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’Missing Link’ Span Begins on Foothills Pkwy.

Mon November 15, 2010 - Southeast Edition
Cindy Riley

In what’s proving to be a challenging task due to rugged terrain, surrounding wildlife and rapidly approaching cold weather, crews in Tennessee are working to complete the so-called “missing link” needed for the Foothills Parkway in Great Smoky Mountains National Park in Tennessee.

Construction began in January 2010 on the roughly $25 million project, which will add an almost 800-ft.-long (243 m) bridge connecting the eastern end of the unfinished Wears Valley stretch of parkway to the Walland segment. Expected to be completed in November of next year, the bridge will be supported by four piers up to 100 ft. (30.5 m) above the ground, placing the road across two ravines on the south slope of Chilhowee Mountain.

“We are building a temporary bridge to get to the piers and to erect the precast segments,” explained Bruce Nicely, senior vice president and partner of Bell and Associates Construction LP, Brentwood, Tenn., the general contractor on the project. “We have a 175-ton crane that will operate on the temporary bridge. We are also purchasing a custom built segment handler that will walk on the temporary bridge and place the 50-ton bridge segments.”

Nicely, whose goal is to have minimal impact on the existing environment at the job site, pointed out, “The work is truly top down. The temp bridge is constructed one span at a time using the 175-ton crane to extend the micro pile drill rig out 36 feet and set it on a drill platform setting on the one to one slope. No access roads are allowed on the site between the abutments, a distance of 900 feet.”

Long-Time Coming

The oldest unfinished highway project in Tennessee, the Foothills Parkway project has been delayed due to funding difficulties since Congress authorized its construction in the mid-1940s. Bridge number two, as it’s known, is another important phase in trying to complete a key unfinished segment of the Foothills Parkway, according to park spokesperson Nancy Gray.

“Bridge number two is on the east side. The parkway’s right-of-way parallels the Park’s northern boundary from Chilhowee Lake in Blount County to Cocke County,” said Gray. “Only 22.5 miles of the parkway are complete and open to the public — a 16.9 mile section on the eastern end of the Park in Blount County and a 5.6 mile on the western end. In earlier years, more road construction was done on certain sections, but work was halted because of structural fill failures and erosion problems in the rugged segment commonly referred to as the ’missing link.’

“Once we get the link completed and funding becomes available to do finishing work to the two partially completed sections to open up another 16.4 miles of the scenic drive, visitors will have another opportunity to enjoy the unparalleled vistas this segment will provide. Viewing mountain scenery is the number one attraction for visitors to the Smokies. The Foothills parkway is also important to the tourism communities as well.”

As for how the work will affect closings in the area, Gray said there won’t be any drastic changes.

“These partially completed sections that flank the missing link are always closed to motor vehicle traffic. We allow pedestrian use — hiking, horseback riding — when construction is not in operation. But once construction begins, we prohibit all use for safety reasons,” Gray explained.

Park Supervisor Dale Ditmanson has described the new bridge as being almost as long as the iconic Linn Cove Viaduct that carries the Blue Ridge Parkway around Grandfather Mountain. The latest bridge is expected to become just as noteworthy, Gray said, “because of it’s engineering design and the way it fits into the environment and accentuates the beauty of the landscape.”

Assembling the Team

John Corven, president of Corven Engineering Inc., Tallahassee, Fla., is leading all elements of the bridge design effort, as well as coordinating activities of the subconsultant team.

“It is a great thrill to once again work for the Eastern Federal Lands Highway Division of the Federal Highway Administration on what I believe will be another signature bridge on one of America’s great parkways. Corven Engineering is the prime consultant for the design/build team. We’ve worked with our subs as a cohesive unit to respond to the RFP, RFQ and preliminary bridge design, and that was what led to our selection.

“Corven Engineering prepared the bridge design, is currently providing construction engineering to facilitate construction and is providing on-site personnel to support bridge construction. Our role is complete when the project is constructed, as-build documents developed and the final bridge load rated,” Corven said.

Details, Details

The new Foothills Bridge No. 2 is a 790-ft. (240 m) long precast segmental bridge that will be be built using the balanced cantilever method, with precast pier and superstructure segments delivered from above, using a unique erection trestle and segment walker. The erection details, developed by VSL, provide access to speed construction while protecting sensitive terrain below the bridge.

