The Next Linn Cove

Like the Linn Cove Viaduct in North Carolina, on which it is partially modeled, the Foothills Parkway Bridge No. 2 in eastern Tennessee is being designed and built to protect the landscape it crosses.

By Deborah R. Huso

Work is continuing on the oldest uncompleted highway project in Tennessee this spring with the ongoing construction of the Foothills Parkway, a scenic, high-elevation drive near the northwest boundary of the Great Smoky Mountains National Park above Chilhowee Lake. Currently, just over 22 miles (35 km) of the Parkway are complete and open for travel. The byway provides breathtaking vistas both eastward into the Smokies and westward into the Tennessee foothills.

The project, which began in January 2010, is the parkway’s so-called 1.6-mile (2.6 km) “missing link.” Connecting the Wears Valley section of the byway with the Walland stretch, it will include a 790 ft (240 m) precast concrete bridge known as Bridge No. 2. The bridge will be supported by four piers crossing two ravines on the southern slope of Chilhowee Mountain. The tallest pier will take the bridge up to 100 ft (30 m) off the ground.

Not unlike the famous Linn Cove Viaduct on the Blue Ridge Parkway (see Precast Solutions Spring 2010), Bridge No. 2 will feature an S-curve design in an environmentally sensitive mountain landscape. “The client did not want to impact the environment,” says Bruce Nicely, senior vice president and partner with Bell and Associates Construction, the project’s general contractor. “We were not allowed to build any access roads under the bridge. We’re basically building it from the top down.”

Challenges: access and environment
Eastern Federal Lands, which is administering the project on behalf of the National Park Service, also made it clear that no vegetation should be disturbed. Nicely says his team left the mountainside untouched with the exception of removing some small trees where the bridge would be located.

“This is very similar to Linn Cove,” Nicely explains, referencing the famous “floating bridge” in North Carolina that was constructed using one-directional cantilever construction almost 25 years ago. In fact, many members of Bridge No. 2’s design-build team worked on the Linn Cove project, including bridge designer John Corven of Corven Engineering, Tallahassee, Fla., who served on the viaduct’s design-build team as a young engineer fresh out of college.

“We actually used Linn Cove as a model for some of the design work on this bridge,” Nicely says. “At the design stage, we felt the government wanted something very similar to Linn Cove. The environmental issues really dictated this job.”

Corven agrees, noting that it was access – getting both people and equipment – to the construction site that drove his firm’s design process. “The environmental concerns are very big,” he says. “The topsoil is very fragile. Once you disturb it, it washes away.” Scott Wilson, project manager for Palmer Engineering, the project’s civil engineer, said one reason the design-build team settled on precast rather than cast-in-place concrete was to reduce the chances of construction-induced soil run-off.

And it’s not just the site’s fragile landscape. The bridge will be located at an elevation of 2,400 ft (732 m) on a nearly vertical slope. “The terrain is so steep that you’re hardly able to walk along the mountainside,” says Corven, who notes the bridge will basically “hang on the side of the mountain.” The elevation has made weather a continuing challenge, too, and Nicely remarks the team had a rough winter with 14 in. (355 mm) of snowfall in the first half of January.

Solution: temporary trestle and balanced cantilever
While the design-build team considered constructing a cast-in-place segmental bridge, there were challenges with pouring 8 ft to 10 ft (2 m to 3 m) bridge sections in the field. Nicely says precast concrete offered the best solution, because the construction site is not close to a ready-mixed concrete plant, and the precast method provided the additional benefit of a controlled manufacturing environment.

Cast-in-place concrete also presented a greater risk of disturbing the surrounding topography and creating more possibility for construction-induced erosion. To reduce impacts on the landscape, the design-build team is constructing a temporary bridge, or trestle, supported on micropiles spaced every 36 ft (about 11 m). The trestle, which was designed and is currently under construction by VSL, will accommodate a straddle gantry crane that will essentially walk down the temporary trestle and set the precast concrete segments in place. The trestle itself is being built from the top down and is supported on micropiles that are 7 in. (180 mm) in diameter. VSL designed the crane that will “walk” the trestle especially for the Bridge No. 2 project.

Corven explains that once the straddle gantry crane puts a segment in place, that segment will then be attached using high-strength cables and sealed with epoxy to make the connection watertight. The closure joints will be cast-in-place concrete.

