U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
|Accelerating Infrastructure Innovations|
Publication Number: FHWA-HRT-06-024
Date: April 2006
The use of geotechnology to build stabilized grass roadway pulloff areas along the Gatlinburg Spur of the Foothills Parkway in Tennessee's Great Smoky Mountains National Park has increased safety along the roadway, while minimizing the impact on the surrounding environment. Owned and maintained by the National Park Service, the Gatlinburg Spur of the Foothills Parkway is a section of US 441 and US 321 that runs between Gatlinburg and Pigeon Forge, Tennessee. The roadway has grass rather than paved shoulders, following standard design for Park Service roads and parkways. However, unlike typical park roads, the Spur has a large volume of high-speed traffic generated by tourist attractions in Gatlinburg and Pigeon Forge.
"Because of the combination of high volumes of traffic, excessive speeds, changes in geometric configuration, and tourists unfamiliar with the roadway, the number of accidents in the area is high," notes Martin Hatcher of the Federal Highway Administration's (FHWA) Eastern Federal Lands Highway Division office. There were very few areas along the roadway where motorists involved in accidents could safely pull off and wait for help to arrive, and where park rangers could pull off to assist them. To solve the problem, the Park Service asked the Eastern Federal Lands Division to design and construct safe and stable pulloff areas to be used by motorists and park rangers.
FHWA constructed eight soil stabilized pulloff areas in 2004. The pulloffs were built at sites designated by the Park Service that are prone to reoccurring traffic accidents. They were typically located at the beginning or end of a horizontal curve or near some feature restricting the shy zone (the space from the edge of the travel lane to the nearest impactable object). The pulloffs were spaced along the length of the Spur, with four located on the northbound side and four on the southbound side. All of the pulloffs are approximately 21-m (70-ft) long, with a 9-m (30-ft) taper on the approach, a 6-m (20-ft) full width area, and a 6-m (20-ft) taper on the exit end. The pulloffs are capable of holding two cars. "The intent was to create a safe pulloff for emergency use and not to create a permanent parking area to be utilized by tourists and fishermen," says Hatcher.
Top: Geoblocks are installed and cut to fit a roadway pulloff area in Tennessee's Great Smoky Mountains National Park. Bottom: The completed pulloff areas blend in well with the surrounding environment.
To determine the best practice for future projects, FHWA used a different stabilization method for each pulloff area. All eight areas were built with a woven geotextile fabric on the subgrade for separation. The backfill consisted of a mixture of aggregate and topsoil. The first area was constructed as a control lot, using only geotextile and the aggregate-topsoil mixture typically used on parkway shoulders. A combination of geosynthetic systems, such as a geoweb made out of polyethylene and sewn together to produce a honeycomb structure; cellular block panels or geoblocks made out of recycled polyethylene; a porous ring and grid system constructed from high-density polyethylene; and a fiberglass grating system, were installed on top of the geotextile fabric and used for soil stabilization in the other seven areas. The pulloffs were then seeded for turf so that they would blend into the environment.
Since construction, "the pulloffs have stabilized the area and are holding up well," says Ken Thornton of FHWA. "All of the systems appear to be viable," adds Hatcher. "Each has its place depending on the desired application." For example, the ring and grid system offers faster and easier construction, while the fiberglass system provides more durability for high-traffic areas. Costs ranged from $33 per square m ($27.60 per square yd) for the control area to $114 per square m ($95.40 per square yd) for one type of porous synthetic ring and grid system. Each of the turf-covered pulloffs blends in well with its environment. "Aesthetically pleasing, environmentally sensitive solutions are critical when constructing features for the National Park Service," notes Hatcher. "It is recommended that one or more of these systems be tried when soil stabilization in an environmentally sensitive situation is required."
For more information on the stabilized grass pulloff areas along the Gatlinburg Spur, contact Martin Hatcher at FHWA, 865-453-7123 (email: firstname.lastname@example.org).