New projects demand higher walls, faster construction, and attractive finishes.

By Janis Keating

In the past, it was common knowledge that retaining walls "had" to be poured, reinforced concrete. Even though concrete retaining blocks have been on the market for decades, there are still some who think blocks cannot do the same job as poured concrete, especially for structures more than a dozen feet high. That's simply not the case. 

By using geogrid, geotextiles, anchors, "stair-stepping," or just sheer weight, concrete retaining blocks are creating bigger and higher structures each year.

"We've done a 40-foot-high wall in a quarry for a stone loading system which fills railcars," relates Joe Powers of Pennsylvania's Easton Block Retaining Wall. "We've also installed low-flow channels–only 12 feet high but a mile long. Sometimes we have to build within tight spaces–for example, within 3 feet of the Norfolk railroad tracks.

"One of Anchor's systems can go as high as 70 feet," Powers goes on, "but I guess one of the most interesting projects we've done is one we've just finished, with 37,000 square feet of retaining wall on a balanced site. The wall, which was retaining earth for a Lowe's Superstore construction, was 27 feet high on its lowest end; then the wall twisted, faced the opposite way, and rose to 34 feet high. We had to cut into the high side of the job to put the soil in the lower end."

Soil Retention Systems of Oceanside, CA, specializes in segmental retaining walls and erosion control systems. Its retaining systems cover the entire spectrum from planter beds to walls more than 60 ft. high. Because the company is based in California, Soil Retention is keenly aware of the need for products that work well in a seismically active area.

"Research and actual events, such as those in Kobe, Japan, and Northridge, California, have proven that mechanically stabilized walls perform very well. They're very flexible," reports Dean Sandry. Soil Retention usually employs geotextiles to further stabilize its walls and the earth behind.

"We've recently built a 40-foot buttress wall, in a high-end residential area, that holds back a whole hillside," Sandry states. "Perhaps our most unusual project was a job for a new golf course, where we built a retaining wall that was eventually totally buried. We also had a few hassles in dealing with the coastline where the golf course was sited."

Of course, there are those who'd say that the concrete blocks are sometimes an unnecessary expense. Now in its 30th year, Vienna, VA's Reinforced Earth Company invented the Mechanically Stabilized Earth (MSE) retaining wall. Reinforced Earth's systems have been used for retaining walls, noise barriers, and precast arches for numerous applications. Its products include precast concrete panels for earth retention, wire-face earth retention systems, modular block retaining walls, precast arches, reflective and absorptive sound walls, and slope stabilization systems.

Reinforced Earth's Dion Gray points out that he works on the precast side of it. "I might question using blocks for larger retaining structures–I'd wonder about their stability. We focus on structures for highways and bridges, 25 feet and higher–runway projects, heavy rail projects. We promote steel as reinforcement, constructing precast facings with tie strips sticking out the back of the panel. We attach a steel strip to this, and insert the steel back into the soil. Our system is made on friction–the soil's mechanical properties are improved by steel reinforcement."

Steel for Strength

For difficult sites, or merely for extra strength, many block systems also use steel reinforcement, such as the Earth Anchor line from Foresight Products Inc. in Commerce City, CO. The company's Duckbill is used for small applications; the Manta Ray and Sting Ray wall anchor systems are used for large retaining walls.

"Most of our customers' structures are 20 feet or less," relates Bob Phillips of Atlanta's Anchors Unlimited, a Foresight distributor. "There might be a tree overhead when they can't use the geogrid, so they use an anchor at the ‘failure point' in the wall. Manta and Sting Rays can also be used to reinforce walls that have failed. The contractors drill holes into the wall and anchor the wall to reinforce it, to keep it from failing more."

Easton Block's Powers has experienced the same problem. "Someone built a wall and installed the geogrid incorrectly; the blocks started to fail. We put 30-foot Manta Rays into the wall to stabilize it, then built a wall in front of it."

Geogrid Gets the Job Done

"We have no problem getting heights," reports Pavestone user Mark North of Mark North Erosion Systems in Fort Worth, TX. "The key is site-specific engineering. Plus, over the past five or six years, a lot of manufacturers have refined their practices and discovered how to make walls with height."

