slump that would flow freely into the forms; and the long set time of at least three hours before the concrete began setting in case of mechanical breakdowns, placement issues, and batching or pumping problems. In addition to these design requirements, the construction team had to meet schedule challenges, specifying rapid strength gain to keep the form traveler cycle time to a minimum.
Strength targets were addressed by a high cementitious material content (800 pounds of cement and 200 pounds of fly ash per cubic yard) and a very low water-to-cement ratio of less than 0.31, which typically achieved strengths of 4000 psi in a little more than a day and more than 12,000 psi in 56 days.
Pumpability and flowability were addressed by use of a water-reducing superplasticizer, which resulted in concrete slump ranges that neared those of self-consolidating concrete. Long set times in excess of 2½ hours were achieved using a retarder.
However, the rich mix design made it difficult to work in such high temperatures. The concrete in its natural curing condition would reach temperatures in excess of 190° F, far above the 155° F limit of the contract specifications. Most typical mitigation methods—such as using chilled batch water or ice chips, shading the aggregate stockpiles, and pouring at night—would not reduce the maximum curing temperature to within the target range.
Only two realistic options remained: circulation of cold water through pipes embedded in the concrete or the use of liquid nitrogen to precool the concrete to a temperature such that its maximum peak curing temperature would be less than 155° F. Miles of cooling tubes had been used to control curing temperatures during construction of Hoover Dam, but the location, cycle time, installation, and maintenance issues involved ruled out their use.
This made liquid nitrogen the only option, allowing the team to drop the batched temperature of 85° F to a predelivery temperature of 40° F. That maintained the temperature at point of placement in the 60° F range, resulting in peak curing temperatures of less than 150° F.
Given the heat of a southern Nevada summer, liquid nitrogen costs often exceeded $100 per cubic yard. But the process eliminates other costs, such as maintaining a consistent water supply, grouting cooling tubes, or leaving forms in place for an extended duration. During the hottest times, the team filled the concrete pumping slick line with chilled water before the pour and wrapped it with burlap soaked in chilled water to reduce heat gain through the placement system.
“Every aspect of each construction contract and the approach to risk management and contract execution was simply a natural extension of the development effort,” says Dave Zanetell, manager of the Hoover Dam Bypass project for the Central Federal Lands Highway Division (CFLHD) of the Federal Highway Administration (FHWA). “The goal simply never was to ‘create a design' or to ‘award a contract.' It was to ‘complete the project' by integrating the world's best design capabilities with those of the world's best construction contractors.”
“The main outcome we want to create is a sense of possibility,” says Zanetell. “Great things can and should be done. It doesn't have to be that great civil works also leave an uncertain fiscal legacy. We can do both—achieve greatness and do it as planned.”
It's the optimism of a man who has supervised the massive undertaking for nearly a decade, an undertaking that has brought together dozens of public agencies, consultants, and contractors who have employed 1200 tradesmen and 300 engineers.
The project also will be completed both on budget and on quality without dispute or claim—a major success of the management team.
“It's about setting in place clear, defined roles of technical coordination and bringing design, construction, and technical expertise together,” Zanetell says. “We've created synergies instead of stovepipes, allowing world experts to bring their vast knowledge yet aligning under a common framework,” noting that a project of this size mandates engagement from the industry's best.
CFLHD managed all aspects of the decision process and integration so that no firm or entity could supersede another. “It was a departure from the all-too-frequent model in which the owner ultimately becomes captive to the decisions of others then is left only to resolve the inevitable disputes and claims,” he says.
The successful project management is the result of FHWA's cradle-to-grave delivery approach in which designers, engineers, and builders have remained part of the process even after their individual tasks have been completed, coming together through specifically structured points of interface and engagement throughout the project's execution. “Major jobs have big turnover. With something of this duration there's always a lot of change over the course of people's careers. They start the project and then move on, but our stakeholders have remained committed to the project.”
Each project management document—the delivery plan, detailed multiagency operating agreement for roles and responsibilities, and programmatic agreement for architectural and historic guidance—was explicitly endorsed by all stakeholders to ensure CFLHD could commission and manage consultant deliverables without risk of costly delay or redirection. The documents also served to further the understanding of the roles to allow the leads from each team to work seamlessly on overlapping and parallel segments of the project.
Zanetell notes that one unique aspect of the plan was to also establish and define an executive committee made up of the heads of each agency, who then served to provide guidance and endorsement of the most sensitive and scope-defining aspects of the project.
For Zanetell, the bypass project furthers the potential for the future of great civil works projects just as the nearby Hoover Dam project did nearly 75 years ago. “It has been a team effort, everyone in every way making this project a reality.”
— David Goodyear is senior vice president of T.Y. Lin International and the design engineer of record on the bypass project. Jeff St. John is the project manager of Obayashi/PSM JV and the contractors' project manager for the bypass project. Michael Fielding is senior editor for PUBLIC WORKS magazine.
Editor's Note: This article appeared in the August 2010 issue of PUBLIC WORKS magazine.