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Seamless Combination of MSE and Gravity Structures

Engineer overcomes site limitations with versatile Redi-Rock system to create space for development access road and bioretention basin

The Challenge:

As metro areas grow, land available for development is scarce. So, when a previously undeveloped parcel that's 2.5 miles (4 kilometers) from a major airport can be transitioned into usable space, it becomes a hot commodity.

This is the story of the development of Preserve Court along Perimeter Place Drive near Nashville International Airport. The hilly terrain and adjacent wetlands of the Sims Branch stream made the land difficult to develop, but civil engineer Seth Sparkman of Barge Cauthen & Associates, Inc. suggested a solution.

Sparkman's plan called for building a Redi-Rock retaining wall to support a road that would provide access to the parcels, as well as create space for a bioswale. The bioretention area would filter runoff from the road, protecting the adjacent wetlands and meeting Metro Government of Nashville's environmental standards.

With the initial plan approved by Metro, Baron Construction purchased the land and began work on the five parcel development. In addition to the inherent challenges of the site, permit timelines and closing deadlines for the parcels created additional urgency for the project.

The Solution:

Using Sparkman's outline for how to meet the challenges of the site as a guide, engineer Mike Mattingly P.E. of Mattingly Engineers got to work on designing the retaining wall to move Baron's vision for the development forward.

"From a design standpoint, the biggest challenge was figuring out a way to allow for a bioretention basin behind the wall," said Mattingly. "They needed to use a special soil mix within a few feet of the back of the wall, so we wouldn't be able to install geogrid behind the wall within the upper few feet of finish grade."

The beauty of the Redi-Rock system is that all of the precast modular block components work together. The massive blocks made of wetcast, air-entrained concrete use patented knob and groove technology to interlock together like giant Lego blocks. The collection of Redi-Rock blocks offer robust design capabilities including: gravity walls with a variety of batter angles; weight-independent reinforcement with Positive Connection (PC) for MSE walls; freestanding wall applications; and accessories like columns and caps. Available in four natural stone textures, retaining wall engineers have the most comprehensive suite of problem solving tools on the market to meet aesthetic demands and design requirements.

To overcome the limitations imposed by the bioretention soil, which is great soil for filtering runoff but generally considered poor soil in retaining wall design, Mattingly seamlessly integrated multiple design structures into a hybrid wall using PC, gravity, and freestanding blocks, as well as caps.

The first few courses of Positive Connection blocks used Mirafi Geogrid 10XT to tie back into the reinforced earth zone filled with crushed gravel.

"The maximum wall height was 20 feet (6.1 meters) so a standard gravity wall was not an option," he said. "We solved the problem by designing a combination wall with Positive Connection geogrid-reinforcement for the bottom half of the wall and a gravity system for the upper half of the wall."

Redi-Rock's PC blocks revolutionized MSE walls when launched in 2011. Unlike friction connections featured in other geosynthetic reinforced wall systems, the PC system uses 12-inch (300-millimeter) wide strips of factory cut and certified geogrid that wrap through a core slot cast into the block. This weight independent connection allows a designer to use ultimate strength as the connection strength independent of the grid layer depth with respect to the top of the wall. In terms of long-term design strength, this is a significant advantage over friction connection which relies on the weight of the blocks directly above the grid layer.

For the reinforced bottom section, Mattingly used the 28-inch (710 millimeter) PC block in conjunction with Mirafi 10XT. The 28-inch PC blocks weighs 1,518 pounds (689 kilograms) and serves as a facing element to the wall, while at the same time working in conjunction with the grid to create the reinforced zone. Mattingly was able to reach a max height of 23 feet (7 meters), as well as account for a 250 PSF surcharge for the planned access road 15 to 20 feet (4.6 to 6.1 meters) behind the wall, by incorporating PC blocks into his design.

Looking to design a retaining wall that combines gravity and MSE structures? Check out Redi-Rock Wall Professional software to streamline your workflow and optimize your design!

