Built in 1936, the bridge that carried U.S. 281 over the Colorado River near Marble Falls, Texas, was considered “functionally obsolete” despite a 1974 rehabilitation of the cantilever deck truss.
The first bridge at this location was built in 1891, but a flood destroyed it in June 1935. Ferry operation took over until the cantilever bridge was completed the following year. It’s not a flood requiring replacement this time. The outdated steel structure can no longer keep pace with traffic on the heavily traveled route through Marble Falls that sees an average of 26,000 vehicles per day cross the river.
Population growth in Burnet County is one of the driving forces behind the plan that targets improved traffic flow as a goal. To accomplish that will require two bridges: one serving northbound traffic, the other serving southbound traffic.
“Once this project is complete, citizens will enjoy two bridges, each with two 12 feet travel lanes for vehicles, large 10 feet shoulders and sidewalks for pedestrians and cyclists,” said TxDOT spokesperson Kelli Reyna. “This project will greatly enhance the safety of the bridge, not only through the addition of shoulders and sidewalks, but by preventing head-on collisions. The added shoulders will provide additional room to maneuver traffic in the event of a traffic accident or routine maintenance being performed on the bridge, which in turn, will lessen delays. Also, in the event that something should happen that would necessitate a full closure, motorists could be re-routed to the other bridge instead of through a lengthy detour.”
Getting Started
The Texas Department of Transportation and the city of Marble Hills first discussed plans for a replacement span in 2005. Funding for the (estimated) $29.7 million project was secured in 2009, using a combination of federal and state sources.
The project was bid in May 2010. Ground was broken in October and major work on the new northbound bridge structure began in December, with preliminary engineering and material acquisition taking place in the interim.
The northbound bridge was open to traffic on December 21, 2012. In February 2013, traffic on the old southbound steel bridge was switched onto the northbound bridge so demolition could take place.
General contractor Archer Western Contractors (part of the Walsh Group) requested the use of explosives to demolish the bridge, said Reyna. The original contract provided $1.25 million to demolish and remove the existing structure. That amount didn’t change based on the demolition method Archer Western chose. TxDOT provided a demolition option in the contract; however, they allowed the contractor to submit a revised plan for approval.
After being reviewed by TxDOT, as well as other regulatory agencies such as the Lower Colorado River Authority, Texas Parks and Wildlife Department, and the Texas Commission on Environmental Quality, the proposed method was approved.
Removing the steel truss was the first phase of demolition and the removal of bridge piers followed shortly thereafter. The two outside bridge spans were removed manually; the concrete deck and the widening done in the late 1970s was mechanically removed prior to the implosion, leaving only 600 ft. (183 m) of the bridge to fall into the water. Of that 600 ft., only the original 1930s truss was demolished using explosives.
On March 17 demolition subcontractor Omega Demolition and explosive engineer Engineered Explosive Services used controlled explosives to demolish the bridge. By performing the demo at one time, it resulted in a safer and more efficient operation.
“Overall, the demolition process went well, drawing a crowd of more than 3,000 citizens to witness the historic event,” Reyna said. “This was the largest steel truss bridge that had ever been demolished with explosives in Texas. Within seconds, a series of scheduled blasts felled the 600 feet of steel that had spanned the Colorado River in Marble Falls for 77 years.”
The use of explosives to demolish the steel truss and bridge piers is a rather unique demolition method here in Texas, she continued. “The purpose of using controlled explosives was to perform the work at one time instead of over the course of several months with the piece-by-piece demolition method, ultimately resulting in a safer and more efficient operation.”
TxDOT evaluations on the bridge detected no asbestos, but test results did indicate levels of lead paint on the bridge; therefore, all lead paint was removed from the areas where the explosive charges were placed.
The implosion, which was felt more than 25 mi. (40 km) away, knocked out 911 and cellular phone service, but otherwise caused no safety concerns. Fiber optic cable next to the bridge was damaged by shrapnel, cutting communications to dispatch call centers. Shrapnel also damaged wastewater lines near the bridge. Fortunately, as Marble Falls Fire Rescue Chief Johnny Caraway said to local media, “Wastewater runs through double lines, so no spillage happened.” Sewage was rerouted until the pipes were repaired.
Some of the steel retrieved from the water was processed and loaded onto trucks to be taken to a recycling facility. Reyna indicated that some remnants will be recycled and made into art beautification projects for the city of Marble Falls.
Bridge, Anyone?
