Standing 38-feet-tall and MASH TL-4-compliant, a new precast retaining wall puts the finishing touches on the Capitol Square project in Atlanta.
By Bridget McCrea
Anyone familiar with Atlanta knows just how critical the I-75/I-85 Downtown Connector is to the region’s transportation infrastructure. Where I-75 is a legacy of the old Dixie Highway (linking the Great Lakes and Florida), I-85 connects Virginia with Alabama. Situated on Atlanta’s southwestern edge, the 12- to 14-lane connector also has an interchange with I-20.
The 7.5-mile-long connector carries more than 437,000 vehicles per day at its busiest point. When the city needed a retaining wall that would stand more than 38 feet tall, extend 283 feet in length, and comprise approximately 8,800 square feet of face area, it put precast concrete at the center of the conversation with Atlanta-based Piedmont Precast manufacturing approximately 270 elements for the retaining wall.
Working in a shoebox
Since the historic Georgia State Capitol dome was located near the job site, the project served two purposes. Not only would it help improve traffic flow, but it would also add aesthetics in front of the capitol building.
Astra Group Inc., the Georgia Building Authority’s contractor for the project, explored its options for constructing the proposed retaining wall, including Earth Wall Products’ Gravix gravity retaining wall system. With panels that are supported by an integrated stem that is backfilled, the Gravix system supports slot construction, making it easier to maneuver on tighter sites like the one in Atlanta.
Involved with numerous retaining wall projects over the years, Earth Wall Products worked with the contractor to develop a preliminary design that was presented to Piedmont Precast, a licensee of the Gravix product.
“My analogy is that it was kind of like working in a shoebox,” said Jason Sailers, vice president of construction products at Piedmont Precast. “It was very confined. And I think that’s one of the big reasons they chose the Gravix system, because you didn’t have tiebacks, they had the room they needed to get the wall in.”
Shawn Springston, senior project manager at Astra Group, said the design made the below-grade work on the project go faster. This was a key benefit for the installers who were working underground within shoring walls next to the heavily traveled interstate.
“We didn’t want to be down low any longer than we had to, and the backfill material in this instance can be a mixture of different aggregate sizes,” Springston said. “It’s not so heavily constrained.
The broad range of backfill also enabled Astra to bring in recycled concrete to quickly bulk up the below-grade work, thus enabling them to beat its estimated construction time by about 33%.
The 38-foot-tall sections of the wall start with panels at the base with 22-foot-long stems extending into the earth behind the wall. The stem length of each row of panels decreases as the wall ascends, with the top row of panels having a stem length of only 8 feet.
Due to an abnormal 130-degree angle, Piedmont needed special blockouts to ensure the stems didn’t run into one another behind the wall.
“Those stem lengths enabled us to build a tall, gravity wall with a barrier on top, and at a bit of an abnormal angle,” Sailers said. “That was the unique aspect of the project from our perspective.”
The face of the wall features an ashlar textured panel and includes a graffiti and weather seal, which makes it easy to erase graffiti and slows the weathering process.
Spillman Company developed the forms for the system in collaboration with Earth Wall. They also collaboratively designed a tilt table for the project, which enabled the precaster to cast precisely angled pieces with ease.
“You set the mold on a tilt table and then tilt it to the angle you need to pour, so the concrete reaches its level,” said Thomas Rainey, president of Earth Wall Products, which licenses the Gravix system. “That’s how you achieve the angled units.”
Spillman also produced a form that included a traffic barrier base on top and a combination form that incorporates both the standard and traffic barrier features.
“The mold tilts rather than the concrete having to be tilted,” explained Ted Coons, Spillman’s EVP of sales. The project’s collaborative approach was unique for Spillman, which rarely sees the site drawings. The wall’s height and the site constraints made the form developer’s participation essential.
Rainey agreed and said the wall was a milestone for his company as it was the tallest one to date. He also noted the project’s quicker construction with fewer crew members due to the system’s modularity.
Once made, the wall and related pieces were delivered to the job site, where the contractor was able to “set it and forget it.”
“All it took was an excavator, a crane and limited crew to get everything in place,” Sailers said. “Precast helped them work within those tight confines without much impact to the general public.”
Meeting requirements, deadlines
The project also entailed adding an extension roadway for employee access near the Georgia State Capitol. Meant to relieve traffic congestion, the extension road required a traffic barrier that was MASH-TL-4-compliant.
Coons said MASH requirements factor into all of the company’s highway form work. The new MASH requirements went into effect on Dec. 31 and apply to permanent and temporary roadside barriers; longitudinal barriers, including those used on precast box culverts and 3-sided structures; and bridge rails.
“We’re aware of the dynamic change in the FHWA requirements and continue to be proactive with our customers about it,” Coons said.
Sailers said the project outcome was very positive with all timelines either met or reduced.
“The contractor pushed on us to get it built very quickly,” he noted. “That was a big selling point for the precast concrete – the ability to get this wall built so quickly.”
Bridget McCrea is a freelance writer who covers manufacturing, industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.