As the Blue Ridge Parkway celebrates its 75th anniversary, its final piece, the Linn Cove Viaduct, still stands as a crowning achievement of how precast concrete can both protect and beautify landscapes.
By Deborah Huso
As the Blue Ridge Parkway, often dubbed “America’s favorite drive,” celebrates its 75th anniversary this year, many of the roadway’s 20 million annual visitors will be showing some special interest in just how this ridge-skimming drive came into existence. And while its construction began in 1935 – in the midst of the Great Depression as one of President Franklin D. Roosevelt’s many public works projects – what many people don’t realize is that the Parkway was not actually finished until 1987 with the completion of the Linn Cove Viaduct, a double S-curve elevated bridge that clings to the side of Grandfather Mountain near Milepost 305.
The Linn Cove Viaduct is the crowning architectural achievement of the Parkway. Even if you haven’t toured the lovely Blue Ridge Parkway, it is likely you’ve seen pictures of it in brochures and guidebooks. The viaduct is something of an engineering marvel even by today’s standards. Designed to prevent the disturbance of Grandfather Mountain’s rare ecosystem from the site disruption of typical road construction, the viaduct is also a prime example of what precast concrete can do in very challenging site conditions. At once a work of art and an icon of environmentally sound bridge architecture, the Linn Cove Viaduct would not have been possible without the use of precast concrete cantilever bridge girder construction.
The conundrum of a fragile landscape
By 1966, the Blue Ridge Parkway was complete except for a 7.7-mile (12.4 km) section curling around the privately owned Grandfather Mountain. The mountain’s owner, Hugh Morton, at first refused to grant right-of-way, fearing harmful environmental impacts on the mountain’s fragile ecosystem. With an elevation of 5,946 feet (1,812 m), the mountain has a diverse ecosystem that includes some 70 varieties of endangered flora and fauna. Among the rare and endangered species that call this mountain home are the Northern flying squirrel and the peregrine falcon. Grandfather Mountain, which is open to the public as a nature park, was declared an International Biosphere Reserve in 1992.
Gary Johnson, supervisory landscape architect for the Blue Ridge Parkway, was one of three landscape architects who worked on the environmental assessment for the last stretch of the roadway to be completed that included the portion around Grandfather Mountain.
“There was a heavy understory on Grandfather, and Linn Cove was a boulder field,” says Johnson. The National Park Service shied away from a typical earth-fill road and bridge system over concerns about changing the hydraulics of the Linn Cove drainage and over the potential adverse aesthetic impact of running a bridge through the area. “It would have been like driving across a big dam. What the Park Service was looking for was a construction technique that would allow us to build a bridge without tearing up the landscape.”
A precast concrete bridge system for the Linn Cove Viaduct was the successful compromise between Morton and the National Park Service and Federal Highway Administration – a bridge that was designed to “float” around the mountain with minimal impact on the landscape. “This was a completely new approach to bridge building,” says Gary Everhardt, who was Blue Ridge Parkway superintendent from 1978 through 2000. “They would have looked at the typical blast-and-fill method, but the Parkway didn’t want to do that here because of the high visibility of Grandfather Mountain.”
Everhardt says several different routes were considered for taking the Parkway along the mountain, including higher elevation routes, but everyone was concerned about the long-distance views of Grandfather Mountain. “For 30 years, this bridge could not be built because conventional methods would have damaged the mountainside,” says Linda Figg, president and CEO of Figg Engineering Group, whose company engineered the bridge.
In the end, Everhardt explains, time worked in Grandfather’s favor. With the onset of World War II and later the Korean War, the Parkway’s funding cycle was interrupted, which is part of the reason why it took more than 50 years to complete the famous roadway’s construction. “The solution for taking the Parkway around Grandfather Mountain was more technologically advanced by the ’70s than it would have been at the time work on the roadway first began some 40 years earlier,” says Figg.
Precast and cantilever construction offer only acceptable solution
Designed by Figg Engineering Group, the viaduct was to carry the road well above the fragile slopes of Grandfather Mountain. Figg says no construction method other than precast was considered for the viaduct. “It was the only type of construction that allowed us to preserve the environment, not remove any trees, or touch any boulders,” she explains. Construction of the bridge began in 1979.
A precast facility was set up about a mile away from the viaduct construction site on land leased from Hugh Morton for construction of the 153 segments that would make up the roadway. Each precast box girder was transported to the site on trucks and then assembled using continuous construction methods, with each segment being built upon the one previously installed to avoid setting up scaffolding on the mountainside. “This was the first bridge in America to be built in onedirectional cantilever,” Figg says. Each bridge segment was brought to the end of the cantilever, then lifted by stiff-leg derrick, turned, and set in place.
