Photo by Onyx Corporation
Massive precast blocks assemble to create New England’s largest precast retaining wall at the Ledges at Woburn.
By Debbie Snidermann
A massive retaining wall is installed at a new housing development in Woburn, Mass., a suburb of Boston. At 18,000 square feet, the wall is 700 feet long and roughly 52 feet high in its highest section and is the largest and tallest free-standing block wall structure in New England.
The Ledges at Woburn project is a gravity wall system built from large precast modular blocks. Because of the height of the wall and its location against a rock ledge face, it needed large blocks for the project to be successful.
“That’s one thing that makes this project unique,” said Eric Opachinski, vice president at MBO Precast in Carver, Mass. “In typical tall retaining wall applications, large blocks are only used on the bottom or first two courses of the wall for stability purposes. In this application, large Stone Strong 24-86 blocks were used for the entire height of the wall.”
MBO Precast has a 20-year-plus relationship with Stone Strong as a dealer and representative for its products throughout New England.
“It’s also unique to use a gravity wall for such a large scale wall,” Opachinski said. “There is no geo grid, nothing keeping this large scale wall tied to the slope behind it, only rebar connections and concrete infill.”
Stone Strong Systems of Omaha, Neb., is the licensor of the Stone Strong retaining wall system.
“Not only is it the tallest wall in Massachusetts, it’s also straight up against a stone façade natural wall, covering the entire hill,” Nathan Gran of Stone Strong said. “With Stone Strong, they managed to accomplish both feats in the same project.”
Site, size and safety: Precast required
The blocks were stripped from the molds with a bridge crane at MBO Precast. Photo courtesy of MBO Precast
Jefferson Apartment Group, the owners of the Ledges at Woburn project, wanted to hold the rock face up and create a safe place for parking and residents of the new community.
“There was supposed to be a free-standing blasted rock cliff,” John Durkin, operations manager of contractor Onyx Corporation of Acton, Mass., said. “But as the excavation began removing rock from beneath the 80-foot towering cliff, it uncovered a degraded rotted ledge face that didn’t have sufficient stability on its own, so a retaining wall had to be built.”
A precast wall offered a creative solution.
“We looked at many designs that incorporated geogrid, steel mesh, rock bolts and other ballasts to support the face, but logistically, the site was tight to the property lines and none of these other solutions were viable,” Durnkin said. “We didn’t have enough room to give up any property on-site or parking spaces. Stone Strong was the only large precast block without geogrid that could go that high.”
Given the engineering challenges of tall retaining walls, there were really no other options. Stone Strong blocks are the biggest in the industry, and the only block of its kind, with a large hollow cell to receive concrete and rebar to create the solid concrete ballast and strength needed to reach that height.
Stone Strong’s patented hollow core precast block design has the ability to be filled with stone and concrete. This wall required both. About 2,500 cubic yards of concrete infill went into the large blocks’ hollow cores in multiple pours in the tallest sections of the wall.
“The fact that the hollow core block doesn’t have much weight allows a smaller or normal-sized machine to handle it without a crane, allowing the concrete infill to be placed later. No other manufacturer could do that and provide the strength that these blocks did,” Durkin said.
The wall required a dowling system of upright rebar to create positive connection with each subsequent course as the blocks were laid on top of each other up against the rock ledge face.
“The hollow core block offered another benefit, allowing us to tie rebar into the block to incorporate into a topping slab that could be bolted into the deteriorating rock face without drilling into the rock,” Durkin said. “It was faster and easier to install, like a bookend with a brace at the top holding the top of the wall.”
Major safety challenges in building the wall also dictated the use of precast.
“In some places, the wall is only 6 inches off of the rock face. We had to use a crash protection box to keep the workers safe while they installed the footings and built the block, so there was no way and nowhere to pour concrete under the box. We had to use precast blocks,” Durkin said.
The job required lots of rock removal and blasting, another reason poured fresh green concrete wasn’t feasible. Precast could be built while blasting operations were ongoing.
Precast also allowed site work to continue in the winter when pouring concrete in New England is unadvisable. The costs for heating and controlling temperature of poured concrete would have been exorbitant.
How precast met the project’s needs
A view of the west wall at the Ledges at Woburn.
Photo courtesy of Onyx Corporation
Producing 850 precast blocks for one project is a large amount from a production and cost standpoint, but that’s what was needed to make the site work. The blocks were produced by MBO Precast, with more than two-thirds of them, about 575 units, being the largest 24-86 blocks, each 8 feet long, 3 feet tall, 86 inches deep and weighing 7,600 pounds. The rest were base blocks, top blocks and Stone Strong’s new corner forms to finish corners on the ends.
“Typically, we have one or two molds, but to meet the project deadline and not wait years for the product, we had to go through a double pouring setup with the forms and rent an additional mold from Stone Strong to pour units twice a day,” Opachinski said.
As product was made, it sat until cured and shipped to the site immediately.
“We stripped units out of the forms early, setting up forms and pouring with special batches of concrete that used accelerating admixtures, hot water, heaters and curing blankets,” Opachinski said. “We did continuous double pouring production and constant delivery as the job progressed.”
For precasters, pouring units daily is a standard procedure. But even though there was “nothing special” about the blocks, the pieces were in stock and it was a process MBO Precast is familiar with, there was nothing standard about producing the precast for this project.
“Double pouring is very uncommon,” Opachinski said. “And, doing it every single day, for this length of time put a strain on the crew and others.”
Production and delivery took roughly six months. Project discussions started in November 2020. Block shipments started in January 2021 and were completed in May 2021.
Despite the production challenges, MBO Precast produced blocks fast enough to accommodate installation of more than 1,000 square feet per day, an impressive rate of productivity, while managing other ongoing work in its standard 8- to 10-hour shift.
Compared to other materials for retaining walls, such as log walls or small residential blocks, Gran said precast blocks are pricier at first but over their lifetime of more than 100 years, they are not expensive. They are not going to fall or cause concern about safety.
“Big precast blocks are easy to install, have a faster installation time, save on production time and labor, and for how long the wall will be there, are the best option,” Gran said. “The big modular block system is a new trend and better concept than smaller neighborhood residential blocks. Smaller modular blocks are not as inherently stable, and for bigger-than-life projects that required filling the blocks with concrete instead of aggregate, like this one in Woburn, no other modular system could accomplish that alone.”
Debbie Sniderman is an engineer and CEO of VI Ventures LLC, an engineering consulting company. She can be reached at [email protected]
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