Project Case Study: Flexible support retaining and pressure-relief walls adapt sloped site to evolving retail development

October 2004 » Feature Articles
On a complex commercial project, where the developer is trying to attract tenants even as construction is getting under way, site designs sometimes have to evolve to keep up with changing client needs and budgets.
Steven Knapp

On a complex commercial project, where the developer is trying to attract tenants even as construction is getting under way, site designs sometimes have to evolve to keep up with changing client needs and budgets. The tricky part is adapting the design to changing circumstances while keeping the budget and schedule under control.

That was the challenge for Manchester, N.H.-based Jaworski Geotech, Inc. (JGI), according to Michael Ciance, the firm’s principal and senior engineer. W/S Development, a New England real estate developer based in Chestnut Hill , Mass., hired JGI in 2000 to create a mechanically stabilized earth structure for The Shoppes at Blackstone Valley . The 800,000-square-foot development was slated to be built on 75 acres in Millbury, Mass. The location is ideal for a retail center, but finding an affordable way to turn the site’s granite outcrops and steep grades into building pads and parking lots was a challenge.

JGI initially proposed a geogrid-reinforced slope. It would have provided a workable solution, but the estimated cost was more than the owner had budgeted. The firm’s revised design featured a 1 horizontal-to-1 vertical (1:1) blasted rock-fill slope built with on-site rock and soils, wich significantly reduced hauling and materials costs.

In 2002, the owner approved the project and awarded the site-work contract to D.W. White Construction of Acushnet, Mass. The company’s crews began the process of blasting the site and segregating the materials into piles for reuse.

They reserved the largest boulders for use as armor stone on the rock-fill slope face, and saved smaller rock to provide fill for zones within the slope.

JGI’s design specified placing the rock-fill slope along the property’s eastern boundary. The northern leg of the slope would run 1,500 linear feet and have wall heights ranging from 10 feet to 35 feet.

The equally long southern portion of the slope would range in height from 40 feet to 115 feet.

Graded-filter blankets also were included within the structure to control stormwater from a large, subsurface infiltration field being constructed in a nearby parking area.

A change in plans

Work on the rock-fill slope was labor-intensive, but D.W. White was making steady progress on site preparations when JGI was asked whether it would be possible to increase the amount of retail space at the site. W/S Development was negotiating with Showcase Cinemas, which was interested in locating a three-story multiplex in the development. To accommodate the facility, the owner needed an affordable way to provide a new grade separation and additional retail space and parking.

JGI considered enlarging the blasted rock structure, but was concerned that insufficient material existed on site to complete the work.

Hauling in stone would have increased costs dramatically. The more affordable option was to stack a different wall system on top of the blasted rock base.

"We thought about using segmental retaining walls or poured concrete," Ciance said. "But the town of Millbury wouldn’t accept those types of structures. They wanted something that fit with the area’s New England heritage." D.W. White contacted John Biondo, northeast regional manager for Tensar Earth Technologies, to see if he could suggest a costeffective strategy for addressing the project’s newest set of requirements. "The SierraScape Retaining Wall System seemed like a good fit for several reasons," Biondo said. "They’d be able to use it in retaining-wall and pressurerelief applications; the work would go in fast, which would help keep the construction on schedule; and it fit their budget." Biondo also thought the system would address the aesthetic concerns raised by the town because the geogrid-reinforced, weldedwire facing units could be filled with on-site stone. It would provide a finished face that blended more naturally with the underlying blasted rock structure, he said.

After evaluating the system, JGI was satisfied that it would meet the project’s requirements better than the alternative solutions, and the firm soon was able to present a new design to the owner and local officials. Ultimately, all project participants agreed that the SierraScape System was the best option for balancing aesthetic, space, budget, and schedule issues.

The revised design called for adding a 30-foot-tall SierraScape Retaining Wall to the project’s northern blasted rock slope. Stacking the geogrid-reinforced, SierraScape forms above the 1:1 rock-fill slope provided the additional space the owner needed without adversely affecting the budget or the construction schedule. A second wall was used to create a grade separation for the new movie theater.

Stacking stones

Installation of the retaining wall structure involved placing the wire-formed facing units on top of the blasted rock slope. Workers tied the adjoining facing units together and unrolled precut lengths of Tensar Uniaxial (UX) Geogrids perpendicular to the wall face. They then used SierraScape connection rods to mechanically connect the UX Geogrids to the facing units.

Heavy equipment was used to spread 9-inch lifts of on-site fill over the geogrids and to place 2-inch to 4-inch stones in the wire-formed facing units.

To establish a separation layer between the wall facing and the compacted fill, workers installed geotextile over the small stones. The process was repeated for each row until the structure attained the design height.

"The mechanical connection between the geogrid and the facing units ensured facial stability and helped maintain alignment over the length of the row," Biondo said. "That’s important when you’re building a structure with rows up to 1,500 feet long." D.W. White’s crews followed the same installation steps for the pressure-relief wall, which JGI designed to address the grade separation associated with constructing the new multiplex theater. Using a SierraScape Pressure Relief Wall instead of a conventional cast-inplace structure enabled the owner to build the theater’s rear wall using conventional masonry block—a much more affordable solution.

"We considered a cast-in-place wall, but the cost would have been tremendous," Ciance said. "Instead, we recommended using a 34- foot-tall SierraScape Pressure Relief Wall approximately 3 feet behind the theater. The structure was strong enough to retain the lateral earth and building loads created by a singlestory retail strip located at the top of the pressure relief wall." Pressure relief walls are a good option for grade separations of more than 70 degrees, according to Biondo. He said, "The positive, mechanical connection between the geogrids and facing units enables geotechnical engineers to design lower-cost walls that are structurally equivalent to conventional methods." The Shoppes at Blackstone Valley project was a team effort. "Collaboration was the key to this project going so smoothly," Ciance said. "We had a lot of design changes right through construction, but everyone worked together to make things go in easier. The SierraScape systems provided an easy-to-construct and cost-effective solution for the client. It was flexible enough to use for pressure relief and retaining."

Steven Knapp is a freelance marketing writer. He has written about engineering, telecommunications, computer hardware and software, satellite communication, and more. He can be reached at steve@knappcommunications.com.

 


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