As the wall approached the elevation of the bioretention basin, Mattingly transitioned to gravity blocks where the massive weight of the blocks holds back the earth without the need for reinforcement. Oftentimes, gravity walls are used in applications where there is limited room for excavation or utility infrastructure exists behind the wall.

In this instance, Mattingly topped the PC wall with courses of 60-inch (1520-millimeter), 41-inch (1030-millimeter), and 28-inch (710-millimeter) blocks--weighing 3,287 pounds (1491 kilograms), 2,309 pounds (1047 kilograms), and 1,613 pounds (732 kilograms) respectively--to create room behind the wall for the bioretention structure. That structure consisted of soil layers high in organic matter above a clay cap that diverted the filtered water into collection pipes before dispersing it back toward the wetland and stream.

Concrete pipes behind the retaining wall will collect runoff that has been filtered through the bioretention area, preserving the integrity of the adjacent wetlands.

While the adjacent wetlands influenced the technicality of the wall, it also played a role in the aesthetic choices for the wall.

"The wall is located at the entrance to a new business park and faces a nature preserve and walking trail," said Mattingly. "It was important to maintain a consistent finished look."

Mattingly chose a standard batter for the PC and gravity portions of the wall and topped it off with a course of freestanding blocks and caps to polish off the look. The wall was constructed in the Cobblestone texture for a smaller-scale, stacked-stone aesthetic.

With the design in hand, Redi-Rock of K.I.T. manufactured all of the blocks and installed the wall to help Baron Construction achieve their objectives before permits from Metro expired or closing deadlines on parcels passed.

"It was a neat project, but a challenging job," said Irvin Vittitow, owner of Redi-Rock of K.I.T. To meet the developments deadlines, the installation needed to occur during the winter, a particularly wet winter for Nashville.

Redi-Rock of K.I.T.'s crew of five led by Glen Vittitow installed all 8,618 square feet (800 square meters) of the wall in 30 days despite the weather conditions. The most challenging aspect of the installation according to Glen was avoiding the bioretention pond and pipe structure with their equipment when setting the gravity portion of the wall.

"It made it a big challenge to reach out and set the wall because we were setting 60s at the base, and then 41s and then 28s, and it made a challenge to reach that far," he said. Of actually combining the reinforced, gravity, and freestanding sections of the wall during installation, however, he said: "It was gravy."

Being able to set 5.75 square feet (0.5 square meters) of face with each pick was certainly beneficial for the crew, especially with the long reach. They used two skid steers, a Caterpillar 314 Trackhoe with a fully hydraulic totem clamp, and a vibratory roller to stage and set the blocks.

The Outcome:

With the 581-foot (158-meter) wall complete, access to the five development sites is now realized. Work on the individual projects has commenced with The Learning Zone, a childcare facility, and plans for a Holiday Inn Express are underway.

"Overall, Redi-Rock's versatility allowed us to meet the technical design challenges of the project while providing a great looking entrance to the business park," said Mattingly.

The Redi-Rock of K.I.T. crew installed the 581-foot wall in 30 days, including the freestanding blocks and caps to polish off the appearance of the wall.

Project Details:

Project: Case 203 - Preserve Court Access Wall Owner: Baron Construction Block Manufacturer: Redi-Rock of K.I.T. Engineer: Mike Mattingly, P.E. Installer: Redi-Rock of K.I.T. Location: Nashville, TN Year Built: 2018


Gravity Retaining Wall Blocks - Massive, one-ton Redi-Rock gravity blocks rely on the sheer mass of each block to hold back the earth.

Positive Connection Wall Blocks - Redi-Rock's revolutionary MSE system, where geogrid wraps through the block eliminating the chance of connection failures

Freestanding Blocks - Redi-Rock freestanding blocks are textured on both sides, giving above-grade walls an unbeatable, finished look for any project you design.

Redi-Rock Accessories - Steps, Caps, and Columns coordinate with Redi-Rock retaining wall products to give your project a cohesive look.

Cobblestone Texture - Redi-Rock Cobblestone blocks feature a great stacked stone aesthetic with the appearance of six smaller blocks.

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