Better Roads Staff
Road Into Bridge
A massive landslide causes Tennessee DOT to rethink its rebuilding
By Tina Grady Barbaccia
Sometimes it’s just better to start over.
That’s exactly how the Tennessee Department of Transportation (TDOT) decided to handle the collapse of a nearly three-mile stretch of State Route 7 in Maury County after a huge landslide following devastating flooding.
Faced with a daunting rebuilding, TDOT dramatically changed course.
Heavy rains saturated the ground underneath the pavement, causing two sections of the roadway to collapse and slide down a steep embankment, closing the roadway for several months. The extreme weather and flooding struck southern middle Tennessee April 30 to May 2, 2010. The stretch of State Route 7 located in the Santa Fe community in the northwest section of Maury County was shut down while a concrete deck bridge was built over the crumbled roadway.
Following the landslide, TDOT determined that constructing a bridge over the damaged section of roadway would be more cost-effective than reconstructing the area.
So, TDOT built a 684.5-foot-long, 62-foot-wide, three-lane, five-span bridge. The project had a total of 338,580 cubic yards of road and drainage excavation, and 547,502 pounds of steel bar reinforcement was used. The bridge work used 2,257 cubic yards Class of A, D and S concrete.
The concrete girder deck bridge first had to be designed, and was then let to contract in December 2010. Construction on the $9.5-million project began in early 2011, and the ribbon-cutting for the bridge was Nov. 30, 2011. TDOT Commissioner John Schroer says that “this was the most extensive repair project initiated by TDOT after the May 2010 floods.”
A Sinking Road
The two landslide areas involved nearly 1,500 feet of roadway. In some places, the road sank 20 feet below its original elevation, prompting TDOT to initiate an emergency contract for geotechnical studies of the soil in the area and for the design of the repairs. “It’s rare to see a landslide of this magnitude,” says TDOT geotechnical engineer Vanessa Bateman.
Before action was taken to begin stabilization of the soil or any repair decisions were made, Bateman noted that “any subsequent wet weather could trigger more slides in this area and could further complicate repair efforts. The ground remains extremely unstable.”
TDOT chief engineer Paul Degges refers to the rain as “a thousand-year rainfall – a very improbable event. Our average annual rainfall is 45 inches.” Degges says that although the state highway transportation system fared well – no bridges washed out – State Route 7 in Maury County was another story. “We had a road riding on the side of a ridge, and all of the earth got saturated,” says Degges. “There was a great big earthslide.” The slide was about 300 feet along the center line of the road, but it slid about 1,000 feet into the valley, he says.
“We had 300 feet by 1,000 feet of unstable soil,” Degges points out. “When you have a big failure like this, you have to know how deep down it is moving. It was 40 feet down and was moving. For us to build the road back, we would have had to dig up all of the dirt, rebuild and then compact it in a stable form.” As the state agency was going through the process of coming up with cost estimates to conduct these procedures, the numbers were coming in at about the $15-million range. “Moving and compacting that amount of earth in today’s financial climate is very expensive,” Degges says.
That’s when he proposed to his staff the idea of just rolling dirt, draining it and then only stabilizing the areas where the piers for a bridge would be. By choosing to drain, stabilize and compact specific areas, and build a bridge instead of rebuilding the roadway, Degges says TDOT was able to save about one-third of the cost, bringing the outlay down to about $10 million.
The job also qualified for emergency relief funds, with TDOT expected to receive about 80-percent reimbursement. Nearly $40 million in emergency relief funds was made available for damage spanning 50 to 60 percent of the state of Tennessee. The May 2010 floods caused millions of dollars in damage to the state’s transportation infrastructure. TDOT worked with the Tennessee Division of the Federal Highway Administration (FHWA) to quickly initiate contracts and complete repairs to numerous roads and bridges across the state. The FHWA (at publication time) had reimbursed TDOT for $39 million in flood-related repairs, with $1.8 million of those funds dispersed to local governments.
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