Hoover Dam Bypass Spans Rocky Terrain

Wed April 19, 2006 - West Edition
Emily Christianson

Thousands visit the Hoover Dam each day in hopes of catching a glimpse of the engineering wonder. Still more travel through the vicinity, braving the heavy congestion, sharp turns and narrow roadway of Highway 93 looking to hit the jackpot in Las Vegas or see the Grand Canyon.

With more than 20,000 cars traveling through the area each day several stakeholders pushed for a solution, including the states of Nevada and Arizona, the Federal Highway Administration, the Bureau of Reclamation, the Western Area Power Administration, and the National Parks Service.

Their efforts led to the Hoover Dam Bypass Project and the Central Federal Lands Highway Division, a branch of the FHWA, was chosen to deliver it.

The approximately $234-million project is fully funded with a blend of federal and state funds and allows through vehicle and truck traffic to bypass the congestion with a new route approximately 1,600 ft. (488 m) south of the existing highway. The 3.5-mi. (5.6 km) corridor departs from U.S. 93 in Clark County, NV, and meets up with the highway in Mohave County, AZ, and includes a four-lane bridge over the Colorado River.

“U.S. 93 serves as the primary conduit between the two fastest growing cities in the United States and it goes directly through the Hoover Dam historic district,” said Dave Zanetell, project manager of the Central Federal Lands Highway Division. We are dealing with a site that is a national historic landmark and never was intended to serve a transportation need and yet it is servicing 20,000 some vehicles a day.

“We have a terrible conflict of visitors, pedestrians and through traffic. Within the project limits the accident rate is three times the accident rate of U.S. 93 on either end of the project. Right within the project there is the conflict of 6 million visitors who have come to the Hoover Dam to simply be on the Hoover Dam and they’re comingling with 20,000 vehicles a day so it’s an unsafe blend of pedestrians and through traffic.”

The most significant portion of the project is Phase Three, the approximately 2,000-ft. (610 m) long bridge needed to carry traffic across the Colorado River. An engineering feat, the bridge will boast the longest arch in North America.

Phases One and Two, the Arizona and Nevada Approach projects, are complete and General Contractor Obayashi Corporation and PSM Construction USA Inc. Joint Venture started building the Colorado River Bridge in January 2005. The contractor expected to complete the $114-million contract in July 2008.

“We looked at a number of different alternatives both in overall bridge type and concept,” Zanetell said. “Once we settled in on the arch as the best solution for the site we then looked at a number of different types of the arch itself and settled on this arch design because of its ability to complement the Hoover Dam, the historic district, reduced life cycle costs and durability and ease in construction relative to some of the other alternatives.”

Building the longest arch bridge, 900 ft. (274 m) above the river, in mountainous and rocky terrain is no small task.

The contractor’s Project Manager Mike Motil, explained how the use of two highline cable cranes will help deliver and place material across the entire bridge span during foundation and arch construction.

“[Each requires] two towers, one on each side of the river, 2,500 feet apart and they are 330 feet in the air,” said Motil of Obayahsi of San Francisco. “On it is a system of cables that allow for a trolley and hook to ride on it from one end to the other and [each] has a 50 ton capacity … and they are centered on the arch. They have seven degree luffing capability. That means these towers can move left and right seven degrees so I can take out a steel girder and luff it over just like a crane would just move it over.”

Engineers at the Joint Venture designed the system in coordination with Nicholson Construction, who assisted with the rock anchor design. It leased the towers and wenches from American Bridge in Pennsylvania. The south tower crane is already up and crews are still working to erect the north tower.

“The roadway comes in from the right as you are coming to the bridge and then exits to the left as you are leaving the bridge and [our highlines need] to be straight,” Motil said. “So the back stay anchors are locatedoff the roadway on both sides of the bridge making construction extremely difficult … we have power poles everywhere and we have to stay inside of them and it’s really changed the geometry of what it looked like on paper.”

Phase Three work is 40 percent complete. California subcontractor Ladd and Associates blasted and excavated approximately 60,000 cu. yd. (45,873 cu m) of rock to prepare for the foundations. Ninety percent of the foundations are in and 400 of the 500 pre-cast column segments are ready to be shipped.

The pre-cast concrete columns are prepared off site in Boulder City where a MiJack Crane on four wheels is used to pick up and move material. Workers will use the highline cable crane to trolley the columns out and place them one on top of the other. The tallest columns are constructed with 30 segments stacked, reaching 300 ft. (91 m).

Once the approaches are complete on each side of the canyon wall, crews will begin building the arch in 25-ft. (7.6 m) segments using traveling forms to make the two parallel arches simultaneously from the four corners out toward the middle of the arch.

“The form travelers are four steel traveling forms,” Motil said. “They have large trusses underneath and a series of steel with wood lined forms to make a hollow box segment, 25 feet long to the dimensions of the arch. We launch this traveler out and we put the reinforcing steel in it, we button it up right, we fill it up with concrete until it reaches a 4,000 PSI strength. We break the traveler free, we hydraulically move it forward, position it again, fill it up, move it forward, fill it up, and move it forward and all the way out, until the closure pour is complete.”

In addition to Ladd and Associates, Obayahshi/PSM Joint Venture also is working with subcontractors Harris Rebar of Phoenix and OlsenBeal of Utah. Harris is furnishing and installing all of the rebar and OlsenBeal will erect the columns and tub girders and put in the SIP deck forms. In addition, OlsenBeal also is erecting the highline cable crane towers.

Currently, approximately 125 employees are working on site including all subcontractor employees and Motil doesn’t expect the number to change much as the project moves forward.

Pedestrians also will get to experience the bridge via sidewalks and walkways and the use of visitor parking and facilities.

“Visitors to the area can now experience the new bridge and while on the new bridge, get a new perspective of the Hoover Dam,” Zanetell said. “We think they will go hand in hand. The Hoover Dam is a historic landmark for its engineering achievements and in a humble and respectful way we think the bridge will complement that because of its engineering challenge and magnitude also.”

Once Obayashi/PSM Joint Venture finishes the bridge construction another contractor will come in for Phase Four and pave the deck to complete the project in 2008.

Previously, R.E. Monks Construction and Vastco Inc. Joint Venture completed Phase One, the Arizona Approach, in 2004. Major components of the $21.5-million project included 2 mi. of four-lane roadway, the Sugarloaf Mountain Bridge, a new traffic interchange at U.S. 93 and Kingman Wash Road, a connection between U.S. 93 and the Colorado River Bridge and wildlife crossings.

Phase Two, the Nevada Approach, included 2.11 mi. of new four-lane highway alignment including six new bridges, a new traffic interchange at U.S. 93 near the Hacienda Casino, retaining walls, wildlife crossings and more. Edward Kraemar & Sons completed the $30.1-million project in 2005.

“We are very pleased with the contractors’ progress to date and we are very pleased with the construction industry’s delivery of all the elements of the bypass,” Zanetell said. “Although the river bridge is getting all the attention right now there is an enormous amount of work that went on just to build all the roads and bridges and transmission facilities getting to the bridge, so we just couldn’t be happier with the response of the industry to the challenge we’ve laid in front of them.” CEG