Of critical importance is the geometry of the precast segments that need to be assembled to match the extreme roadway geometry, as the parkway hugs the mountainside. The alignment includes 262 ft. (79.8 m) radii, grades of eight percent and cross slopes of as much as 7.8 percent.

“The precast segments are being produced by Ross Prestressed Concrete, [Knoxville, Tenn.] Geometry for the 92 unique precast segments is produced by Corven Engineering. Control procedures are those developed by personnel of Corven, and have successfully been used for over 30 years. The precast segments will be stored in the Ross casting yard until they are needed for construction. One by one, they will make the nearly 30-mile trip to the project site and be erected in the bridge,” Corven stated.

The 9 ft. (2.7 m) deep segments will be delivered to the beginning of the bridge where a segment walker will pick the segments from the delivery trucks. The segment walker will then transport the segments over the erection trestle and place them in their final location in the bridge.

The segments are assembled with temporary post-tensioning bars and permanent cantilever post-tensioning tendons. Completed balanced cantilevers are jointed together by cast-in-place closure joints and continuity post-tensioning.

VSL Post-Tensioning & Specialty Reinforcement Systems is furnishing the temporary work trestle, post-tensioning system, and segmental erection equipment, according to project manager Ryan Redman.

“We also are providing the labor to install and erect the work trestle, precast segments and post-tensioning. VSL will be involved through the completion of the segment erection and disassembly of the temporary works trestle, and the work should be completed by September of 2011,” Redman said.

“The construction team has developed and implemented a construction scheme where equipment for installing the temporary trestle will be elevated on working platforms eliminating the need for access roads along the permanent bridge alignment. Top-down construction poses particular access challenges for our crews because we have to make sure we provide safe access to progress the work in a timely and efficient manner.” Redman continued.

“This is a unique job. The scenery and beauty of this bridge location are something not to be taken for granted. Since we have arrived on-site we have seen wild turkeys, bears, bobcats, deer and various other animals. It is not your typical bridge construction project.”

Strong Foundation

As specialists in deep foundation design, construction and testing and slope stability problems, Dan Brown and Associates PLLC (DBA), Sequatchie, Tenn., designed the foundations for the bridge, both the permanent and temporary structures. The foundations also address slope stability concerns at the pier locations due to the intense terrain.

“We started last spring, as the foundations are always the first concern and we are always on the critical path,” said DBA president Dan Brown. “We will be involved until the foundations are complete and the superstructure is under construction. When they are out of the ground, the hard part is done and our work is finished.

“One of the major challenges on this job is the difficult access conditions. Also, a major consideration is to limit the impact of construction activities on the environment. For these reasons we designed the foundations to utilize micropiles, which are small diameter [less than 10 inches] drilled piles that can be installed using a very small mobile drilling rig. The small, portable micropile drill rig is unusual for bridge construction. This type of equipment is normally used for underpinning buildings because of the light weight and mobility.”

Keeping it Real

Knoxville, Tenn.,-based Hedstrom Design is serving as the landscape architect for the project, and will be on the job until completion. According to owner Sara Hedstrom Pinnell, “One of our main challenges is management of invasive plant materials during construction, and maintaining slope stability. Final plant material must match the local genotype, so some of it will need to be grown specifically for the project.”

Hedstrom explained, “Minimizing impact to the site has allowed us to have stable slopes during construction. Minimal disturbance allows vegetation from the existing seed bank to fill in naturally around trees that were coppiced. We prefer to have native vegetation stabilize the soils rather than an introduced seed mix such as a fescue blend.

“Our strategy to blend with the natural landscape is to maintain this diversity with new plant material. The undisturbed forest is rich in many different types of plants,” Hedstrom continued.

“Hedstrom Design has been working in the area with for quite some time now on projects with similar challenges at sensitivities and it was a natural fit for us. The Bell team also is just as committed to protecting natural resources as we are.

