Bridging greatness

Work begins on the Colorado River Bridge, the longest arch span in the U.S. and the fourth longest in the world that’s part of the $234 million Hoover Dam Bypass project.

by Clair D. Urbain

Visitors to the Hoover Dam still marvel at  the engineering and construction feats  that it took to build this superstructure 70 years ago. Today, another engineering and construction marvel is under construction just 1,600' to the south of the dam – the Hoover Dam Bypass. When finished in 2008, it will feature four miles of four-lane highway that takes a straight shot across the mighty Colorado River with a nearly 2,000' long bridge supported by the longest arch in North America.

Burgeoning growth in the Las Vegas area and traffic congestion makes travel on Highway 93 a slow and, in some ways, dangerous drive across the Hoover Dam and the Nevada-Arizona state line.

Heavy traffic on the narrow, two-lane road with several switchbacks does not mix well with the mass of tourists visiting the dam.

Highway 93 is also designated as a North American Free Trade Agreement (NAFTA) route, so the congestion and security issues were no longer acceptable.

To alleviate the traffic and improve safety, the Central Federal Lands Division of Federal Highways is the lead agency responsible for management and delivery of the project. It is coordinating the efforts of Federal Highway Administration (FHWA), the states of Arizona and Nevada, the Bureau of Reclamation, the Western Area Power Administration and the National Park Service on this three-section project to build the bypass.

The Arizona and Nevada approach projects, which total $51.6 million, are complete and in February 2005, work began on the Colorado River Bridge, the longest concrete arch bridge in North America and the fourth longest in the world.

The 88'-wide, four-lane bridge with pedestrian crossing will tie the completed approaches together. This engineering wonder is aesthetically pleasing and combines several building technologies to achieve a cost-effective and well-engineered solution to traffic problems.

General contract work for the bridge is under the control of a joint venture between Obayashi and PSM Construction USA. Obayashi is known for its tunnel and bridge work around the world, completing projects in Atlanta, Seattle and portions of the Big Dig in Boston. PSM Construction USA is known for its pre-stressed concrete construction experience, says Mike Motil, Colorado River Bridge project manager.

“This is a challenging job. There was nothing here one year ago and since then, subcontractor Ladd and Associates has been blasting and removing rock for the bridge foundation and abutments. This is a $15 million subcontract out of the $114 million bridge project. They have done an excellent job of safely and efficiently removing 640,000 cu. yd. of rock to make room for the columns and abutments that will support the bridge,” he says.

Environmental concerns
Protecting the environment is a high priority on the project. From landscape to wildlife, careful monitoring assures that work isn’t adversely affecting the area.

“Before we started removing rock from the canyon walls to make room for bridge foundations, we monitored normal vibration in the area for one month to establish a baseline. The 26 seismometers are still in place, monitoring vibration from construction. They are positioned 110' left and right of the bridge centerline on each substructure unit. This is fractured rock, which means blast effects can be unpredictable. Because of this, crews have been using small charges to remove rock,” says Motil.

To prevent rock from falling into the river, crews install extensive rock containment systems before removing any material. Once rock is removed in some areas, rock bolts are inserted back into the mountain to stabilize the remaining rock.

One of the largest herds of desert bighorn sheep in the United States reside in the area, and the spec requires they aren’t disturbed or displaced by the project. Field supervisors report sheep sightings so they can be tracked and to assure work doesn’t affect their habitat.

Barriers have also been built around the jobsite perimeter to prevent tortoises and other wildlife from entering the finished roadway. Both approaches are designed with wildlife crossings so animals can cross under the bypass to reduce chances of being hit by motorists.

To minimize site contamination, the contractor uses a 100 percent biodegradable bond breaker on forms used for column construction.

Bridge construction
Designers chose a composite concrete deck arch design for the bridge, which will cross Black Canyon about 1,600' south of the Hoover Dam. This design addresses cost, schedule, aesthetics and technical excellence required to meet the demands on the site, says Motil. “The concrete composite alternative blends the best of concrete and steel, using compressed concrete for the arch and lighter steel for the upper structure,” he says. The bridge decking will be cast in place.

This bridge will be the fourth largest arch in the world and will rise 900' above the river. Its cast-in-place concrete arch will be built using custom form travelers built by NRS in China. “This design also allows work to simultaneously take place on-site during arch erection and in the shop for superstructure steel fabrication,” he says.

