Sustainable Land Development Today - April 2008 - (Page 34) RETAINING WALLS New Solution for Old Geotechnical Challenge Retaining wall systems help geotechs meet the unexpected. By Steven Knapp 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. 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 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 34 April 2008 Sustainable Land Development Today 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
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