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Geotextiles cut costs, protect the environment and increase safety on one of America’s deadliest highways.
By Jake Kulju
The warm summer sunshine, winding blacktop roads and pristine saltwater marshes of South Carolina are more often associated with car commercials than they are with car crashes. But in 2005, the South Carolina Department of Transportation (SCDOT) recognized U.S. Highway 17, a heavily traveled road in the eastern part of the state, as one of the deadliest roads in the nation. From January 1997 through February 2005, nearly 1,000 automobile accidents, including 33 fatalities, were recorded. The long-awaited U.S. 17 widening project was designed by state and county officials to improve safety for a stretch of highway from Gardens Corner to the Combahee River. The project became a priority for the SCDOT Commission after a series of fatal crashes.
Also known as the Ocean Highway, U.S. 17 runs 1,189 miles along the southeast coast from Virginia to Florida. More than 200 of those miles pass through South Carolina. Previous construction projects in the state improved 173 of those miles, but a 6-mile stretch of heavily traveled road from Beaufort County to Colleton County was left unimproved. In 2004, funding was provided to finish the highway project. To stay within budget and provide a safe and long-lasting road, the SCDOT turned to geotextiles.
Geotextiles lower highway expansion costs
The project is straightforward: the two-lane highway is being expanded to a four-lane highway, in three phases. The unique setting and budget constraints are what make this a challenging project. SCDOT Project Manager Chris Hernandez says the uniqueness of the area makes it vital to protect its natural beauty. Designers looked for ways to create a long-lasting road that would have little impact on the environment.
The project was divided into three phases to accelerate construction and reduce the impact on motorists. Construction has begun for the three miles in Phase 1 and two miles in Phase 2; the preliminary design is currently underway for Phase 3. This 6-mile design and construction carries an estimated $80 million price tag and will have a significant impact on motorists. Project designers chose a woven polypropylene geotextile from Thrace-LINQ Inc., Summerville, S.C., to assure that no premature maintenance during the life of the road would be necessary.
Because most pavements fail prematurely due to base contamination and the subsequent loss of strength and drainability, prudent designs today achieve quality construction by starting with a separation/stabilization geotextile beneath the road. The geotextile lowers initial project costs and reduces the need for future road maintenance.
For the upgraded U.S. 17, two lanes of traffic in each direction are separated by a 100-foot wide median. Because of the weak, silty subgrade and the potential for high precipitation and groundwater levels in this marshy region, the geotextile was placed on the prepared subgrade. The road base aggregate was then placed directly onto the geotextile for improved stability and drainage.
The geotextile layer enabled the permanent separation and filtration of the subgrade and base aggregate to keep the subgrade fines from migrating up into the aggregate base while allowing the base layer to drain. This important function maintains the long-term strength and drainability of the aggregate base. In addition, the added layer enhances the stabilization of both the subgrade and the base aggregate through confinement and local reinforcement.
The design and cost benefits gained from using a geotextile are impressive. The geotextile layer used for this project costs no more than 1-2 inches of aggregate and provides significant life-lengthening and maintenance-saving qualities to the road. Weston Newton, chairman of Beaufort County Council and the Beaufort Country Transportation Advisory Group, says the widening of this stretch of U.S. 17 has been a priority for Beaufort County for some time and that the cost-effective long-term vision of this project is what made it possible.
Protecting the environment
U.S. 17 cuts through the lowland area known as the ACE Basin, where the Ashepoo, Combahee and Edisto rivers converge and meander past old plantation homes, cypress swamps and tidal marshes. This valuable habitat provides food and shelter to hundreds of different plants and animals. Additionally, the low country basin is one of the largest undeveloped estuaries on the East Coast. State and county authorities naturally wanted to preserve its pristine nature during reconstruction of the highway and in the realized design.
Bald eagles, short-nosed sturgeon, loggerhead turtles and other endangered species are among the wildlife that call this area home. The nearby Nemours Plantation also protects a large 9,800-acre area of diverse habitats including remnant rice fields, fresh and brackish water marshes, pine groves, hardwood forests and large stands of cypress trees.
In an effort to maintain the integrity of the habitat, part of this project will include a 100-foot median to preserve valuable tree canopies, says Hernandez.
“We did a tremendous amount of tree surveys because this area has a large number of significant and specimen oak trees,” he says. “We made a commitment to save as many of those as possible.”
Along the route, engineers will be installing several guardrails and possibly tree wells, which stabilize trees and protect their root structure. Silt fencing is also being installed, which will protect the area marshland by providing erosion control, Hernandez says.
To allow animals to continue to migrate through the wetlands, two 100-foot flat slab bridges will be erected. Informally named “critter crossings,” these thoroughfares will be used by snakes, deer, armadillos and alligators.
Former SCDOT project manager Wilson Elgin oversaw the development of the environmental documents and permitting at the beginning of the project.
“I think it’s a worthwhile project because it addresses safety as well as the environmental concerns,” Elgin said.
The agency coordination team included several state and federal agencies, such as the Environmental Protection Agency, U.S. Geological Survey, U.S. Fish and Wildlife, U.S. Department of Defense, and a local presence, including the ACE Basin Task Force.