It’s not news to geotechnical engineers that site designs sometimes have to evolve to keep up with changing client needs and budgets. That’s particularly true on a complex commercial project where a developer may be trying to attract tenants even as site work is getting under way. The tricky part of these assignments 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/senior engineer. JGI was hired by W/S Development to create a mechanically stabilized earth (MSE) structure for The Shoppes at Blackstone Valley. The 800,000-square-foot development was slated to be built on 75 acres in Millbury, Mass., just west of Route 146. The location was ideal for a retail center but left a lot to be desired from a constructability standpoint.

Finding an affordable way to turn the site’s granite outcrops and steep grades into building pads and parking lots was a major challenge from the start.

Blastrock Slope Seems to be the Answer
“We had to find a cost-effective solution to meet the owner’s space demands while keeping costs down,” Ciance said. “Our first proposal was a geogrid-reinforced slope. Then a blastrock fill slope to reduce costs. When a tenant changed the space requirements, we recommended a composite blastrock slope in combination with the SierraScape® Retaining Wall System.”

JGI’s initial design for a geogrid-reinforced slope would have provided a conventional solution, but the owner baulked at the construction estimate. To make the budget work, they needed a strategy that would meet anticipated space requirements with less cost. The firm reconsidered the available options and came back with a design for a 1 horizontal to 1 vertical (1H:1V) blastrock fill slope built with onsite rock and soils. Recycling onsite materials had the desired effect of significantly reducing hauling and materials costs.

Site Work Gets Under Way
The owner gave the project a green light and awarded the site-work contract to D.W. White Construction of Acushnet, Mass., and 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 at the blastrock fill slope face and saved smaller blastrock to provide fill for different zones within the slope.

David York, the firm’s chief estimator, calculates this portion of the operation produced a half-million cubic yards of stone and an equal amount of fill.

“We were basically working with a big hill,” he said. “The crews had to flatten the top off so we could have material to start building the blastrock slopes.”

JGI’s design specified placing the blastrock fill slope along the property’s eastern boundary. The northern leg of the slope would run 1,500 linear feet and have wall heights of ten to 35 feet. The equally long southern portion of the slope would range in height from 40 to 115 feet. Graded-filter blankets were also included within the structure to control storm water from a large subsurface infiltration field being constructed in a nearby parking area.

The armor stones had to be lifted into position with heavy machinery and then hand-chinked to ensure a tight fit between stones. The work 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.

Geotechs Have to Revise ­Strategy
JGI considered enlarging the blastrock structure, but they were concerned that there was insufficient material 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 blastrock base.

“We thought about using segmental retaining walls (SRWs) 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.”

Since JGI met resistance from local officials, D.W. White decided to contact John Biondo, northeast regional manager for Tensar International Corporation (TIC), to see if he could suggest a cost-effective 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 pressure relief 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 since the geogrid-reinforced, welded-wire facing units could be filled with onsite stone. It would provide a finished face that blended more naturally with the underlying blastrock structure, he said.

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

The revised design called for adding a 30-foot retaining wall to the project’s northern blastrock slope. Stacking the geogrid-reinforced, welded-wire forms above the 1H:1V blastrock fill slope provided the additional space the owner needed without adversely affecting the budget or schedule. A second wall would be used to create a grade separation for the new movie theatre.

Ease of Installation is Key
With the project once again on track, D.W. White’­s crews returned to the task of completing the site earthwork. Installation was easy according to York, even though his crews were new to the SierraScape Systems. “Once we gained experience putting it together, the installation moved along quickly. It took a lot less time than the boulder walls which really helped us gain ground on our original schedule of the overall ­project.”

Installation of the retaining wall structure involved placing the wire-formed facing units on top of the blastrock 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 connection rods to mechanically connect the UX Geogrids to the facing units.

At that point, heavy equipment was used to spread nine-inch lifts of onsite fill over the geogrids and place two- to four-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 height specified in the design.

“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.”
Savings on Theatre ­Construction Costs
D.W. White’s crews followed the same installation steps for the pressure relief wall, which JGI designed to address the soil separation associated with constructing the new multiplex theatre. Using a SierraScape Pressure Relief Wall instead of a conventional cast-in-place structure enabled the owner to build the theatre’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 wall approximately three feet behind the ­theatre. The structure was strong enough to retain the lateral earth and building loads created by a single-story retail strip located at the top of the pressure relief wall.”

“It’s a good option for grade separations of more than 70 degrees,” Biondo 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.”

Project Benefits From­ ­Cooperation
With most of the project behind them, Ciance, York, and Biondo share the feeling that The Shoppes at Blackstone Valley project was very much a team effort.

“We definitely had to work together to meet the owners needs,” York said. “We were thinking outside the box all the time and sharing ideas to keep the project moving along.”

Ciance has a similar observation. “Collaboration was the key to this project going so smoothly. It also helped to have Tensar as a support group at every phase of work. We had a lot of design changes right through construction, but everyone worked together to make things go in easier.”

“We all worked closely together,” Biondo agreed. “Everybody contributed to solving the design and constructability issues.”

As for the system that played such a prominent role in helping the Shoppes at Blackstone Valley get off the ground: “The SierraScape Systems provided an easy-to-construct and cost-effective solution for the client,” Ciance said. “It was flexible enough to use for pressure relief and retaining. It’s a product I will look at again.”

York reported that D.W. White has already used the system on a second project. This time the SierraScape Systems are supporting a big-box development being built in Reading, Mass., at the site of a former landfill. SLDT