The viaduct was built using cantilevered construction, meaning the builder begins at one end of the bridge, joining precast concrete segments to already completed sections of the bridge, eliminating the need for scaffolding and thus protecting the fragile ecology of the mountainside. While using special support systems in the construction process was commonly used when building bridges over water, it was an unusual method, at the time, for building bridges on land from above without any support. Instead of scaffolding, workers used an elaborate system of elevated wooden catwalks to put them above the mountain’s rock formations. “It was amazing to watch it being built with all that weight hanging out at the end,” Everhardt says.
Construction of the bridge began at the south abutment with the bridge segments jutting out over the landscape. “The only disturbed areas were where the piers went into the ground.” Otherwise, explains Everhardt, “the construction crew didn’t touch the ground.”
“There is usually a lot of construction equipment on the ground with traditional bridge-building techniques,” Johnson says, “but because of the technique used with the viaduct, we had very tight construction limits.” The result was that on the day the bridge was opened for traffic, mature trees were still standing directly adjacent to the viaduct, making it look as if the structure had been there for years.
Foundation piers were placed as the bridge segments reached out over the landscape, not in advance of construction. The idea was to avoid sending crews and equipment down onto the mountain soil. “When they got to the end of the bridge, they would drop equipment down over the edge to the pier site,” Everhardt explains. “There was no equipment whatsoever rolling over the mountain terrain. Workers were treading very lightly on the land.”
Figg says the piers were placed from the top down, meaning the seven or so pieces that made up each pier were lowered to the mountain floor from the bridge rather than being transported in from the bottom. “We built in one-directional cantilever until reaching the location of a pier,” Figg explains. “Then the pieces for the piers were stacked in place.” The tallest pier, consisting of seven pieces, is 65 feet (20 m) high.
The only work for the Linn Cove Viaduct done on the slope of the mountain was the necessary drilling of holes for the seven support footings that would make up the bridge’s foundation. There are 180-foot (55-m) spans between each pier. “Built in an S-shape, the bridge features every type of alignment geometry used in highway construction,” Figg says. “It went from a 10 percent slope in one direction to a 10 percent slope in the other direction in less than 200 feet (61 m).”
The bridge was built on a constant curve, which presented a challenge to the precaster and builder, because each segment of the bridge had to be designed to fit its own individual part of the curve. The viaduct consists of 153 concrete segments, only one of which is straight. The segments, each weighing 50 tons (45.4 Mg), are joined together by epoxy and massive steel post-tensioning tendons. The entire weight of the
structure is supported by seven concrete piers.
“All of the segments had the same basic shape,” Figg explains. “The difference was the geometry.” The precaster was thus able to use the same machine and form to build each segment, but had to rotate the form to match the individual geometry of each piece. Figg says precast construction made the job easier. “Through precasting, we could create a common, repetitious approach using the same form, just adjusting it slightly for each piece.” Jasper Construction also added black iron oxide to the concrete to help the bridge blend in with the mountain boulders.
A durable public and precast icon
A Blue Ridge Parkway icon today, the Linn Cove Viaduct is 1,243 feet (379 m) long and 35 feet (10.7 m) wide and is located at an elevation of 4,100 feet (1,250 m). The total cost of the project was $10 million. The viaduct was completed in November 1982, having taken three and a half years to construct, and the last link of the Parkway opened five years later, more than 50 years after construction of the roadway began.
The bridge has won more than a dozen national design awards, including the 1984 Presidential Award. The Linn Cove Viaduct is one of only five bridges to ever receive the award. It has also been featured on the televised program “Modern Marvels.” Figg says if her firm were hired to do this project today, it would still take the same design approach used in the 1970s. “The general principles we used on the Linn Cove Viaduct are still the best solution.” And it’s a solution the firm has replicated in many bridge constructions since. “We use precast concrete and cantilever construction to span long distances with minimal use of foundations whenever we have an environmentally sensitive site,” she adds.
Everhardt, who served as superintendent of the Parkway for almost two decades after construction of the viaduct was complete, says he believes the precast bridge has performed tremendously well over the years. “There has been no extensive maintenance work required so far as I know,” he says, “and it has become a major draw for the Blue Ridge Parkway.”
Since the Linn Cove Viaduct’s construction, it has become a model for environmentally sensitive bridge design. “The most noteworthy thing about this project is the care that was taken in building it and protecting the Linn Cove drainage system,” Everhardt says. “Everybody was dedicated to making as little environmental impact as possible.”
Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.