People often think of the landscape architect arriving at the end of a project to ’green it up.’ The reality is that we are part of the team from the very beginning, participating in general aesthetics of the impact of the bridge, understanding the existing hydrology and soils and how we work within the disturbance to ensure the plants survive. For example, dissipating runoff with site boulders into areas of new planting. All of this is thought of during the design process and then worked through during implementation.”

Top Down Construction

The Foothills Parkway Bridge will be located between Pigeon Forge and Townsend, Tenn. In a somewhat unusual move, construction is being performed from the top down, using a trestle mounted gantry crane and work platforms. Proper planning was critical early on.

Scott Wilson, project manager of Palmer Engineering, was responsible for the structural aspects, while Matt Everett headed up the roadway, hydraulic and environmental aspects of the project for Palmer.

“As far as roadway design, our role was to provide the roadway plans incorporating safety appurtenances, drainage and cross-sections for the bridge and approaches,” said Wilson. “The bridge design included the abutments and associated retaining walls.

We also had to design adequate means for addressing rainfall. This included providing drainage for the bridge deck and roadway approaches. As for erosion prevention and sediment control, our responsibilities included providing the storm water pollution prevention plan, as well as EPSC plans. Environmental compliance is another concern. To help ensure that the EPSC measures are installed and functioning as intended, Palmer will conduct 10 visits to the site throughout the duration of the project. Two of these inspections have already taken place.”

Wilson added, “In addition to the resource impacts stemming from release of sediment, the project site also has a high probability of encountering acid-bearing rock. In the late 1980s and early 1990s, mitigation for release of sediment, acidic runoff and dissolved heavy metals was prompted by improper implementation of erosion prevention and sediment control measures during construction. As a result of these impacts, it was agreed the National Park Service would perform the future design and construction for the remaining sections [8B-8E] and site specific plans for areas of disturbance would be developed for the initial, intermediate, and final phases of construction.

Runoff potential is high as the project site consists mainly of rock and class D soils, and flows will ultimately enter a tributary to Brickey Branch designated as a ’Known Exceptional Tennessee Water.’ With the steep terrain and limits on project area, providing adequate storage area for sediment-laden runoff is difficult. Therefore, the prevention of erosion is a much more viable option. The method of construction proposed by the Bell team works to minimize areas of disturbance and thereby reduces the amount of runoff requiring treatment.

“Ultimately, all aspects of the design are important as they must work together for a successful project,” Wilson continued. “The retaining wall design was challenging. In addition to the steep slopes, rock is very near the surface of the ground. Our team felt it would be beneficial to minimize the amount of rock excavation required for the construction of the cantilevered retaining walls [some taller than 40 feet]. We utilized rock anchors that enabled the use of footings with a smaller footprint than what would typically be required for the stability of cantilever retaining walls of this size.”

Wilson acknowledged that the rugged beauty of the project site and dense vegetation does create obstacles for crews accessing areas where ground work occurs. Though the drainage areas are fairly small, flows do develop significant shear forces when concentrated due to the dramatic slopes, increasing the erosion potential post-construction.

“Our team has made great efforts to minimize negative impacts to the natural landscape and topography while providing a safe facility meeting the goals of all parties involved. Hopefully this section of the parkway will afford visitors views that they might not otherwise have been able to enjoy,” Wilson concluded.

A Little History

The Foothills of the Great Smokies reportedly began drawing tourists as early as the mid-19th century. Most visitors came for the mineral-rich mountain springs, which were believed to provide health benefits. Construction of resort hotels quickly followed, making the region a popular summer-time destination.

Over the years, the Foothills Parkway project has been considered somewhat controversial, because of the high volume of traffic passing through the Smokies each year. Opponents have claimed the parkway would add to tieups, while others say it would actually ease congestion by drawing off some of the traffic. Some bloggers question the need for new construction, concerned about increased noise in the area, exhaust fumes and aggressive driving; however, advocates of the project say the elevated roadway is long overdue.

The National Park Service, which manages the parkway, also has announced the Federal Highway Administration recently awarded a contract to continue construction on the Walland end of the ’missing link,’ working eastward from Bridge number eight towards Wears Valley. Funding will need to be obtained to build the last two bridges, as well as money to perform finishing touches such as striping and guard rail installation. The segment will most likely be completed in time for the National Park Service’s Centennial in 2016. CEG

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