While Bridge No. 2 will share many features with the Linn Cove Viaduct, its construction method is different. Rather than a linear cantilever construction process used for Linn Cove, Bridge No. 2 will be built using a balanced cantilever method. Corven says he originally considered progressive linear construction but could not manage the less than two-year construction schedule with that kind of design.

With the balanced cantilever method, the construction team will go out to the first pier, put one bridge segment on the left and one segment on the right, and then add to each side. Nicely explains that it’s like building a “T” with a 90-ft (27-m) bridge on either side.

Corven says Bridge No. 2 will actually be a bit easier to build than the Linn Cove Viaduct (and substantially quicker, too), because the construction team will have access to both sides of the high-strength cables. With Linn Cove, builders had to actually go inside the bridge to make attachments. Corven says use of the temporary trestle will save the Park Service a substantial amount of money, and he anticipates total project time to be 680 days – almost half the time it took to construct Linn Cove.

One of the major challenges of the Bridge No. 2 project is that the bridge is designed with a double curve, making for a total total 250-ft plus (76-m) horizontal curve, meaning each bridge segment is slightly different in orientation. To address this, Ross Prestress is constructing the bridge segments using match casting. Located 35 miles (56 km) away in Knoxville, the precast manufacturer came to the Foothills Parkway bridge project with no previous experience in segmental precast.

But with Corven’s help, the company purchased a bridge form through Southern Forms in Jasper, Tenn., and is manipulating that single 37 ft by 10 ft (11.3 m by 3 m) form to produce all 92 of the bridge’s 37 ft by 8 ft 8 in. (2.6 m) precast segments. Each box girder weighs in at about 50 tons (45 tonnes). While Corven notes there is no economy of scale here since every bridge segment is unique, the match-casting process allows for use of the same form, employing hydraulic jacks to articulate each new segment to accommodate the bridge design’s unique geometry. “One form makes all the pieces for this bridge,” says Corven.

Basically, as the segments are constructed, the form stays in the same place, and each newly constructed segment then moves into the match-cast position for creation of the next segment. Four hydraulic jacks at the corners of the form adjust it for vertical elevation, while four hydraulic jacks on the sides adjust the horizontal geometry, Rick Merritt, Ross’ vice president and general manager, explains. Merritt says his company has definitely had a learning curve on this project, noting “the geometry is so tight.” He says the grade tolerance is only 1/32 in. (0.8 mm).

Tight timeframe, tight geometry
Greg Hunsicker, VSL’s division manager, agrees this project has been a challenge, even though he’s accustomed to building bridges over rivers, bays, ravines and roads. “The geometry is pretty extreme in terms of superelevation, grade and the radius of the bridge.” VSL will not only be erecting the bridge segments but also the four piers. “It is a very unique project in terms of access and working on the side of a mountain.” The bridge has a radius of 262 ft (80 m), an 8% grade and cross slopes of more than 7%.

The precast concrete also features a dark charcoal color designed to complement the color of the rocks on the mountainside. Merritt says the color matches all of the precast concrete projects his company has performed for the Great Smoky Mountains National Park.

As of February 2011, the design work on Bridge No. 2 is complete and just more than one third of the precast pieces for the project has been manufactured by Ross Prestressed. Merritt says delivery of the precast concrete segments will begin this summer.

“We’re just getting geared up in the field,” says Nicely, pointing out that construction has begun on the first pier. The first bridge abutment has been built, and VSL has begun construction of the temporary trestle.

Corven says working on Bridge No. 2 has offered the opportunity for him and the rest of the design-build team to pull together a lot of tried-and-true components from different projects into one job. “The challenge has been in taking established methods and making them work in concert,” he says. And it’s been a learning experience for some of the team, too. Neither Bell and Associates nor Ross Prestress had ever completed a segmental precast project previous to the Foothills Parkway bridge.

The collaboration among the members of the design-build team has been critical for this complex bridge project. “There are a ton of directions you can go for decisions in a project like this,” says Wilson, who appreciated that all the designers, builders, engineers and product manufacturers were on the job at the beginning. “It streamlines it to know what equipment everyone has and what limitations everyone has,” he says. “The whole team was there right from the start, all the way down to the landscape architect.”

The $25 million Bridge No. 2 is scheduled for completion by November 2011.

Deborah Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.