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Concorde Flats Apartments

Project engineer sees opportunity in challenging building lots

Year Completed: 2022

Gregg Matlock, project engineer with Sandman Structural Engineering in Brainerd, MN, had worked with tire-derived aggregate once before when he recommended it as a possible solution for the future site of Concorde Flats, an affordable senior and market-rate apartment building in Burnsville, MN.

Matlock had a couple of challenging issues to address at the building site. “There was a severe imbalance of soil at the site,” Matlock explained. “One side had two stories and the other had none. The lateral pressure induced by that soil basically made it so we couldn’t resist it without using an alternative backfill that would reduce the pressure on the building.”

From his experience, he knew TDA could be the alternative backfill material. Add to that, the TDA had to cooperate with another part of the design—a 20-foot-tall retaining wall on another side of the building that would abut to the TDA.

TDA wasn’t a slam dunk. Sandman Structural Engineering, the engineer of record on the project, offered two choices to the contractor, in addition to TDA. A geo-grid system was quickly dismissed because of building site constraints. The third option was foam blocks, which would have been more expensive than TDA.

“I enjoy bringing our experience to the table to apply to a project,” Matlock said. “We’re always trying to educate and get the best end result.”

The final design calls for approximately 1,200 cubic yards of TDA in a 170-foot trench, 10 feet deep, that is eight feet wide at the bottom and 18 feet wide at the top. The wedge will support a part of the building that has the most pressure on it. The TDA will be invisible on the surface. The building will open in summer 2023.

These kinds of engineering challenges are becoming the norm, according to Matlock. “Ten years ago, all the building sites were flat, but now it seems like almost every building has some amount of unbalanced fill,” he said. “Worse changes in grade. Squeeze them in as tight to the boundaries as possible—it’s getting more and more complicated all the time.”

TDA can address many of these challenges, according to Monte Niemi, CEO of TDA Manufacturing in Isanti, MN, the supplier of the material for Concorde Flats. “Concorde Flats demonstrates just two of the ten properties TDA offers—reducing lateral load and lightweight—to support needs in engineering, sustainability, and performance,” Niemi said.

TDA reduces lateral loading by 50% compared to mineral soil, making it a superior backfill material, Niemi said. In addition, it weighs one-third the amount of traditional soils. The other eight properties Niemi promotes of TDA are hydrophobic material, 50% void space, sheer strength

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