Traffic in a Nampa, Idaho — considered the state's fastest growing community — just couldn't keep waiting for "the little train that could" to pass through one of the busiest intersections in town. In the last decade, southeast Nampa's Kings Corner evolved into being one of the city's worst traffic bottlenecks. Seven roads — including Amity Avenue and Southside Boulevard — converge at the Union Pacific Railroad tracks. The combination of cars and trains caused long backups over 40 times a day. Switching operations blocked the crossing for 10 to 15 minutes at a time, causing traffic to back up for miles in all directions.
"Transportation largely defines quality of life for many people," said Tom Dale, Nampa mayor. "Can you get from point A to point B in a reasonable amount of time and in safety?"
In 2003, the citizens of Nampa overwhelmingly responded to the menacing traffic problem by supporting a general obligation bond to pay for the construction of two bridges intended to provide safe and efficient vehicular travel over the railroad tracks. The project was named the "Kings Corner Railroad Overpass Project," and the main goal was to separate automobile and train traffic, both for safety reasons and to alleviate traffic delays when trains block cars at the intersection.
Layton Construction, out of Sandy, Utah, is serving as the general contractor of the $18-million project. J-U-B Engineers of Boise is the project engineer of record, along with fulfilling additional responsibilities including: project manager, survey, right of way, public involvement, structural design, initial horizontal and vertical alignment, quality control/quality assurance, city service utility plans, and construction engineering and inspection.
"This is a heavy civil transportation project consisting of the construction of a 360-foot, three-span plate girder steel structure and 608-foot, four-span plate girder steel structure," said J-U-B engineer Chris Canfield. "Both structures include deep (driven) pile foundations with reinforced concrete piers and a continuously reinforced concrete deck."
Construction began in June 2005, and at the time of PB&E's November 2006 visit, the first overpass taking King's Road over the railroad tracks and connecting with Amity and Southeast was complete (Sept. 6, 2006, open date).
The second overpass was slated to open on Dec. 15 and is being built over the railroad tracks at Amity Road. Traffic is being rerouted to the completed King's Road overpass. Throughout the construction of both bridges — as promised to the citizens of Nampa — traffic flow, with the exception of short duration closures, has been maintained throughout the project (to cars and trains). Upon completion, the project design creates a standard right-angle intersection of Kings and Amity controlled by a traffic light. The tracks run north-west and south-east; the Amity Road alignment runs east-west and the Kings Road and Southside Boulevard alignments run north-south.
"The Kings Road Structure crosses the tracks at a 90-degree angle as it was feasible to reduce the span required to cross the tracks in this way," said Canfield. "This option was more efficient (versus running at a skew to the tracks) as it could be done on this alignment. The Amity structure is more of a skew with the tracks at about a 45-degree angle."
Canfield said the Amity overpass was designed like this for two reasons, including: 1) This is a major east-west route providing better continuity; and 2) It is adjacent to commercial property making this option the best for the city of Nampa.
Site preparation for the second overpass was concurrent with the completion of the first overpass (September 2006). Utilities had to be relocated, including gas, water, telephone, storm drains, irrigation canals, and above-ground power lines. Hurricane Katrina affected the site prep for the first bridge last September.
"All of the linesmen were down in the Southeast trying to deal with the aftermath of Katrina, and there is still a shortage up here today," said Tom Coughlin, Layton's senior project manager. "Materials were also short in some cases during that time, but we worked around the situation and were able to stay on schedule."
Soil condition was also a significant factor in site preparation.
"The soil was lousy," said Andy Coleman, J-U-B's on-site project inspector. "We had to bring in large amounts of fill because the site soil was made up of a mixture of clay, sand and water. We could go about 2 feet a day up to about 25 feet, allowing optimal time for settling."
An inclimeter, a piezometer and a correlating survey were all utilized to ensure that the fill had properly settled. Once the fill was settled out, crews came in and drove the H-piles for the rebar and concrete abutment walls before placing the steel piers. Concrete was poured onto the abutment piers and once it broke strength, the bearing plates (to help the bridge move for contraction and expansion) and the steel girders were swung into place.
"Steel was used for the girders to provide for the span requirements and reduce the depth of the substructure," said Canfield. "This allowed the design to meet the height requirements for the trains while reducing the vertical profile change in the roadway."
Design and material consideration for the project also took into account load anticipation. Jim Porter, J-U-B's transportation manager, noted the overpasses were designed to handle the stress of live and dead loads versus the unlikely earthquake load, due to the geographic location.
Once the girders were in place, cross members and connecting plates were then installed. A survey was then conducted, and once it was determined that proper elevation had been achieved, the bolts were torqued down. More than 900,000 pounds of rebar and 5,000 cubic yards of concrete were used on the job. Coughlin noted that concrete supply had been an issue at times, but did not notably delay the project.
Lifting on the site was significant due to the nature of the project, and Inland Crane of Boise had what might be described as a "crane-fest." Dave Frei of Inland Crane identified that more than eight pieces of equipment were used over the course of construction — a large number compared to the typical project.
"We haven't used that many cranes on a site since we worked on the massive Treasure Valley WYE Interchange project a few years ago," said Frei.
Included in the list of cranes utilized were a 257-ton Krupp (girders), a 150-ton Demag truck crane (girders and piling), a 110-ton P&H crawler crane (girders), and a 32-ton Grove, a 30-ton Drott RT, and a 25-ton Manitex. The last three were used to feed iron inside of the braces to hang the diaphragms. For drilling and piling, a modified Link-Belt log loader was utilized (modified by Inland Crane for the project).
After the bolts were secured, crews proceeded with forming up the false shoring for the bridge deck and the cantilever for the sidewalk and parapet wall. Rebar was tied and concrete was pumped onto the bridge and then sealed with a concrete compound sealant. Once the deck was down, the sidewalk and parapet walls were constructed followed by lighting installation. A unique design of the MSE wall on both bridges was implemented. The design allows for conformity to soil settlement versus anticipating settlement.
The last tasks were fencing, roadway stripping and final site cleanup — getting ready for the mayor to declare the second overpass to be open to traffic on Dec. 15, in complete avoidance of the railroad tracks. The final date of completion for the entire project is scheduled for summer of 2007. The $18-million price tag includes design, construction and right-of-way costs.
Thanks to citizen support and the cooperation of the construction and engineering team, the little train that could will still pass through Nampa. The Kings Overpass Project serves as a great example of how a community can work together to find thoughtful solutions to the challenges of growth.