The water along the North Carolina coastline is beautiful—unless it is filling up your cast pile holes faster than you can drill them. As part of the $95 million Military Cutoff Road Extension project in Wilmington, North Carolina, Balfour Beatty is constructing 170,000 square feet of sound wall on each side of the roadway. The land in this area is quite marshy, and at an elevation of 12 feet above sea level with a water table at 10 feet, setting concrete piles for the sound wall means securing each hole from water and soil seepage.
This issue is so prevalent along the North Carolina coast that the North Carolina Department of Transportation (NCDOT) was finding it difficult to attract bidders for the complex installations. This gave our Southeast team a unique opportunity to find a solution that would help our client and give us a competitive advantage.
“The NCDOT standard requires control of the water coming into the pile hole and sets the flow rate at less than six inches of water seepage in ten minutes before you pour concrete,” says Jay Boyd, operations manager for the Military Cutoff Road Extension project. “The rate of seepage along the coast of North Carolina is much higher than this, so the method used for installing the concrete piles before the Military Cutoff Road Extension project was driving a casing the full length of the hole. This method poses a lot of risk for a contractor.”
The temporary steel casing method involves multiple steps and requires costly equipment and materials. Once the casing is driven and the hole drilled, the concrete post is placed in the casing, the casing is filled with concrete, then the casing is pulled out of the hole and re-used. The method is time-consuming and expensive.
A Proven Approach is New Again
After learning of a more efficient method for constructing concrete piles on coastal sound wall projects in Florida, Jay and the team visited the construction sites of three different contractors in the state to view the process.
This method, the continuous flight auger (CFA) post system, uses a hollow auger drill to drill the hole. Concrete is pumped through the auger and down to the bottom as the shaft is removed. As soon as the auger is removed from the hole, the pile is inserted. This method prevents water and soil seepage and is completed in one step, requiring less equipment and a smaller crew. The Florida Department of Transportation (FDOT) has used this faster, more cost-effective method as a standard for more than ten years.
Convinced that the CFA post system was an ideal solution for the sound walls on the Military Cutoff Road Extension project and that it could be self-performed by the team, Jay presented the method to NCDOT and the Carolina Associated General Contractors of America (CAGC). NCDOT accepted the method as an alternate specification in their special provisions with the exception that the design needed to be professionally engineered and approved by the NCDOT engineer.
STV was working with Balfour Beatty on other projects and was a natural choice to provide the engineering design for the sound wall auger cast pile hole. The design lengthened the pile panels from 15-foot spacing to 20-foot spacing, which made the materials more economical. The FDOT standard uses a concrete pile with a rebar base. The rebar base is set into the grout up to approximately one foot above the bottom of the concrete. The NCDOT standard requires a pile fully encased in concrete. However, this displaced too much of the concrete grout in the hole.
The new design combines the two standards and uses a rebar base with a larger diameter and number of bars to maximize the connection throughout the entire concrete section. By leaving the rebar uncovered at the base, the concrete grout fills the spaces within the rebar and creates a bond equal to the NCDOT requirement with less concrete waste.
The project team designed and constructed templates in-house to hold the piles in place during the installation. Combining ideas from the templates they had seen in Florida with their own experience, the team developed a user-friendly template that can manage a variety of cast pile sizes with built-in safety features.
The CFA post system has proven itself as a successful solution on the Military Cutoff Road Extension sound walls. The project team uses a REV Drill with an auger mounted to a CAT 336 hydraulic excavator for the construction pile installation with drilled holes ranging from 30 to 36 inches in diameter and 12 to 25 feet in depth. The team worked with NCDOT to help update the specification and determine inspection procedures for the new solution.
The Military Cutoff Road Extension is the first NCDOT project to use the CFA post system method, and it has resulted in significant time and budget savings. With less equipment, fewer materials and a smaller crew, the team estimates that it has saved up to 20% on cost and 20% on schedule compared to the casing installation method while providing a high-quality construction service.
The results are immediately visible. The team can set up to 10 posts a day, whereas the previous standard averaged an installation of six posts a day. Self-performing the work has also increased site safety, further lowering risk on the project.
Balfour Beatty is currently the only contractor providing this service in North Carolina, and the Southeast team is working to apply this innovative solution on other projects. They are also continuing to explore other ways to leverage select self-performing services to increase their competitive advantage.
Looking out across the forest of set posts, Jay remarks, “It looks really good to watch the project coming together. This is what we do. We are always looking for a better, more efficient, safer and faster method.”