It is said that saving time lengthens life. Thanks to a Colorado Department of Transportation (CDOT) bridge replacement project, life expectancy just got longer across the Big Thompson River.
A goal is a dream with a time limit. So when CDOT sought to replace two 70-year-old metal truss bridges that carry State Highway 34 across the Big Thompson River, the goal was clear. So was the time limit. After prescribing a traditional construction window of 16 to 18 months-which required a six-month partial road closure and a temporary detour across environmentally sensitive lands-CDOT went looking for a project team to help execute their plan. The selected project team worked closely with CDOT, employing innovative technologies and fast-track methods that would advance the project's timeline, reduce traffic impacts, protect the environment and save taxpayer dollars. Through a dramatic redesign, CDOT’s $2.6 million bridge replacement project became a textbook example of how fast-track construction and design innovation can optimize a project solution.
"The bridges were scheduled for replacement because of their age; they had reached the end of their useful life," explained Rick Gabel, program engineer for CDOT Region 4 North. "Originally we designed the project based on traditional methods of construction. Because of the restriction in the canyon, we would have had to build a detour around each location, come in and remove the old bridges, and construct new bridges while maintaining traffic on a one-lane, signal-operated detour during peak-use periods for State Highway 34. We knew this would be an inconvenience for travelers, especially during the summer, but we had few options; the bridges had to be replaced. In light of all the project complexities, the project was awarded to Lawrence Construction with DMJM Harris. And the project team immediately developed a value engineering change proposal (VECP) that created a whole new approach to the project.
"They came to us with a proposal for fast-track construction, a proposal that would dramatically expedite the project. But this wasn't just about speeding up the schedule. This was a major perspective shift in terms of environmental mitigation, community convenience, cost savings—the whole project. For example, rather than close the road for six months, they were looking at a road closure of less than two weeks. There were some challenges, and a few construction methods involved that we had never used before, but as a department, we're very open to trying new ideas. And we quickly saw the benefits of this method."<
Thompson twins
The project site is just west of Loveland, Colo., near the entrance to the steep-walled Big Thompson River Canyon. The original contract documents called for the construction of two bridges over the Big Thompson River (a 149-ft two-span bridge and a 219-ft three-span bridge), along with two temporary detours, stream embankment mitigation and roadway improvements. This was all supposed to be done during a six-month bridge closure. A lifeline route was considered to be essential for area residents, since approximately 7,500 vehicles use the bridges each day. And during the summer the traffic increases significantly as tourists flock to the Rocky Mountain National Park and Roosevelt National Forest in the Estes Park area. But traffic was not the only issue.
"The shorter closure period proposed by the team not only reduced the traffic impacts to the community, but also minimized the environmental impacts associated with the longer schedule-reducing the number of trees removed, the impact on the wetlands and fish habitats, and the impact on the river itself in terms of silt buildup," explained DMJM Harris Project Manager Gary Maji, P.E. A noted transportation architecture and engineering firm, DMJM Harris was contracted by Lawrence Construction to provide design and scheduling expertise and limited construction oversight on the project, as well as serving as the project engineer of record. Maji continued, "But there were some considerable design and construction challenges associated with implementing such a shortened schedule.
"First, the project required staged construction techniques to build cast-in-place and precast concrete bridge components under traffic. Partial construction of the abutment structures was accomplished with the use of open-trench boxes, prefabricated reinforcement cages and one-lane road closures. And cast-in-place and precast straddle pier caps were used for the intermediate supports. These caps were supported by pier columns located beyond the limits of the existing structure edge of the deck. Precast side-by-side box girders that were laterally post-tensioned and a 3-in. asphalt and waterproofing membrane topping allowed for quick superstructure construction. Concerns over the ability of the deck protection system to provide the desired 75-year durability resulted in the team's decision to provide a 6-in. cast-in-place concrete deck on the box girders, with a 3-in. asphalt and membrane topping (in lieu of using laterally post-tensioned box girders and a 3-in. asphalt and waterproofing topping).
