Nestled into the South Carolina Department of Transportation’s (SCDOT) recent $60-million rehabilitation of a 15-mile stretch of Interstate 385 was a very difficult erosion control assignment. More than 300 acres of steep slopes would need vegetation established quickly to keep soil from eroding in areas dissected by two dozen streams. Furthermore, much of this had to take place during winter and spring with heavy rains throughout construction—not exactly an ideal scenario for such a large erosion control application.
The erosion control work had to follow a tight eight-month time frame, further complicating matters. With a project that typically would take two years to complete, that meant there was no room for second chances. The state of South Carolina mandated that the project be fast-tracked because the stretch of interstate being revamped was so well traveled (about 21,900 vehicles travel I-385 each day) and more than $34 million in savings would be realized with a condensed construction schedule.
The interstate was closed to one direction of traffic to hasten construction, which began in January 2010. It was the first time an entire direction of traffic was closed on an interstate in the state’s history. In addition, the contractor tapped to handle the project, McCarthy Improvement Co., faced a $50,000-per-day fine for each day the roadway remained closed to traffic past mid-August.
Specifications
On top of the time crunch, McCarthy Improvement also had to contend with strict control product specifications and performance expectations. In January, SCDOT had initiated new hydromulch specifications for all state highway projects. The size, cost and importance of the I-385 project called for the most stringent of the state’s four hydromulch categories to be used: fiber-reinforced matrix.
Project officials trusted the job to Profile Products LLC’s Flexterra high-performance flexible growth medium (HP-FGM).
“There were no ifs, ands or buts. The Department of Transportation said the road will be open no later than Aug. 15,” said Tris Waystack of ACF Environmental, the distributor that supplied HP-FGM on the project. “They had to align themselves with people and products that were going to work the first time, and that’s what they did.”
“The state considered the project very sensitive and high profile,” said Austin Childers, regional manager for Profile Products. “Flexterra HP was approved by the SCDOT for that category.”
Childers said the project started with Flexterra FGM but later stepped up to the new and improved formulation to establish vegetation and prevent soil erosion on the more than 300 acres within the construction right-of-way. The solution’s wet-bond strength and ASTM-documented functional longevity make it stand out, he said.
“It’s going to hold up against more rainfall and stronger downpours,” Childers said. “There were several creeks and streams in the area, and we wanted to make sure we didn’t release any sediment into these water bodies.”
Waystack noted that the products utilized to establish vegetation had to work right the first time because of the stringent timelines to which the contractor was bound. Each working day, the contractor prepared (cat-tracked) the slopes to be revegetated. Then, the seeding crew treated the slopes before the close of day. This minimized the amount of slopes potentially exposed to storm events and allowed McCarthy Improvement to progress along the I-385 alignment in a highly controlled fashion.
“The contractor had such a tight time frame that as they were spraying Flexterra, others were putting up guardrails,” Waystack said. “The contractor knew there was no way at all they could come back and redo anything. If the product failed, it was going to be extremely difficult to finish on time.”
Application
HP-FGM is hydraulically applied and designed to immediately bond to soil, even under wet conditions, making it an ideal solution for controlling erosion on the exposed rolling hill slopes within the I-385 project corridor. In independent ASTM lab tests, the medium was shown to be 99% effective; vegetative establishment was documented to grow eight times faster than with bare soil and twice as fast as with rolled erosion control blankets. The solution also is faster and less expensive to use than blankets, as no fine grading is required to smooth the slopes before application. HP-FGM is entirely biodegradable and manufactured from 100% recycled wood fibers and naturally derived biopolymers.
A two-step application technique was chosen to best prevent soil from possibly eroding into the numerous waterways and to maximize vegetative establishment over the entire 300 acres to be treated. This method employs the placement of seed, fertilizer and agronomic amendments with a small amount of HP-FGM in a first application, followed by a separate layer to achieve the specified application rate in the second pass.
First, the product was mixed with ACF Environmental’s SlopeShield, a specially designed erosion control grass seed mix, and fertilizer. Next, workers applied it.
“You get that seed-to-soil contact with that first tank load that’s so important,” Childers said.
After the first pass, McCarthy Improvement went back and applied the remaining solution two separate times in opposing directions to cover and protect the first layer of grass seed that was applied previously.
An Early Finish
Aside from a few localized, concentrated flows after particularly heavy rains, the application was unscathed and allowed the project to finish early.
The interstate was reopened to motorists on July 23, almost three weeks ahead of schedule. Those involved with the project said the contractor’s precise use of the hydraulic erosion control solution helped the work to be completed on time.
When all was said and done, according to Waystack, the technology stood up to all the potential pitfalls of the project—the tight time frame, heavy rains, sheer amount of application—and delivered on its promise to keep soil out of the streams and produce vibrant vegetation adjacent to the new interstate improvements.
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