Building An Environmentally Friendly Trail
Pervious concrete, steel bridges and attention to the environment have been key elements during construction of the Bob Callan Trail, a new 2.1-mile multi-use trail along the Chattahoochee River and Rottenwood Creek in northwest Atlanta. The trail extends from Interstate Parkway and Interstate North Circle near I-285 and I-75 and continues into the Chattahoochee River National Recreation Area (CRNRA) off U.S. 41.
CRNRA draws visitors year-round, and hiking along the river and through the surrounding countryside is a popular activity. This $3.6-million trail project enhances their experience by providing a greatly improved trail surface, and the pervious concrete used in trail construction will be a major factor in controlling runoff and thus minimizing river impacts during rain events.
Lewallen Construction, based in Marietta, Ga., served as the contractor on the project. Moreland Altobelli designed the project, which is part of Cobb County's trail system.
"Projects like this one are fun projects," says Larry Lewallen, "but they can be challenging too."
In fact, the very thing that makes this area so popular with visitors — its decidedly urban yet relatively remote and ruggedly scenic topography — also brought challenges to the construction team. This became apparent from the moment of the initial walk-through and remained a factor throughout the course of the project.
Pervious Concrete Trail
Pervious concrete, like conventional concrete, consists of cement, coarse aggregates and water. However, unlike conventional concrete, pervious concrete mixes include little or no sand. The result is a porous mix with an open-cell structure which allows water to quickly pass through and then soak into the underlying ground instead of running off quickly into nearby waterways. Because of that ability to regulate runoff, pervious concrete is ideal for projects in environmentally sensitive areas where surges of storm runoff must be minimized.
During the course of this project, pervious concrete was used to construct much of the 2.1 miles of new trail. The main portion of the trail, running along the Chattahoochee River, has a width of 12 feet; a side trail following Rottenwood Creek is 10 feet wide.
To construct the trail, contractor Lewallen Construction first cleared and graded the trail route. Along the Chattahoochee River, the new trail follows the route of an earlier trail. However, the Rottenwood Creek portion of the trail involved new construction — and with it some challenging clearing work.
As a section of trail was cleared and graded, crews placed and spread 6.5 inches of graded aggregate base. Then, in preparation for concrete placement, crews installed forming strips along the edges of the trail route.
The pervious concrete used on the project — a total of about 1,400 cubic yards was required over the course of the project — was supplied by Thomas Concrete. It arrived at the site in ready mix trucks, which carefully backed down the trail route to the work area and discharged the mix directly onto the prepared base.
As soon as the concrete was discharged, Lewallen's crew went to work with a custom-made screed to level and finish the concrete.
Lewallen's custom screed, which the contractor fabricated for this job by modifying anexisting screed, traveled by riding on the edges of the form boards. The screed included a vibratory system, though only a small amount of vibration was required during placement. Winches and cables pulled the screed along as the concrete finishing moved ahead. Final finishing was achieved using long-handled steel rollers. Proper finishing of the pervious concrete was important, too, as too much finishing can bring the paste to the surface and adversely affect the pervious qualities of the finished pavement.
After final finishing, the newly placed concrete was then covered with plastic, which remained in place for seven days to allow the concrete to cure.
Bridges, Boardwalks, Walls, And Culverts
In addition to construction of the new pathway, this project included construction of three major bridges on the Rottenwood Creek portion of the trail. These single-span structures were constructed by subcontractor Steadfast Bridges, based in Fort Payne, Ala. The bridges rest on concrete abutments, and the structures themselves were set using cranes from Phoenix Crane.
The first bridge, which crosses Rottenwood Creek very close to its confluence with the Chattahoochee River, has a length of 85 feet and replaced an older bridge at the same location. New abutments were first constructed alongside the existing bridge. The bridge itself then arrived at the site in two sections, one section at a time, and each delivered by a semi.
Because of the tight quarters, the construction team faced a challenge when it came time to get the delivery semi turned around and on its way. To meet that challenge, here's what was done:
The semi carrying one of the bridge sections arrived at the site.The bridge section was lifted from the trailer and set on the ground.The trailer was disconnected from the tractor.The crane then picked up the trailer.The tractor backed out.The trailer was turned around in the air, then lowered back to the ground.The tractor backed in, hooked to the trailer and returned the way it came.
This procedure was repeated for each of the bridge's two sections. The sections were then bolted together to complete the bridge, which was then set by Phoenix Crane.
The second bridge, Bridge 2, has a length of 125 feet and is located in very different terrain. At this bridge's location, rock was a significant factor. In fact, about 50 cubic yards of rock had to be removed to allow construction of the end abutments. However, due to the close proximity of a major sewer line, no blasting could be used.
Some of the rock was removed using a trackhoe outfitted with a hammer. Other rock, however, had to be removed using Dexpan, a material which is mixed with water, placed in drilled holes, and then expands overnight to break rock apart. At the end of the day crews would drill 1.25 inch holes 24 inches deep, mix the Dexpan and pour it in. By the next morning the Dexpan had done its work, giving the construction team plenty of broken rock to remove — without any blasting being involved.
First, concrete abutments were formed and poured. The bridge itself was then delivered. This bridge also arrived at the site in sections. The upstream truss was then set, followed by the downstream truss. Cross pieces were then bolted in place, and finally the concrete deck was poured.
Bridge 3, the longest of the new bridges, has a length of 135 feet and posed the greatest installation challenges. Following concrete abutment construction, this bridge was assembled on nearby Cumberland Boulevard. Phoenix Crane then set the completed 72,000-pound bridge as a single piece using a 240-ton crane positioned above the site on Cumberland Boulevard.
The project also includes close to 500 feet of boardwalk construction. Near bridge three, for example, a 215-foot-long boardwalk carries the trail across some particularly rugged terrain. Boardwalk construction was handled by R Construction, Lawrenceville, Ga., with Martin Robbins Fence Co., Atlanta, taking care of handrail installation. Meadow Creek Landscaping handled the landscaping portions of the project.
In addition to the trail and bridges, the project included construction of a total of seven walls along the Rottenwood Creek portion of the trail — among them a 120-foot-long, 30-foot-tall mechanically stabilized earth (MSE) wall. MC Precast constructed the MSE walls.
Two significant box culvert extensions were also included. One involved construction of a 58-foot-longextension to an 8-foot by 8-foot box culvert; the second extension, also to an 8-foot by 8-foot box culvert, had a length of 80 feet.
A final project element was paving of part of the parking area near the trail head. C&S Paving handled the asphalt paving.
Positive Community Response
Because of the large number of visitors using the area for recreation, this project received a great deal of public attention. Further attention came from users of the river itself as fishermen, boaters and others with an interest in the river kept a close eye on the project. But thanks to the contractor's careful attention to factors such as silt fence maintenance, impacts on the creek and river were minimal. This was confirmed by water quality monitoring, which provided yet another check.
"At first I didn't know what was going on," one frequent trail user said. "But now that I see what they're doing with the trails and bridges, I love it."