SHOWCASE 3
Pigeon Falls
Parking area and trolley access Environmental solutions prove to be critical for a new trolley access parking area and tourist destination. Environmental and economic concerns are often difficult to align. However, when the economy of a region depends on the quality of its environment, the two must go hand-in-hand. Pigeon Forge, in east Tennessee, relies upon the beauty of the Great Smoky Mountains for the tourism that drives its economy, but the congestion from the other kind of driving is endangering the balance. When GS&P was charged with providing a trolley center parking lot that would help ease the congestion created by the necessary tourism, the team was confronted with a startling set of challenges. The site not only lay in a floodplain, it sat adjacent to the U.S. EPA-protected Little Pigeon River that also included a Native American burial ground. The team’s solution illustrates top-notch problem solving and creativity that promises sustainability and community empowerment.
Pigeon Falls Development – Trolley Access Par king Area I N T E RVI E W /
J ason B ra d y , M i k e F l att , M ichae l J en k inson , J onathan H ay craft
You faced a number of challenges in the development and engineering of this lot. Let’s start with the river itself.
Jason: With the Little Pigeon River, we had a good portion of this project that was inside of the regulatory floodplain. It is a 303(d) stream and the parking lot is part of a greater Tourism Development Zone (TDZ). Through all of the planning there were wetlands and streams to be impacted, and there were a slew of environmental constraints. One thing that we brought to the table was that we could build the parking lot in such a way that we could address water quality and mitigate the streams and wetlands. There was constant bargaining with the Tennessee Department of Environment and Conservation (TDEC), so we volunteered to make this a green parking lot. We wanted to give a little bit back for what we had to take to complete the rest of the development. Mike: We had to come up with a concept that the permitting agency would sign off on. They thought they put so many restrictions on the site that no development could happen. It is in a floodplain and in a floodway. It is also in an area where you can not excavate because of
archaeological concerns with Native American burial grounds.
projects that helped us gain confidence from our client in what we were doing.
How did you manage the challenge of the Native American burial site?
With the burial site itself, the human remains were right at the surface. We think a developer might have removed some top soil out of there in the past, and the archaeologist protected it by thinly covering it back up.
Jason: We had one known burial area and the entire site became an archeological zone. It was then deemed a ‘can’t cut’ site, so we literally could not do anything during construction of the parking lot that required us to do any cuts. We had actually designed the entire parking lot with a closed drainage system and storm pipes, and a lot of those pipes were built in the cut. So that created a need to come up with an engineering solution to build this parking lot without having to make any cuts and just work with the land as it lay. That affected the parking lot and water quality design. To place fill in a ‘can’t cut’ area is kind of a risky proposition, so we worked with a local environmental firm, and the city on a solution to build on top of the ground without a cut. It required the use of a geogrid and a stone base. Luckily, the whole parking lot is in six-plus feet of fill so that helps to “bridge” bad material. Our local team member designed some geotechnical solutions using engineered
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More than meets the eye showcase 3
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Our solution was to remove what the archaeologist had put down. The remains were in the fetal position, and did not take up a lot of room. We took two pieces of concrete pipe and placed the lower section of pipe down around the remains. Then we built the pipe up to below the level where the parking lot would be to help protect the body instead of just placing fill on it and running bulldozers over it. We filled the pipe full of sand by hand so that the force of the weight on it
With Native American remains near the surface, concrete pipes were placed around the remains and filled with sand to lessen the impact.
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fabric ROCK GRID fabric The system was designed with a sandwich of geotextile fabric, geo grid reinforcing mat, and free draining stone constructed directly on the ground and connected to vertical drains. A traditional closed system design with engineered water quality structures was quickly found to be impossible. The resulting design utilized porous concrete pavement strips in non-drive aisle areas to collect stormwater and eliminate the need for catch basins and pipes.
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would be evenly distributed as opposed to trying to fill it up with gravel or another material that might create a point source load. Our solution was well received by all reviewing agencies. Mike: They took grass tile and laid it out in such a way to separate handicap parking. Now you think it is just there because it needs to be there. You would not know that there is a Native American burial underneath it, and the Native American council we checked with wanted it that way. Where did you go from there?
Michael: The next issue became how to handle the water quality requirements set forth by TDEC and to determine the easiest and most efficient way that provided the best cost benefit to our client. We investigated the traditional scenarios using pipes and conventional methods to treat the water— store the water and treat it underneath the pavement—but these avenues were cost-prohibitive. We came up with the idea of using #57 stone for the last foot or foot and a half above the existing ground to attenuate the water. We performed stormwater volume calculations to determine the acreage needed for storage below the engineered fill. The required acreage of #57 stone allowed it to percolate through the stone and into the existing ground without disturbing the burial areas. The geogrid and stone layers also bridged the fill over the existing poor soils. Was this the right thing to do?
