“The existing bridge was an extremely narrow overpass with one travel lane in each direction and no accommodations for pedestrians,” explains Jody Braswell, GS&P senior transportation engineer. “There was also poor sight distance from the bridge approach because the vertical roadway curves leading up to the overpass didn’t meet today’s standards. Coupled with the existing side roads’ close proximity to the bridge— which made it difficult for motorists to see drivers coming from both sides of the overpass—these safety issues resulted in a large number of crashes, either on or near the bridge, that included multiple fatalities. So improving safety was a top priority with this project.”
In addition to enhancing safety, planning for future expansion was another key design driver.
“It was imperative that we coordinate with the work being done by the Georgia Department of Transportation (GDOT) on I-85 because that work would ultimately determine I-85’s future footprint,” says Kent Black, GS&P senior vice president and principal-in-charge on the project. “Once that footprint was determined, we could start designing the new bridge and its span to accommodate future plans for the interstate from both a vertical and horizontal perspective.”
Putting America Back to Work
As part of the American Recovery and Reinvestment Act signed into law by President Obama in 2009, it was critical that the project was delivered on schedule.
“The project was part of a President-mandated stimulus plan that was designed to jumpstart the economy, and it was a huge investment in the local infrastructure,” says Black. “All eyes were on this high-profile project—from the media to the Federal Highway Administration, from GDOT to Gwinnett DOT—and it was absolutely critical that we came through for the County. Gwinnett DOT had made a commitment to GDOT to have the project ‘shovel ready’ so they could deliver on the stimulus program. Because the final design and construction letting were on an accelerated schedule and we were able to deliver as promised, the project received the federal funding under the TIGER grant stimulus program. It was certainly a unique project component that ended up being a home run for everybody.”
Staging and Construction
Another distinctive element that sets the $12 million project apart was the staging of the bridge’s construction. As Gwinnett DOT was adamant that construction take place with minimal traffic disruptions on both SR 324 and I-85, the new overpass was designed to accommodate an innovative two-phase construction process.
“Because of the high traffic volumes the corridor handles, the new bridge had to be built in two separate phases while maintaining traffic flow,” says Braswell. “This involved keeping the existing two-lane bridge open, and designing the new overpass so that two out of the four lanes could be built next to the old bridge. As soon as the first two lanes were built, traffic was transferred onto those lanes, the old bridge was demolished and removed, and the two remaining lanes were constructed.”
Once the two phases of construction were complete, the bridge segments were attached with cross bracing, and a concrete slab closure pour was executed.
To help support heavy traffic volumes and consequently reduce crash potential, the new bridge was designed with two 12-foot travel lanes in each direction—a huge upgrade from its predecessor, which only accommodated one lane in each direction.
In addition—at 466 feet—the new overpass was designed to be longer than the old bridge and with a flatter curve, improving both sight and stopping distances.
“Our design solution gives drivers the ability to see far enough ahead to stop before colliding with something in the roadway—whether it’s a car, a pedestrian or road debris,” says Braswell.
GS&P’s bridge design also implemented curbs, six-foot pedestrian sidewalks and a 32-foot-wide raised concrete median (designed to be removed during construction of the anticipated full-service interchange), which increases traffic separation and reduces the likelihood of head-on collisions.
To further improve sight distance and safety, two adjacent side roads—Camp Branch Road and Morgan Road—were relocated to intersect SR 324 further from the bridge. The relocation of the roads allows for future exchange ramps to tie to SR 324 while maintaining median open spacing.
Coordination between Gwinnett DOT, GDOT and the Federal Highway Administration was conducted throughout the design of the bridge to ensure that future widening on I-85—as well as the addition of future collector-distributor/HOV/toll lanes—was planned for. To achieve this goal, the new overpass was designed so that intermediate piers for the bridge were located between the future widened travel lanes and the potential collector-distributor travel lanes. Vertical walls that support each end of the bridge span were designed to allow for the greatest width possible underneath the overpass, and a typical section of I-85 was used to develop the drainage ditches underneath the bridge.
“There’s a grand plan for I-85,” says Braswell. “It currently has four travel lanes—two in each direction—but in the future it will be able to accommodate up to eight general purpose lanes, four CD lanes and eight managed lanes.”
Widening the existing two-lane rural roadway to an urban four-lane highway for just under a mile across I-85, the new SR 324/Gravel Springs Road bridge completes Gwinnett County’s link of four lanes along SR 324 from SR 20 to SR 124. The signature structure effectively accommodates future plans for I-85, and offers enhanced safety for both motorists and pedestrians.
“Many times in our business the best feedback is no feedback,” says Black. “Since the new SR 324 bridge has been open to traffic, there have only been two minor accidents reported—neither resulting in injuries—through this section of roadway, and no crashes on the bridge. So there have been no reported safety problems like the ones that previously hampered the corridor. All parties involved have identified that it was a successful project, and that it has a good legacy in the county.”
As a result of this success, Gwinnett County has asked GS&P to develop a concept for making SR 324 a full-service interchange with I-85. Due to GS&P’s innovative design solutions, minimal work will be required to the existing bridge or roadway to accommodate such a full-service exchange.