In 2019, Costco Wholesale set out to open a new, 152,000 square foot warehouse and gas station complex in Eagan, Minnesota. With the groundbreaking in the Spring, the aggressive completion deadline was set for the Fall, just in time for the holiday shopping season. However, the land the company selected had more than a 50-foot grade difference from the South to North end of the site, making a flat surface a challenge to create for the warehouse, gas station, and a 725-car parking lot. The elevation change required varying segmental retaining wall heights across the site, with the need to design two vehicle ramp access ways at either end of the site. Adding to the challenge: the crews building the retaining walls and those moving the dirt had to choreograph the build in less than 90 days to meet the grand opening schedule. Belgard Diamond Pro HD blocks (8”x18”x12”) were chosen for the segmental retaining walls because they can be installed quickly, look great, and are durable enough to withstand Minnesota’s harsh winters. “The project came with several challenges, the first being it was an export site, meaning there was more soil on site than we needed,” says Joseph Kowalski, PE. He explains mechanically stabilized earth (MSE) walls being constructed for Costco are comprised of alternat- ing layers of compacted granular backfill affixed to the Diamond Pro wall face. “Most MSE walls require an excellent sand and gravel mix as backfill,” he says. “Typically, in an export site, we have the native material removed and then import clean sand. We were fortunate that when this soil was tested, it was relatively clean sand and gravel and we didn’t need to bring in thousands of tons of new material. This is good environmental stewardship as well as a cost-saving move.” The very thick layers of sand and gravel on the site were interrupted by layers of lean clay. The General Contractor, Novak Construction, had to be careful not to mix the clay in with the sand when digging, or the sand would have been contaminated and required removal. “They did a very good job with that. We were able to reuse all the sand as intended, within the geogrid-reinforced section of the walls,” says Kowalski. “This was a significant cost saving.” Kowalski worked closely with Structures, Inc., the firm responsible for constructing the MSE walls. The company is well-known nationally for providing earth retention and segmental pavement system solutions for some of the largest and most complex projects in the United States. They had previously worked on Costco projects with Novak and were familiar with the products being used. Belgard ® Diamond Pro HD Blocks Key to Coordinated Effort to Build Seven Segmental Walls for Large Costco Complex
Walls were backfilled using alternating layers of eight- to 16-inches of sand and gravel and Mirafi® geogrid reinforcement. Where the slope was particularly steep, the sand and gravel below the walls were undercut and replaced with layers of geogrid reinforcement within the replaced soil. This reinforced soil zone below the wall improved its overall global stability. In some places, the geogrid layers were up to 55-feet long and extended several feet inf front of the wall face. The geogrids are extended to near the face of the Belgard Diamond Pro HD blocks. “The downward lip at the back of the Diamond Pro also provides an extra mechanism to connect the geogrid to the block,” said Craig Vardy, construction manager of Structures. “That’s one of the reasons we enjoy working with the Diamond Pro – you just grab the rear lip connector and the block goes in more easily.” Kowalski agreed that the design and ease of Belgard Diamond Pro block was a big time-saver for this job. “The blocks are ideal for demanding commercial applications like this,” he added. The block’s cores were filled with an angular stone. The combined weight of the blocks and the stones pushing through some of the apertures in the geogrid keep the material firmly in place. Diamond Pro is also approved by the Minnesota Department of Trans- portation based on its higher resistance to freeze/thaw degradation. “We wanted a block that has excellent freeze/thaw resistance when tested using saline water,” explains Kowalski. “These blocks are denser, have less absorption, and a higher compressive strength. We are confident they will provide sufficient resistance to the freeze/thaw climate changes experienced in Minnesota.” With a rough-hewn face style, the natural rock texture and earth-tone colors blend easily into any environment, while functionally, the block wall gives landscape substance and stability. “It’s all about the plant and how they produce the look and aesthetic and needing to make sure that we get the block from a consistent run,” said Vardy. “The Belgard plant did a great job to make sure it matched up.” Experienced in coordinating foremen and crews, Structures was able to quickly begin the early site work. “We had to get the big wall up and
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FEBRUARY 2021
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