The process of erecting sound barriers is one of the more straightforward types of highway-related construction. Though the scale of the project will vary, in most cases a contractor will drill holes, set posts, erect the walls and move on. However, add in issues such as limited onsite accessibility, the presence of a granite layer in your drill zone and a staggering volume of traffic, and matters become anything but simple. For J.M. Wilkerson Construction, all three of these factors combined to challenge the Marietta, Ga.-based firm in its efforts to construct nearly three miles of sound barrier alongside a southbound stretch of I-75 in Atlanta. Planning, persistence and the use of some specialized equipment have helped keep a potentially troublesome project on track.
Tight quarters
The I-75/I-85 corridor in Atlanta is one of the busiest traffic routes in the nation, carrying more than 340,000 vehicles daily into and out of the downtown area. With a good portion of that volume being truck traffic, it is easy to appreciate how welcome a sound barrier would be to area residents whose homes abut the interstate. A multi-million-dollar design-build project to do just that was awarded to J.M. Wilkerson which, by project’s end, will have drilled more than 1,100 holes—each about 18 ft deep—for wall posts. According to Project Manager Matt Bunch, the job has already presented its share of challenges.
“We started the project in November of 2004, but we knew from the outset that our work area was going to be very tight. Because of the extremely high volumes of traffic that pass through this area on a daily basis, Georgia Department of Transportation (GDOT) wanted to minimize lane closures. As a result, we are working solely on the shoulder of the interstate; that’s about a 12-ft-wide area in which to operate.”
Bunch said the logistics of simple things such as getting equipment and personnel into and out of a specific work area can, in themselves, be trying. “We have to carefully plan in advance when and where each piece of equipment is going to be needed. If, for example, we need to get a skid-steer loader to a particular area and an excavator is blocking his way, the skid steer has to be loaded onto a trailer, taken out into traffic and brought into the work area from the other direction. It can be a time-consuming process if we are not prepared.”
Hard-rock headaches
Logistics aside, equally challenging to Wilkerson have been the uneven terrain and a layer of granite that sits about 12 ft below the surface. “The terrain rolls and lifts in some areas,” said Bunch. “So that, coupled with the tight work area, made use of a traditional drilling rig impossible. Also, there is a shelf of granite below the surface—residual debris from when the interstate was first being constructed. We had a drill onsite that, while it was fairly steady drilling in soil, really bogged down when faced with rock, so we knew we had to make a change in our drilling technique. We contacted a local distributor that handles both standard and specialty tools, Driller’s Choice, also from Marietta, and made arrangements to rent a Lo-Dril mounted on a Cat 325BL.”
According to Bunch, the difference in down pressure between the two drills has been dramatic, allowing them to make steady progress, even in the hard granite. They equipped the drill, manufactured by Bay Shore Systems Inc., Rathdrum, Idaho, with both 24-in. and 42-in. rock augers to meet the dimensional needs demanded by GDOT for the post holes.
“The diameter of each hole is dependent upon what function the post itself will serve, with standard post hole diameters at 24 in. and corner post and end posts demanding 42-in. holes. Spread footings measuring 12 ft x 4 ft x 21?2 ft have been authorized by GDOT for situations in which the rock is simply unforgiving.”
Easy to get down
The drill, which has helped keep the project on pace for its December 2005 completion, affords crowd forces as high as 21,000 lb, a crowd stroke of 66 in. and torques as high as 24,000 ft-lb—all with an overhead clearance of 18 ft 6 in. (without attachment). Telescoping Kelly bars allow drilling to depths as great as 80 ft. Although area power lines had already been moved for the project, the clearance feature still proved beneficial when dealing with trees and other obstructions as well as when simply making a move with the excavator in the limited work area.
Bunch said Wilkerson is installing three different types of sound barriers throughout the distance of the project, again, with each type dictated by the environment into which it is being installed.
“The overwhelming majority of work for this project involves installation of a ground-mounted barrier in which holes are drilled, rebar cages are set, concrete is poured and posts are installed. However, we are also doing a fair amount of walls in which the column for the wall actually passes through an existing concrete side barrier, generally in an elevated area. Additionally, there was some existing barrier in place at several points in the project, but it was not designed to support the required wind loads. So we removed it and will construct a new barrier that can support the 80 mph wind loads specified by GDOT.”
Despite these challenges, Bunch said, the project has managed to move along nicely.
“There’s no denying we’d like to have some things be different,” he said. “Ideally we would really like to have another lane to work with, but if you spend any time in Atlanta at all, you know why that isn’t going to happen. There aren’t enough lanes out there to handle the traffic as it is. However, we’re managing nicely given all that, and the equipment we’ve brought in for some of the tougher material, specifically the drill, has played a role making that happen.
