Crossing the Colorado
October 1, 2006
At the time of its construction, the Hoover Dam was the world's tallest dam and the largest hydroelectric producer of its kind. It is considered an engineering marvel, most notably for its ingenious concrete column construction, sheer size and speed of construction. Just a stone's throw away from this magnificent structure, another engineering masterpiece is now emerging. The Colorado River Bridge, initiated by the U.S. Bureau of Reclamation in 2004, is the first concrete-steel composite arch bridge to be built in the United States and the final segment of the $234 million Hoover Dam Bypass Project.
Located just 1,600 feet from the Hoover Dam, this four-lane bridge stretches 1,900 feet across the Colorado River, which flows 900 feet below the bridge deck. Its purpose is to improve travel times between Arizona and Nevada, reduce congestion, improve safety and reduce the potential impact of an attack or accident at the dam site. The project is led by the Central Federal Lands Highway Division of the Federal Highway Administration (FHA), and a multi-agency team consisting of the Nevada Department of Transportation (NDOT), Arizona Department of Transportation (ADOT), Western Area Power Administration (WAPA), Bureau of Reclamation (BOR) and National Park Service (NPS).
The initial control survey for the bypass project was performed under an agreement with the FHA, and was then extended to additional locations throughout the project by a local survey company. From that point on, all geometry control on the project has been performed by the joint venture of Obayashi Corporation and PSM Construction Company Ltd. These two global engineering and construction firms, both headquartered in Tokyo, combined their financial and employee resources to build the third phase of the Hoover Dam Bypass--the Colorado River Bridge.
Just as the Hoover Dam construction in the 1930s was dependent on highly trained surveyors and engineers, so is this extraordinary bridge. From constructing and aligning the precast columns that will support this structure to positioning the massive arches, surveyors are one of the key elements to making sure every aspect of the construction fits together as it should.
"From a geometry control perspective, this is a particularly technical project," says Nick Carter, field engineer on the bridge project for Obayashi/PSM Joint Venture. He continues, "Field crews are not expected to fully understand how the structure will behave as construction progresses, so there needs to be a tie between the actual construction of the bridge and the engineering that is involved. We help provide that link." In fact, equipped with advanced robotic total stations, the geometry control team has been able to exceed precision requirements while performing a majority of surveys with speed and accuracy.
Column Care and PrecisionConstruction on the Colorado River Bridge began in January 2005. The primary design elements include two 1,060-foot arches and 30 precast columns consisting of more than 400 segments, as well as support structures that form the foundation for the roadway across the river.
Early on, one of the first--and most technical--survey responsibilities was to control the geometry of the precast column segments. The design includes 440 pre-cast column segments, most of which are 10 feet tall by 15 feet long. The widths of the segments range from 5 to 12 feet wide at the base of the largest segments. Each of these columns is constructed offsite at a precast yard approximately 12 miles from the Hoover Dam.
The positional tolerances for the precast column segments are based on vertical angular breaks between the segments. The maximum angle break from one segment to the next (vertically) is .002 radians (6.875 minutes). Casting of the 492 column segments was completed in mid-June.
As the columns are built, geometry control crews constantly monitor and project the alignment of the 280-foot-plus tall columns. Carter adds, "The most challenging task so far has been fine-tuning the geometry control system at the precast yard and then carrying that control over to the jobsite to get the columns erected within tolerance."
Robotic ResourcefulnessAs the column segments are completed, they are moved to the bridge site where construction crews have already begun to lift these columns into place. At this point, one- and two-person crews align the columns across a canyon that is more than 1,000 feet wide and almost 1,000 feet deep utilizing a Leica Geosystems (Norcross, Ga.) TCRP 1201 robotic total station and TCR 703 automatic total station. The crews must prevent even slight errors in the alignment of the base segment because they significantly multiply as the column is raised.
For the first segment of each column, the geometry obtained at the precast yard at the time of casting is reproduced at the jobsite. The segments are erected and compared to the theoretical geometry. Construction crews first place the bottom segment as accurately as possible--to within .0005 radians (1.72 minutes) vertically. Two additional segments are then placed on the base segment and the three segments are post-tensioned to the footing to assure that the match-cast joints between each segment close.
The geometric relationship between the first three segments is then adjusted to match the casting geometry by slightly changing the alignment of the base segment. After the geometry of the first three segments is established, the stress is released from the segments and a permanent grout pad is poured under the base segment. After the grout pad has cured, the top two segments are removed and the base segment is tensioned to design stress.
Because of the very steep terrain, multiple control points are required for each column. Performing accurate survey on the base segments requires crews to bring control into nearly every pier. On some columns, the first three segments require multiple setups to obtain proper line of sight. Once the columns begin to rise out of the canyon, the alignment of each segment is checked from a common control point. From this point on, segments are set and stressed one by one.