Michael: Yes, and it is also the most efficient way to do it. We looked at paving large areas with pervious concrete and
decided to incorporate strips that would act as conduits for the stormwater that flowed across the parking lot—to infiltrate that into the existing soil or into the riverbed. This solution cooled the heated stormwater, detained the increased flow, and introduced the water back into the ground at a more natural rate. Jason: We put down geotechnical fabric and geogrid, then a layer of rock, and geotechnical fabric on top of that. It actually comes all the way up to the toe of fill so that water in a heavy rain event actually has the ability to leach out of the site anywhere along the toe of the slope. I think the whole parking lot is a little more than 2,000 feet long. The whole side of the parking lot that faces the river is free-flowing stone built up to the towing slope, and that is built out of riprap so the water has the ability to leach out anywhere depending on how much pressure there is. The water naturally finds its way to the low spots and discharges out there, but if there is enough of an event and the underground area becomes saturated, it can leach out anywhere along the toe of slope. Jonathan: A good thing about being in a floodplain was having the rock face on the slope and all the storage under the parking lot. If the river came up, it had somewhere to go—under the parking lot—instead of flooding it. Michael: Also in that same vein, the pervious pavement gives any backwater pressure under flood conditions a way to surcharge through the pavement so that there was no undermining of the pavement that is currently there. So the stone and pervious pavement act like a release valve.
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Mike: Also, car tires turning on top of the pervious concrete will cause it to pop, unravel and chip off from the top. It is placed in an area where there is no turning of the wheels—just under a front bumper—so you do not turn until you get back out of the parking spot. Between that and the laying out of the parking lot to drain all in one direction and capture that water was important to allow us to put in the underground detention in the stones. Michael: Another advantage of pervious concrete is that motor oil adheres to concrete much easier than asphalt. If you ever look at pervious concrete versus pervious asphalt you will see oil spots on concrete. This shows the oil drippings do not go into the gravel backfill or soil below. By using concrete, pervious or standard, you are reducing the amount of oils being introduced into the environment. Jason: From an environmental standpoint, it works like a water quality pond in that it slows the water down and allows it to settle out particulates. All of these things are happening underground. By the time the water moves down to the pervious, through the rock, through the geotextile, out of the toe of the slope and overland flows into the river, it has a long time to cool itself off which is good for the river. Water coming off a 110-degree asphalt parking lot and going straight into a river via a pipe is not a good thing. It was important for Pigeon Forge’s economy that you find a solution so that they could get this parking lot and trolley center built.
Mike: This parking lot fits into the bigger plan, and it is a very critical area to the state of Tennessee for tax revenues
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because of the tourism industry. The plan is for the municipal parking lot to be in the center of new development and people can come, park and have trolleys pick them up and take them all around town. Visitors can come and spend the day and not have to move their cars, which is brand-new thinking for Pigeon Forge. We had to think about the circulation of people wanting to park there to shop and to go everywhere around town. We had to lay out the parking lot to suit circulation for a tram to pick up people and drop them off, and still have cars coming and going. We also had to bring that dynamic on top of finding a way to build this parking lot so that we have 15 or 16 acres of asphalt without one single bit of overland runoff leaving the parking lot. It is not only the green aspects of the design that Michael was talking about, but it is also the green aspects of trying to reduce congestion, trying to have fewer cars buzzing up and down Pigeon Forge to go eat, shop or go to a show. Visitors can park their cars and eventually utilize the advanced trolley system to get all around town. What do you find the most gratifying about this project?
Jason: I am most proud we got the parking lot built for the city. The permitting and engineering and construction issues were daunting, but this is really what the city needed. Our charge from the client was to get them a quality parking lot built and get it built quickly. Michael: Since we were allowed to do this design correctly, by obtaining additional geotechnical investigation for the porosity of the existing soils, the final product
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represents innovative engineering. This was very exciting to design. It goes a long way to say that this is one of the biggest success stories I have worked on regarding underground retention, underground water quality, and pervious pavement. Everything seemed to work together in concert, it was the perfect application for this area, and it was the solution Jason needed to achieve his goals with the city. Jonathan: I think what I am most proud of is the parking lot layout and how we incorporated the pervious pavement strips, knowing that we had to get so much pervious pavement, knowing that we had to fit so many spaces into that defined area, and that we were able to accomplish those things. The parking lot works. Mike: There were so many stakeholders involved. They had their needs and their wants at the top of the list. Trying to appease everyone and come forward with a design that could actually get permitted and built affordably was the ultimate challenge. And the design we chose has the green elements but still allowed us to accomplish what everybody wanted in support of parking for shopping, for the trolley center, and to get around town. Jason: One thing we always pride ourselves on as a company is to be able to work from one office to the next. I think this project might have been one of our finest moments in being able to manage a project in one place, design it in another place, then come back and build it where it started and to have that happen nearly seamlessly.
Water will flow from the porous concrete parking surface and through the stone beneath to the original ground level. From there the water flows through an additional stone layer and exits near a stream.
The lot serves Pigeon Forge tourists by offering parking for trolley rides, which alleviates traffic congestion and emissions.
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City of Pigeon Forge Pigeon Forge, Tennessee civil engineering site planning TEAM P rincipa l - in - char g e
Michael A. Flatt, P.E.
P roject M ana g er
Joseph L. Vance, Jr., P.E. Jason Brady, P.E. Jay J. Cameli, RLA, ASLA
P roject P rofessiona l
Jonathan Haycraft, P.E., CPESC
A d d itiona l T E A M M E M B E R
Thomas J. Carr Michael Jenkinson, P.E., CPESC Jeffrey McElroy Chris Brown
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