Prior to demolition of the 1-84 westbound bridge, crews began tentative work on the GRS system in the median to prepare for building structures in the middle of I-84. At the top levels of the GRS landscape, shown here, 4 in. layers of soil are sandwiched between layers of geotextile. Utah Department of Transportation
Two projects, one in Utah and one in Colorado, will be the first to use geosynthetic-reinforced soil for the abutments of interstate highway bridges. The method is saving time, money—and headaches.
September 3, 2013—Geosynthetic-reinforced soil (GRS), a methodology that has often been used to build mechanically stabilized earth walls, is now being used for the first time to construct the abutments of bridges that are part of the interstate highway system. The Utah Department of Transportation (UDOT) recently completed twin bridges carrying Interstate 84 over a county road near Echo Junction, a small town 50 mi east of Salt Lake City. And the Colorado Department of Transportation (CDOT) has received a $2-million grant from the Federal Highway Administration (FHWA) to construct the first multispan interstate highway bridge in the nation, on I-70 near Aurora.
A GRS abutment is constructed on a relatively shallow reinforced-soil foundation by building up layers of compacted soil sandwiched between sheets of geogrid fabric to the desired height. The FHWA is promoting the use of the method as part of its “Bridge of the Future” initiative, stating on its website that the method reduces costs by 20 to 60 percent over conventional methods, is easy to build with commonly used equipment, and is easy to maintain because it comprises fewer component parts.
Construction of the $5-million Utah project began on April 22—the second of the twin bridges completed August 17, when crews slid its bridge deck into position. The project was fully completed on schedule on August 22, four months from the start date.
Construction of the Colorado project begins this month and completion is scheduled for 2015. The multispan, three-lane bridge will cross over both Smith Road and the Union Pacific Railroad lines, replacing existing two-lane bridges completed in 1965.
The GRS walls for the I-84 bridges, like the median wall shown
here, are faced with nonstructural, decorative concrete block to
create an aesthetically pleasing structure. Utah Department of
Phased construction is helping both projects keep traffic moving while construction takes place. The UDOT planned to construct the twin 50 ft long, 18 ft high single-span bridges in three phases to minimize traffic disruption. In the first phase, a GRS wall was constructed in the median and the future eastbound bridge was built on it. In the second phase, the existing westbound lanes were closed, traffic was moved to the new structure, and the westbound GRS wall and bridge were constructed. In phase three, traffic was shifted to the new westbound bridge and the median bridge while work was completed on the eastbound GRS wall. Finally, the median bridge was slid into its final position on the GRS abutments to create the new eastbound bridge.
“Using GRS technology we built two abutments in three phases, completing each phase in seven days—about a third of the time it would take using conventional steel and concrete construction,” says Matthew Zundel, P.E., a resident engineer for the UDOT. The two GRS abutments extend across the median under all four lanes of I-84, allowing both bridges to be carried on the same abutments, Zundel explains. At the lower levels, soil layers are compacted to a depth of 8 in. Soil depth is reduced to 4 in. for the final load-shedding layers beneath the bridge beam seat, Zundel says.
“Ohio—which leads the country in GRS bridge construction with 23—and other states have been using an open-graded soil that is relatively free-draining,” Zundel says. “We elected to go with UTBC [untreated base course] soil that is finer and beefier for this application.” This choice, he says, is “more conservative.” Compacting the soil by 96 percent during construction eliminates settlement as a long-term concern, he adds.
In the project’s final phase, construction crews demolished the
eastbound bridge structure and moved traffic into a temporary
configuration in the median while they built the permanent
eastbound structure. Utah Department of Transportation
In another groundbreaking move, the I-84 project represented the first time a precast bridge was slid into place over a GRS wall, to the best of UDOT’s knowledge. The precast bridge deck elements were delivered to the site, connected to their end diaphragms, and posttensioned before being slid into their final position. The UDOT now has completed more than 40 bridge slides, the largest number in the country.
Although such bridge decks can be placed directly on the abutment mass, for this project the UDOT placed them on concrete blocks on top of the abutment. The completed I-84 abutments are faced with nonstructural concrete block for aesthetics.
GRS technology simplified logistics for the I-84 project, Zundel notes. “GRS is ideal for rural areas like this project where concrete and steel would have to be hauled long distances to the construction site,” he explains. He also notes that soil and the geotextile are comparatively inexpensive and easy to transport, and GRS requires only simple equipment and no special training for the crew.
The FHWA says that GRS construction blends the roadway into the abutment without requiring any joints or cast-in-place concrete, creating a smooth, “no bump” transition from road to bridge. This was an important consideration for the CDOT in designing the twin I-70 bridges, because the 454 ft long, three-span structures will require multiple foundation systems that will settle at different rates. Caisson foundations will be used for piers 2 and 3, driven piles for one of the abutments, and the GRS system for the other abutment. This mix was required in part because of clearance requirements from the railroad.
The UDOT raised the height of the bridge deck to meet clearance
standards that would accommodate the local farming and ranching
community. Utah Department of Transportation
The bridge will utilize two large, trapezoidal GRS zones to support abutment 4, says Shingchun Trever Wang, Ph.D., P.E., the bridge engineer for the CDOT’s regions 1 and 5. The GRS zones are both approximately 70 ft long at the riding surface and 25 ft deep behind the face of the abutment. Their pronounced trapezoidal shape creates sides as long as 200 ft for abutment 1 and 133 ft long for abutment 2. The GRS zones will reduce the need for excavation by approximately 50 percent, he says.
The piers are expected to exhibit no settlement, but the approach embankment is expected to settle as much as 3 in., Wang says. These variable rates of settlement can create what is known as distinct bump along the riding surface that can be distracting to motorists and can create issues with water ponding. The GRS abutment is projected to settle just 1 in., creating a smoother transition. This reduction in settling compared to the approach embankment can be attributed to both the large GRS zones supporting it and that fact the abutment will be built atop soils already compacted by the existing bridge’s abutments for the past 48 years.
“We use the trapezoidal GRS zone to improve the condition of the embankment and provide a stiffness transition,” Wang says. “We can eliminate the possibility of the bump problem.”
The bridge girders will be supported on spread footings supported by the GRS zones. The GRS zones will comprise 4 in. thick layers of soil, divided by strong layers of low-cost synthetic fabric to provide structure and support. Unlike the Utah project, this one will employ sheet piling for the facing.
Construction crews used cranes to engage a powerful pulley
system that would successfully slide the bridge deck 55 ft to its
final position. Utah Department of Transportation
“The new bridge is double the width of the existing riding surface,” Wang explains. “So we have a much bigger footprint. The sheet pile accommodates staged construction better.” Aesthetic concerns about the appearance of the exposed, undulating steel sheet piles were abated when the design team developed a paint treatment in a vivid color. The sheet piling will be anchored via deadman tiebacks.
The CDOT plans an accompanying research project to document their experiences with the GRS system. “This one is not normally covered in our soil mechanics or geotechnical textbooks,” Wang says. “The textbook looks at the bearing capacity of a soil. This is not a soil. It’s not a pile. This is a very closed-spaced reinforced soil. It is a reinforced soil pad that I think provides a reasonable bearing capacity.
“We want to know more about that bearing capacity, earth pressure, and deformation,” Wang explains. Instrumentation will be installed in the abutment and the CDOT’s research department will collect data to better understand the GRS system’s performance in this application. The research project will lead to a report, detailing lessons learned.
For his part, Zundel admits he was skeptical of GRS at the outset but now champions its benefits. “GRS allows for the elimination of the entire concrete substructure of a bridge and replacing it with inexpensive materials without compromising performance,” he notes.