"This process involves a lot of repetitive work, but is necessary to assure proper geometry," Carter says. "By using this procedure to align the column we can much more accurately "aim' the column in the right direction. It's like shooting at a target with a rifle as compared to a pistol--the longer barrel allows for more accuracy. By using three segments instead of only one we can be much more accurate."
Due to the drastically varying terrain--structure elevations range more than 470 feet from the top of the temporary pylons to the bottom of the skewback footings--crews are currently utilizing 15 different points to gain line of sight to all portions of the almost 1,900-foot-long structure.
If the as-erected geometry of the columns does not agree with the as-cast geometry, geometry control crews and engineers work together to develop corrective actions that satisfy the geometric requirements as well as the final structural requirements. "The hard work put forth at the precast yard appears to be paying off, though," Carter says. "Four columns have been topped-out and multiple other columns have been started. To date, 101 segments have been permanently placed. Obayashi/PSM's erection crews continue to improve productivity, placing as many as four segments in a 10-hour shift."
Towering TransportPrior to the column construction, crews also prepared the framework for the majority of the columns and the two arches. This included the construction of two 50-ton tower cranes on each side of the river. These cranes span the canyon and enable crews to move the precast column segments and arches into place and deliver other necessary materials. Each tower is stabilized by a backstay cable and two sidestay cables. A large trolley runs along two 3-inch cables extending between the towers and supports the weight of the load. A separate line is connected to the trolley and pulls the load back and forth across the canyon. The final line, called the "load line," runs to the trolley and then down to the load block to raise and lower the load.
The geometry control crews and the design engineer worked hand-in-hand throughout the installation of the cableways, relying on the speed and flexibility of a one-man operation utilizing the robotic total station. The south cableway was completed in February 2006 and the north cableway was completed in May 2006; both are currently being used to set the bridge's precast column segments on the footings.
All together, the geometry control team has precisely located the 14 foundations for the bridge and 35-plus foundations for the two cable crane systems, aligned post-tension bars in the 14 approach columns and gathered large amounts of topographic information.
"The robotic total station made the geometry control operations easy; the hard part was getting around the job, back and forth across the river, and up and down the canyon sides. Since the beginning, we've struggled with limited access and dangerous conditions because of the steep slopes, loose rock, intense heat and frequent high winds," Carter says. As the structure progresses, better access and proper work areas are being created, but some areas will continue to be difficult to access. To avoid the heat, some of the crews have switched to night shifts, although additional crews still work through the heat of the day to maximize the use of the two cableways. When wind speeds are in the range of 30 mph, the cableways are shut down until wind speeds fall to allow for safe working conditions.
Construction of the arch footings/skewbacks is underway and steel tub girder fabrication is taking place offsite. Once the columns are erected, the tub girders and concrete deck surface will be constructed on the approach spans to pave the way for arch construction.
Arch AccuracyAs the focus changes from constructing the footings to column and arch erection, the geometry becomes more technical. The arch portion of the Colorado River Bridge is a composite concrete deck arch and steel superstructure with an arch span of more than 1,890 feet. The crossing includes two 1,060-foot arches. Placing them once again required geometry control crews using robotic total stations with Automatic Target Recognition (ATR) capabilities. The positional tolerance for the bridge structure is +/- 1/8 inch and the positional tolerance for arch construction is +/- 1/16 inch.
When arch erection began this summer, geometry control crews worked very closely with the engineers to properly project and align the arches as they progressed. Because most of the arch segments are supported by temporary cables, many different aspects of the construction environment need to be monitored in addition to the actual arch. These details include temporary tower position (a temporary tower supports the arch stay cables) cable elongations, environmental effects (temperature, wind, etc.) and dead load deflections (downward movements due to the weight of the structure) that vary from those expected in the design.
Both the engineering and construction of this bridge are very challenging; careful planning, extensive communication and proper use of the available technology will allow the construction team to be successful. "The combination of tools and techniques used to meet the needs of this project are unprecedented," Carter says. "We've been able to carry out all of our geometry control operations with ease and accuracy--and oftentimes with just one man using a robotic total station."
The final roadway surfacing for the bypass will be performed under a separate contract and will be completed in conjunction with the opening of the bridge. The bypass is scheduled to be completed by late 2008.
Visit www.hooverdambypass.org for additional project information.
Sidebar: Hoover Dam HistoryThe most common cause of death during the building of the dam was being hit by falling objects as surveyors and construction crews scaled the canyon walls. As a result, the crews improvised and coated cloth hats with coal tar, ushering in today's "hard hat."
The first man to die on the Hoover Dam project was a surveyor named J.G. Tierney. Tierney drowned while trying to locate the optimal site for the dam. On February 6, 1935, exactly thirteen years after Tierney's death to the day, another life was lost at the Hoover Dam site. Patrick Tierney, J.G. Tierney's son, fell from an intake tower and drowned.