Once the project is complete, each of the two new concrete bridges will feature two main lanes for traffic, two shoulders and a sidewalk for use by pedestrians and bicyclists, allowing for safer travel.
Designed by Finley Engineering Group for Archer-Western Contractors of Arlington, the new dual bridges are three-span variable depth cast-in-place segmental bridges with a main span of 410 ft. (125 m). The spans of the bridge are spans of 274, 410 and 274 ft. (84, 125 and 84 m), with a bridge deck width of 47 ft. (14 m). The box depth will vary from 23 ft. (7 m) at the interior piers to 6 ft., 6 in. (2 m) at the end spans with variable super elevation of up to 5.5 percent.
Project length is 0.568 mi. (.91 km); the bridge is approximately 950 ft. (290 m).
Finley performed a time-dependent staged analysis of the structure to monitor stresses and anticipated deflections during construction, revised the segment layout and optimized the post-tensioning, which reduced costs and construction time. Finley’s analysis included a model using Bridge Information Modeling, which allowed the synchronization of the CADD shop drawings with the analysis model, to produce the details quickly to meet the contractor’s fast-pace construction schedule.
The structure is being built using the balanced cantilever construction method, with the end spans constructed on falsework and consist of 5,000 cu. yds. (3,823 cu m) of concrete with a weight of 11,000 tons (9,979 t). The deck will support a 10-ft. (3 m) outside shoulder, a 3-ft. (.9 m) inside shoulder, a 6-ft. (1.8 m) sidewalk, a 1-ft. (.3 m) barrier from the sidewalk to the travel lanes and two 1-ft. outside rails.
The first bridge was completed in March 2013 as part of phase one. Phase two, which includes removing the old bridge and replacing it with the second new bridge, is expected to be completed in 2014.
Challenging Work
Construction on the new bridge began in early April 2013 and construction of the new southbound bridge is expected to be complete in fall 2014.
The project is currently on schedule to finish in the fall of 2014, according to Reyna. “We are currently in the beginning stages of building the second bridge. We have begun work on the abutments and retaining walls on both ends of the project and our drillers have started drilling the shafts.”
Weather could impact the schedule, though. “The weather plays a large role in determining when concrete pours can take place,” Reyna said. “Crews spend a large amount of time and effort controlling the heat of hydration in the high-strength concrete during the hot summer and winter months.”
Since this is a cast-in-place, post-tension bridge, the vast majority of the bridge is Class H (high early strength) concrete and American-made steel strand cable and rebar. Using standard construction equipment typically used on most construction projects, with the exception of the Form Traveler System, which is new in Texas, approximately 30 to 35 active workers plus approximately 5 to 10 supervisors are on site during a normal day when they’re building segments of the bridge or during the demolition process.
The use of the post-tension cast-in-place construction method seems to be an excellent fit for this project, Reyna said. “While cast-in-place segmental bridges have been used for a number of years around the world, they are just now starting to be used in Texas.”
An unexpected challenge during drilling for the placement of shafts on the north side of the river involved previous man-made obstructions that were roughly 10 ft. (3.05 m) deep, as well as very strong, hard rock that the driller had not anticipated, based on his review of the bore logs. The drilling operation was improved by adding additional weight to the drill bit, drilling additional smaller holes within the large diameter hole, and using a diamond rotary tooth bit.
Another unexpected challenge crews encountered was the possibility of endangered mussels in Lake Marble Falls in the area of the bridge. The smooth pimpleback mussel is a state-protected species. To receive clearance to remove the old bridge, TxDOT had to coordinate with multiple regulatory agencies and conduct a detailed survey of the bottom of the lake. After extensive surveys, none of the endangered mussel species were found, Reyna said.
“TxDOT and Archer Western worked diligently with multiple regulatory agencies such as the Lower Colorado River Authority, U.S. Army Corps of Engineers, Texas Parks and Wildlife Department, and the Texas Commission on Environmental Quality to determine wildlife and environmental concerns,” Reyna said. “We performed mussel surveys to search for endangered species and contracted an aquatic biologist to provide an 11-point plan to safeguard wildlife. TxDOT is committed to preserving our state’s natural environment and we have and will continue to take every precaution necessary to ensure the safety of motorists, wildlife and the environment.”
Maintaining a safe transportation system is a top priority at TxDOT, according to Reyna. “We are constantly working with others to provide safe and reliable transportation solutions for Texas. The construction of the U.S. 281 bridge will greatly enhance the safety of the bridge and we are excited for the completion of this project, which will allow the city of Marble Falls to enjoy their two new bridges and sidewalks.”
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