The Interstate 15 (I-15) Brigham Road Bridge replacement is one of many Utah Department of Transportation (UDOT) projects constructed with precast concrete, a material that brings so many benefits to these infrastructure investments.
The design features precast concrete partial-depth deck panels to replace the formwork and half of the concrete and rebar. Historically, these project elements have been cast-in-place on the job site, but a cast-in-place design would not meet the project goals, says Spencer Stephenson, project engineer with Horrocks Engineers. UDOT and the design team developed an aggressive 11-month construction schedule that required construction be substantially completed before the winter season. “Choosing precast concrete shortened the construction duration so that the critical-path construction schedule elements could be met and the project completed on time,” Stephenson says.
The construction timeline was further complicated because the bridge is built over the Virgin River floodplain, which provides critical habitat for several threatened and endangered (T&E) species, including the southwestern willow flycatcher and the Virgin River chub. The construction teams had to be mindful of breeding seasons and minimize disruptions to the ecosystem of the delicate environment. “The biggest risk in meeting the tight timeline was getting critical bridge substructure elements installed within those environmental constraint windows, while also dealing with the flooding events from the Virgin River,” Stephenson says.
To complete the project, the river had to be shifted from its original course so the drilled shaft foundations could be placed, and construction could not take place during summer months when the habitats were most vulnerable. The project team installed the substructures and columns prior to the restricted period and sequenced construction to avoid impacts during sensitive breeding and migration periods for the T&E species.
To shorten the duration of disruption, the Horrocks team developed a strategy with the girder fabricators to complete the inside lanes of the new structure in the spring and the outside lanes in the fall. “The strategy was contingent on a quick start, with drilled shaft foundation installation for both phases of construction being the critical path,” explains Lee Cabell, project manager for Horrocks. The phasing and methodology used to complete the bridge structure efficiently allowed the team to work within the environmental constraints and keep two lanes of travel on mainline I-15 open at all times throughout the duration of the construction. This strategy reduced delays for motorists and ensured the safety of the traveling public by virtually eliminating back-up on I-15.
Once construction was completed, the team restored and revegetated the floodplain habitat with willows and natural vegetation, and installed T&E-friendly voidless riprap for erosion protection during flood events.
Going forward, the decision to use precast concrete will have lasting benefits, Stephenson notes. “Using precast concrete girders eliminates the need for routine maintenance to the superstructure elements,” he explains. These long-term benefits lower the overall cost of the project and will protect the sensitive river channel for decades to come.