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Magazine » June 2012 » Features » PROJECT CASE STUDY

Stringent schedule for slope protection
Large-scale interstate project includes extensive erosion control effort.

McCarthy Improvement Company crews hydraulically applied Flexterra HP-FGM in a two-step technique to best prevent erosion on the slopes and to maximize vegetative establishment.

Wrapped up in the $60 million rehabilitation project of a 15-mile stretch of Interstate 385 (I-385) recently constructed by the South Carolina Department of Transportation (SCDOT) 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. And much of this had to take place during the winter and spring seasons that saw heavy rains throughout construction – not exactly an ideal scenario for such a large erosion control application.

The project had to follow a tight eight-month timeframe, further complicating matters. With a project that would typically take two years to complete, that meant there was no time 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 had been closed on an interstate in the state's history. In addition, the contractor tabbed to handle the project, McCarthy Improvement Company, would be fined $50,000 for each day the roadway remained closed to traffic past mid-August.

Strict specifications
On top of the time crunch, McCarthy Improvement Company also had strict erosion control product specifications and performance expectations to contend with. In January, the 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 selected Profile Products LLC's Flexterra High Performance-Flexible Growth Medium (HP-FGM). "The Department of Transportation said the road will be open no later than August 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."

Slopes were directionally tracked and then seeded the same day.

"The state considered the project very sensitive and high profile," said Austin Childers, regional manager for Profile Products LLC. "Flexterra HP was approved by the South Carolina DOT for that category."

Childers said the project started with Flexterra FGM, but then later stepped up to the new and improved Flexterra HP-FGM formulation to establish vegetation and prevent soil erosion on the more than 300 acres within the construction right-of-way. The product's wet bond strength and ASTM-documented functional longevity makes it stand out, he said.

"It's going to hold up against more rainfall and stronger downpours," he 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 the products utilized to establish vegetation had to work right the first time because of the stringent timelines the contractor was bound to. Each working day, the contractor prepared (directionally 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 to progress along the I-385 alignment in a highly controlled fashion.

"The contractor had such a tight timeframe 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."

How it works
HP-FGM is hydraulically applied and designed to bond immediately to soil, even under wet conditions, making it a good solution for controlling erosion on the exposed rolling hill slopes within the I-385 project corridor. According to Profile, in independent ASTM lab tests Flexterra HP-FGM was shown to be 99-percent effective while vegetative establishment was documented to grow eight times faster than bare soil and twice as fast as rolled erosion control blankets. It's also faster and less expensive to use than blankets, as no fine grading is required to smooth the slopes before application. HP-FGM is the next generation in the Flexterra product line, working much like its predecessor, only it's 100-percent biodegradable and manufactured from 100 percent 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 that were 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 of the HP-FGM to achieve the specified application rate in the second pass.

Specific to the I-385 project, HP-FGM first was mixed with ACF Environmental's SlopeShield, a specially designed erosion control grass seed mix, and fertilizer, then applied. "You get that seed-to-soil contact with that first tank load that's so important," Childers said.

After the first pass, McCarthy went back and applied the remaining HP-FGM two separate times in opposing directions to cover and protect the first layer of grass seed that was previously applied.

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.

Waystack said that the HP-FGM stood up to all the potential pitfalls of the project – the tight timeframe, the heavy rains, the 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.

"We're calling this project our Chia Pet. It's green and it has more hair than a dog's back," Waystack said.

Jeff Salem is a public relations associate at Swanson Russell, based in Lincoln, Neb.

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