Top Down Boardwalk Construction | Flat Branch Preserve: Charlotte, NC

Posted: July 19, 2016

Concrete Boardwalks Through Wetlands at Flat Branch Preserve

Boardwalk construction is nearing completion this week at Flat Branch Nature Preserve, located at the intersection of Tom Short and Ardrey Kell Roads in south Charlotte, NC. The recreational trail now includes two PermaTrak boardwalks (323 LF and 44 LF walkways) through wetlands and heavily wooded areas (mostly Willow Oak trees as well as some Overcup Oaks), as well as observation decks that can serve as outdoor classroom spaces, called "wetlands classrooms." flat_branch_preserve_permatrak_boardwalk_construction_1.jpg

According to Mecklenburg County, the 44-acre Flat Branch Preserve site is "one of the largest upland mafic depressions (swamp forests) remaining in Mecklenburg County."1 The preserve also includes a restroom/shelter area with picnic tables.

Two Concrete Boardwalks Through Sensitive Wetland Areas

The PermaTrak boardwalk was designed in collaboration with HensonFoley, Inc. of Huntersville, NC, a landscape architecture, civil engineering and surveying firm.


The Mecklenburg Country project was primarily managed by Chris Matthews (Nature Preserves and Natural Resources) and Bert Lynn (Landscape Architect in Asset and Facility Management). The general contractor for this project was TG&R Landscape Group, a landscape architecture group based in Charlotte. 

While pressure-treated wood decking was also considered for these walkways, PermaTrak's installed boardwalk cost came in at less than a 10% premium compared to this higher maintenance wood option. With these minimal upfront cost differences in mind, Mecklenburg County project managers chose PermaTrak after analyzing the long-term cost savings with a maintenance-free PermaTrak concrete boardwalk.

The boardwalk and trail alignment were rerouted through the heavily wooded area of Flat Branch Preserve after a preliminary alignment was shown to be too intrusive- requiring too many trees being taken down. With a revised alignment to minimize tree disturbance in place, the two PermaTrak boardwalk segments now extend 323 LF and 44 LF, respectively.permatrak_boardwalk_at_flat_branch_preserve_12.jpg

The 323 LF walkway through the heavily wooded areas includes a unique outlook/observation deck area that will be used as an outdoor classroom for local students, especially those enrolled in Polo Ridge Elementary School. The 44 LF boardwalk segment crosses an "upland depression swamp forest" area and leads to Tom Short Road, directly across the street from the elementary school. 

What is an upland depression swamp forest?

We spoke with Mecklenburg County's Chris Matthews on the topic of upland depression swamp forests, sometimes referred to as "perched wetlands." These impermeable areas are made up of dense, heavy clay and exist in just a few locations across Mecklenburg County. As a division director for Nature Preserves and Natural Resources, Matthews manages a division of folks that take care of 26 nature preserves, across about 7,500 acres in Mecklenburg County. His team also takes care of locations that have protected or endangered species, or rare or unusual planned communities or cultural resources. Matthews filled us in on why these upland depression swamp forests are so important to the local ecosystem.

"These wetlands, basically they fill up with water in the wintertime when the leaves are off the trees and it's just raining. Then, the water sits in there over the winter into the early spring. By the time the trees begin to leaf out and the weather gets warmer and you get this evaporation and evapotranspiration through the leaves, and the water ends up disappearing.

But, in the meantime, over the wintertime, these are real important areas for amphibians to breed. We typically get a lot of frogs and salamanders that are laying their eggs in these upland depression swamp forests. And these are not associated with creeks or streams. They simply exist because the soil there is super heavy clay and it's very dense and impermeable. And so the water basically just kind of sits in what is almost like a "plate" sitting on top of the ground. That plate holds water in it for only a certain amount of time during the year."

Boardwalk Foundations in an Upland Depression Swamp Forest

"And so we really wanted to get a boardwalk system that we didn't have to put six-by-sixes into the ground, because these clay layers, they aren't the same depth. You get portions of the wetland that you may have this real heavy clay layer that's three feet deep and, fifty to sixty feet away, it's seven feet deep. So we wanted to have a system that would sit on top of the ground. We didn't want to puncture this clay lens that's holding the water and then have this potential seepage or drainage during the wintertime, to where the water might get through this clay lens and then hit groundwater and get pulled away. So, that's why it was real important to have this kind of setup at Flat Branch Preserve where we had the boardwalk sitting on the surface of the ground and not penetrating it like a typical boardwalk setup might be. 

These amphibians, specifically marbled salamanders, spotted salamanders and various frog species, like still water, so they need to find a situation where there's water that isn't moving around a lot. They may lay their eggs in a floodplain area with pooling water that's flooded from the creek, but they will not lay their eggs in moving water. So, even though there may be a creek or stream or a river nearby, they will not lay their eggs into those locations because the eggs won't survive the movement of the water."

Protecting these areas is a priority for Mecklenburg County, which is how PermaTrak's precast pier foundation system was brought into the boardwalk design discussions. Project managers from Mecklenburg County and HensonFoley wanted a boardwalk system that didn't require driving the standard 6'' x 6'' wooden posts into the upland depression swamp forests. Doing so would create the potential for water to drain through the interface the support member is driven into the ground, which could seep down and cause a dewatering effect.

Matthews continued, "We obviously don't want to dewater because this is a critical habitat for the amphibians for breeding, so we want to try to keep it as intact as possible." -Chris Matthews, Nature Preserves and Natural Resources, Mecklenburg County

For this Flat Branch Preserve site, PermaTrak's precast piers made a good solution by spreading the load of the boardwalk out, without penetrating and introducing seepage or water flow through the clay lens. Thanks again for the explanation, Chris!

Using PermaTrak's Precast Concrete Piers for Environmentally Sensitive Areasflat_branch_preserve_permatrak_boardwalk_construction_3.jpg

We have been fortunate to be a part of many projects where a landscape architect designer uses PermaTrak to avoid tree root disturbance. The same is true for this site at Flat Branch Preserve - PermaTrak's precast pier foundation system gave Mecklenburg County and HensonFoley a durable foundation option that avoids disturbance to the sensitive wetland (upland swamp depression forest) area. Furthermore, a top down construction methodology satisfied construction requirements to protect the perched wetland area.

At 6'-0'' wide (PermaTrak tread length), the installing contractor, USA Drilling, used a piece of light construction equipment, referred to as a "dingo," to weave through the trees with PermaTrak's concrete components. This more nimble piece of equipment allowed the contractor to carry in the PermaTrak beams were up to 10'-0'' long within a very tight construction corridor. 

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Are you interested in seeing this project for yourself? 

Last week I hosted several landscape architects from Woolpert's Charlotte office who were able to come out to Flat Branch Preserve to watch some of the installation and walk through the site. If you are interested in seeing the site or the boardwalk construction progress, please send me an email or give me a call to schedule. I'd be happy to show you around!


Jason Philbin, P.E.

1Mecklenburg County, NC - Flat Branch Nature Preserve

Topics: Boardwalk Construction, News