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 alternating 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.
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 Transportation 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 get the site elevated, so they could lay out the pad for the building; that was the number one priority,” explains Vardy, referring to one of the two largest walls on the property. It extends along Blue Gentian Road and is 1,350 feet long and approximately 30 feet high, stretching from one entrance ramp on the East side of the site to the other entrance ramp on the West side of the site. The wall was designed to take the live load of large semi-trucks and firetrucks.
“It’s not that common to have walls that tall,” explains Kowalski. “We had no structural concerns, though, because we were working with good material and in accordance with the Federal Highway Administration’s design methodology for wall design; it results in a resilient and conservative structure.”
The largest wall on site was 32 feet high, but not nearly as long as the one along Blue Gentian. It retains the earth and supports the back, northeast corner of the warehouse. There are three different types of Mirafi geogrid in this section (5XT, 8XT, and 10XT), and the grids are 33 feet long where the building gets closest to the wall. “With walls this size, we had two loaders at one point feeding directly to backfill,” explains Vardy.
Structures worked with the company moving the dirt, with each company’s foreman coordinating with the other as truckloads of dirt were moved around the site and dumped into the geogrids. “We laid 1,000 sq ft. of blocks a day, so we needed to be in sync,” Vardy says. The process of back-filling and building the wall simultaneously, as well as working around utilities and storm pipes on-site, created complications. “There were manhole structures we had to work around, pipe culverts we had to bridge over, and a lot of utilities—conduits for electrical lights and for the gas station. With the size of the parking lot, many storm utility pipes would need to be placed within the reinforced zone of the retaining walls, as well as the functionality of a new stormwater management pond,” notes Vardy.
Once the first large wall was complete, Structures Inc. prioritized building by when the site or utility contractor was finished and ready to support another wall. At times, they had two or three crews working onsite at one time.
To maximize space for curb and parking stalls, a Sleeve-It® SD-1 pre-engineered fence post anchoring solution was used to install the four-foot fence directly behind the wall block, along with a guardrail.
Structures began construction of the walls in April of 2019 and Costco opened its new facility on Friday, November 15, just in time for holiday shopping.
“Building on a site like this with cast-in-place retaining walls would have been cost-prohibitive and many developers would not have attempted it,” noted Kowalski. “The technology of using these types of walls has become more advanced and Civil Engineers and Developers are more aware of what can be done with reinforced soils.” For the plethora of large warehouses and distribution centers being constructed on large sites, Kowalski said even a relatively flat (two percent) grade across a 1,000 square foot site may likely require a 20-foot-tall retaining wall.
For future segmental wall projects of this magnitude, both Kowalski and Vardy have the confidence in Diamond Pro HD blocks to bring the project in on time, with durability and aesthetics–even in the most severe climates. For more information, visit www.BelgardCommercial.com or call 1-877-Belgard (235-4273).
Belgard, part of Oldcastle APG, offers a complete collection of paver and wall products for plazas, terraces, parking areas, roadways, rooftops and retaining walls. Available in a range of styles, premium Belgard products have been found in the nation’s finest developments and award-winning commercial and retail properties since 1995. Oldcastle APG is part of CRH’s Building Products division. As the largest building materials company in North America, CRH provides a single-source solution for commercial construction projects with a full portfolio that also includes structural masonry, masonry veneers, dry mix products, hardscape jointing sands and sealants, stormwater management systems, concrete infrastructure, architectural glass, lawn & garden products, and composite decking.