"Finally, to minimize work in the river, precast concrete block and cable scour mitigation blankets provided necessary hydraulic and embankment protection along the river. Through considerable analysis and constructibility testing, we ensured that all of the precast and cast-in-place elements would fit appropriately when and where they were needed. The project team took a lot of time and effort to make sure that all of these elements would work together. We had to have great confidence that the pieces of the puzzle would fit. Without that certainty, we could not ensure the schedule would be met."
Staging, though, was not the only key to optimized delivery. Use of the design-build approach allowed design and construction to commence simultaneously. This meant that once CDOT approved each "release for construction" package, the project team could begin work in the field for a particular segment. For example, it was critical that pier caissons be constructed first, because of concerns that the spring thaw would raise the river’s water level. As the pier design and construction progressed, designers prepared construction sketches and design details for future construction. And there was another positive by-product of this working method.
There is speed in team
This incremental submittal process facilitated open discussion of proposed design solutions, identification of critical issues and expedition of the design approval process among all project team members in subsequent construction phases. Through this close partnership, the project team was able to execute design and construction concurrently. And the overall process was actually enhanced because of the pressing need for close, clear communication to make simultaneous advancement possible and productive. This is graphically illustrated by some of the unexpected challenges that arose during the process.
In one instance, girders placed after the road closure were not sitting squarely on the abutments. The project team leveled the girders at the pier supports with shims and pressed on. When unexpected rain softened subgrade soil, more suitable soil was brought in quickly to stabilize the soil for paving. Near the upper bridge, a rock fissure channeled water into the roadway subgrade. The project team worked together to design a drainage system that redirected the water. While project teams often encounter unexpected obstacles, what set this project apart was the team’s collective ability to respond quickly and effectively. According to DMJM Harris Project Principal Mark Mehalko, P.E., that degree of partnership and cohesiveness is a direct result of the VECP process.
"From environmental issues to road closures to costs, the whole VECP concept was used to examine potential risks and devise the most advantageous plan for this project," Mehalko explained. "But we had to work through some pretty new technical details on the actual design and construction of the bridges. We had precast elements that had to fit-like the precise pier cap and precast abutment cap-elements that we normally don't use. These elements had to fit perfectly to the substructure out in the field. It took a little extra care in terms of dimensioning and detail to make sure that we had a proper fit. But failure to do so could have meant extending the road closure and schedule. That would have negated the value of the VECP. We looked at the components well in advance and made sure that everything was right. And that speaks to the close collaboration across the entire project team. When unexpected challenges presented themselves, the project team was in a perfect position to handle them."
That collaboration extended beyond the project team as well. To facilitate the temporary detour for the two-week closure, CDOT worked closely with Larimer County on issues concerning county road access. After agreeing to take responsibility for signage and maintenance, CDOT was granted use of county roads as a detour alternative. Again, that speaks to the general spirit of cooperation on the project. And it's a spirit that CDOT intends to use again. According to Gabel, CDOT knows a good process when it sees one.
"Obviously, we were really happy with how the project turned out. And we were equally happy with the cooperation that was forthcoming from Lawrence and DMJM Harris. They helped us through our concerns and in the end gave us a very nice project. And as a result, we developed a new value-engineering specification for CDOT. The state already had value-engineering specifications. However, they excluded value-engineering proposals on bridges. The reasoning was that it was counterproductive to discard the considerable amount of time, effort and energy our bridge and design staff put into a bridge design. At least, that was the thinking when that exclusion was made. As a result of this project, though, we re-evaluated that thinking and developed a new specification. We established controls to determine what types of bridges are subject to value engineering. But even within those new limits, we're always open to innovation on these structures. And much of that is a direct result of this project.
"This project is a good example of how we use innovation on difficult jobs, bringing in outside expertise to tap new ideas. Good ideas come from everywhere. And outside experts often have developed new ways to do things. As an agency, we want to be open to that. It’s how public/private partnerships should work."
A goal is a dream with a time limit. When CDOT established the goal of replacing the two bridges over the Big Thompson River, they also followed conventional wisdom and set an industry-standard process and time limit. But through innovative design and value engineering, that timeline changed, dramatically reducing traffic impacts, better protecting the environment and saving both money and time. And if saving time lengthens life, CDOT's bridge replacement project just lengthened life expectancy across the Big Thompson River.
