THE CHALLENGE: Excavation revealed different geotechnical conditions onsite from original design
Sometimes, best-laid plans can go awry. That’s the case with the retaining wall needed to create space for a new hangar for the governor’s airplane at John C. Tune Airport in Nashville, Tennessee. Excavation for the cut wall opened up a can of geotechnical worms.
“It was originally scheduled to be a gravity Redi-Rock retaining wall. That’s what it was spec’d at,” said Irvin Vittitow, owner of Redi-Rock of K.I.T., the manufacturer and installer for the project. However, when site contractor Civil Constructors, LLC, started excavation it was questionable whether a gravity wall would still be possible.
Civil Constructors unearthed that an unstable fill material had been used on the development of the adjacent property, which created an issue with the crest slope of the wall. There were also buried boulders instead of solid rock in some locations, and the limestone rock cut wasn’t as tall as originally anticipated.
“When we originally had blasted the wall and excavated it out to the parameters needed to install the wall, we had our QC go out there, Redi-Rock go out there, and everybody met on site,” said Dalton Clark, project manager for Civil Constructors. That quality control group decided to go back to the design phase because they were going to need a larger wall to overcome the conditions found onsite.
Retaining wall design engineer Clint Hines, P.E., principal at JC Hines & Associates, LLC, was brought into the fold for the redesign of the project. Hines explored options like a cast in-place solution, which proved to be too cost-prohibitive, and a Federal Highway Administration (FHWA) Stable Feature Wall, which wasn’t possible without a few more feet for geogrid in front of the rock cut.
Hines was actually able to achieve the necessary height for the redesign with a Redi-Rock gravity wall using Redi-Rock XL Hollow Core Retaining Blocks.
“The only way we could get up and down at the heights that were now required was really going to be with the XL units,” said Hines.
THE SOLUTION: Redi-Rock XL Hollow-Core Retaining Blocks used to optimize redesign
Redi-Rock XL is a new, hollow-core retaining block that is twice the height of standard Redi-Rock blocks at 36 inches (914 millimeters) tall, and available in 52, 72 and 96-inch (1,320, 1,830, and 2,440-millimeter) widths. The three block sizes round out the Redi-Rock system to help build taller walls in tight spaces while using less concrete.
“The XL units really help us get above the 15-foot [4.6-meter] height with maintaining the standard Redi-Rock 5 degree batter and helps us build taller structures in a narrower footprint,” said Hines.
For this wall in particular, Hines was able to reach a maximum height of 25.5 feet (7.8 meters), using 806 XL blocks of various widths, then transitioning to standard Redi-Rock blocks at the top of the wall to optimize the design. With the new wall being much taller than originally bid, as well as going back to the design phase of the project, the wall was becoming more expensive.
“There was a lot of pressure on us to make sure we optimized the design as completely as we possibly could,” said Hines. To do so meant creating very accurate geotechnical models of the varied soil conditions onsite.
With the excavation of about 32 feet (10.7 meters) up to the neighbor’s property line now exposed, Hines worked closely with the project geotechnical engineer to gather new, accurate data.
“They knew what we needed from them, and they could give us recommendations based on their experience and some verification testing on what shear strength parameters to use and how to model these soils. Because when you get to these taller structures like this, accurate modeling is essential,” said Hines. “As we got up into the wall, the location of the changing soil strata and the strengths of those layers as we went up became very important to optimizing the design.”
“We had the benefit of the Redi-Rock software, which is tremendous,” continued Hines, who used Redi-Rock Wall+ to design and analyze the wall.
Once the wall design was optimized to the point where it was acceptable to the Metropolitan Nashville Airport Authority in regard to cost, it was time for Redi-Rock of K.I.T. to manufacture the 979 blocks in the Ledgestone texture and install the 10,214-square-foot (949-square-meter) wall.
Redi-Rock of K.I.T. had manufactured and installed a few smaller projects that included Redi-Rock XL units, but this was the first project of a larger scale.
“It’s a learning curve just like everything else,” said Vittitow. The crew of five used the same equipment to install the wall with XL blocks that they use for installing standard Redi-Rock blocks, including two skid steers, a Caterpillar 314 Trackhoe with a fully hydraulic totem clamp, and a vibratory roller. After setting the base course — which is the most important course on any Redi-Rock wall, not just walls with XL blocks — they gained some momentum and efficiencies.
“We had a few hiccups up front,” said Clark of the need for the redesign. “But it’s been moving pretty well ever since [Redi-Rock of K.I.T.] got out there.”
THE OUTCOME: 25-foot-tall wall successfully modeled and constructed
Despite the initial hiccups, the wall is now complete, and the rest of the hangar construction project could proceed. Clark had a positive experience working with Redi-Rock.
“I’d definitely work with them again; they’ve been really helpful throughout this process — the whole redesign process,” said Clark.
Hines said the success of the redesign process was twofold.
“Having that whole system of products available and then having the Redi-Rock software that you could model such an intricate geotechnical model to make sure that you had everything covered, it was really the marriage of the two — the software and the product,” he said.