To build the span, Motil’s crews will use twin 50-ton luffing highline cable cranes specially designed for this job. “The cable way is anchored into solid rock. It will be used to build the foundations, place the bridge columns, erect the tub girders and then service the four form travelers that will be used to build the arch,” he says. After the arch is built, the cranes will place the spandrel columns, tub girders and then place the bridge deck.

The twin towers have heavy 3"-diameter lines that run across the canyon. The luffing crane design allows the highline to move from side to side to maneuver the wide variety of building materials into the canyon.

Designing the luffing crane towers was challenging because the road comes in from the right and the bridge turns left as it crosses Black Canyon. “There was little space in which to anchor the cranes. We are anchoring off the mountain on both sides,” he says.

Preliminary foundation work continues with two other cranes positioned on each side of the river. “Ladd and Associates is subcontracting this work. They are blasting fractured rock, which can be difficult to control. There is a chance a blast will take out too much or not remove anything at all, but what’s left gives you a solid foundation.

“There are power lines all around the cranes. There is a 230 KV line on the Arizona side and a 2.3 KV line on the Nevada side. We have the cranes configured so they can’t turn into the power lines,” he says. Before construction could begin on the approach contracts, several transmission lines had to be relocated and upgraded to make room.

Most concrete will be poured at night. “The temperatures here can reach 120 F and winds can reach 100 mph. Our equipment is engineered so that we stop work when the winds reach 30 mph and must lash down the towers in 50+/- mph  winds. The FHWA supplied five years of wind data to help engineer the construction,” he says.

While this is a big job, there is room for only about 100 workers on the site. “This is like doing a $25 million job four years in a row.  There is not much in each foundation or traveler, so we have many smaller crews,” Motil says.

Many of the workers are at the pre-cast site at this time, building the column sections. “As columns begin to be transported to the site and placed, some workers will move from the pre-cast yard to the bridge site. In both locations, we demand 100 percent fall protection on the job, whether they are working 6' off the ground or 900' over the Colorado River,” he says.

Column construction offsite
“The support columns rising from the canyon walls must be constructed with extreme accuracy. Their design is also aesthetically pleasing,” Motil says.

In total, the project requires 440 pre-cast concrete pier segments and 64 concrete pylons. The columns are engineered to slightly taper as they are stacked on one another. They are built sequentially in the pre-cast yard, one on top of the previous one, to assure a perfect fit. Each piece is numbered and matched perfectly so they can transported, placed and then doweled together on-site.

“At the pre-cast yard, we are building two to three column segments a day. We are working ahead because when we begin placing them, we hope to place four to six sections each day,” he says. Each column segment weighs as much as 40 tons and each pour is approximately 15 cu. yd. Every concrete load is checked for slump and other performance factors.

The pre-cast yard has three forming/pouring areas. Two are for regular column segments and the third is for the top or base segments and the pylons. “We always pour on top of the previous column segment and move in a figure 8 fashion. In the curing area, up to 14 segments are held for seven days and watered to allow for an even cure. The segments are moved around the yard with a 50-ton mobile crane.

In one area of the yard, crews use a template to build the rebar structure for column segments. They place forms precisely, guided by geometric controls that are checked for accuracy every day at 2 a.m. Once they’re checked, the forms are repositioned for another pour, working off the previous day’s segment.

The column segments are made with 6,000 psi concrete; the arch will be made of 10,000 psi concrete and the foundations are 4,000 psi concrete.

Once the column segments are set in place, they will be joined with a 10'-long bar that will be slipped in from the top of a column segment and bolted to the segment below.

“This is an engineering feat and we have great engineers who are planning it,” Motil says.

While building the arch, a temporary cable stay system, much like what’s used on suspension bridges, will support the arch and traveling forms. Once the arches are completed, the cable stays will be removed and the arches will be freestanding. Then, the precast columns will be installed vertically on the arch.

Steel tub girder construction requires a special certification for installation. “That will be handled by Utah-based steel construction contractor OlsenBeal,” says Motil.

Watch the progress
To watch progress on this project, go to www.hooverdambypass.org. There, two Web cams monitor the site. It also features project images and a virtual bypass drive-through that shows how much earth was moved to create this project.

Published in the March/April 2006 issue of Contractor Tools and Supplies magazine.

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