North recently began using blocks from Pavestone Company of Grapevine, TX, in an existing creek channel in Irving, TX, under stressful conditions exacerbated by this summer's flooding. "We're putting up 55,000 or 60,000 square feet of walls, with a maximum height of 12 feet, 8 inches. When finished, we'll have put up 7,331 linear feet of wall, gridded with 25 square yards of geogrid–3XT Mirafi plastic materials."

The first task was diverting Delaware Creek, which eventually drains into Trinity River, so work could proceed on its eventual site. "We made coffer dams and excavated the stream; a silt program will have to be put downstream," North explains. "Some grade control structures will be built into the creek, a small lake effect, a series of dams–this is for flood control–but those will be made out of concrete."

Not only will these walls have to channel the creek, but also a portion of the walls will be submerged. "We're creating tiered walls for this natural creek. The tiers will probably be grassed, with maybe some landscaping. The lower walls for sure will be in wet condition–3 or 4 feet will be under water most of the time. There's a concrete foundation or leveling pad for the walls, and the fill material behind is free-draining sand," North adds.

Work continues apace; North expects to have the project completed by the end of the year. "We've scheduled 77 working days with a 15-man crew. One limiting factor: not how many blocks we can set in a day, but how much heavy clay embankment can we process in a day?"

Although Delaware Creek is not used for water recreation ("except maybe by ducks," North notes), it runs through a park setting, so the walls need to be attractive. With help from Pavestone, North is fitting the bill.

"We're using 120-pound Anchor Vertica Pro split block, which is more like natural stone," he continues. "And after talking with the city, Pavestone spent lot of time and effort to make a special Irving Blend block, which uses multicolors to simulate Milsap Stone, a sandstone indigenous to this area. Quarried in and around Milsap, about 45 miles west of Fort Worth, it comprises earth tones ranging from light tan to brown with traces of rust. The Irving Blend is a rust, tan, brown, earthy stone."

For Stability, Size Does Matter

Due to their massive size, Redi-Rock blocks must be lifted into place mechanically.

Sometimes just the sheer bulk of a wall will keep it stable–and that's the premise behind Redi-Rock based in Charlevoix, MI. Where most retaining wall blocks hover around the 100- to 200-lb. range, various Redi-Rock blocks weigh between 1,100 and 2,600 lb. With these massive blocks (averaging 46.5 in. wide x 18 in. tall), geogrid tiebacks are not required for many Redi-Rock walls.

This summer, Redi-Rock of Colorado Springs, a subsidiary of EAM Manufacturing., finished a 500-ft.-long, 13-ft.-high retaining wall in Douglas County, near Denver. Because the site involved unusual problems, for once the company had to use soil anchors.

"The project entailed widening a roadway, taking the steepness out of the grade at the side of the road," recalls Doug Schanel, president of Redi-Rock in Colorado. "We didn't want to have to move the power lines. This segmented wall system was tied back with wall anchors. As Redi-Rock is so much bigger and substantial, most applications only require geogrid; we used soil anchors, a threaded rod, like those used in highway underpasses. We figured a way to anchor them into soil behind the wall; we used a bracket and cable to attach the wall to the anchors. We drilled into the wall and grouted, of course. 

Soil anchors were used to hold the Redi-Rock to the slope.

"The client considered doing a cast-in-place wall, but that would have needed too much excavation," Schanel continues. "Also, that would have caused safety problems, cutting the slope back. We had less safety worries because we wouldn't be behind the wall, only the machines would be, setting forms and so forth. We had the ability to put the wall up in segments and tie it up to slope. For our process, there was no need for oversized excavation; we would have had to excavate on private property if we had used geogrid. To our knowledge, this has never been done; they didn't have a big enough wall system. Because of the anchoring, we could tie back the slope, we didn't have to excavate for geogrid, and we didn't move power-line systems. Plus, with a three-person crew, including the truck driver, we erected the wall in a week–five working days!" 

Redi-Rock is produced in seven block styles. Including the standard set–bottom, middle, and top blocks–producers also can create top or middle half blocks, planter blocks, drain ditch blocks, and end blocks. Producers can customize textures and colors to match regional rock environments. Although Redi-Rock's Classic Face and Split Limestone Face are ideal for most applications, local or specialty rock formations also can be replicated.

John Gran, general manager of Lincoln, NE's Stone Strong Systems, uses a lot of Redi-Rock in his soil erosion work for the city. "We were working on storm sewer and drain areas in the Pioneer Woods/Holmes Lake area," he reports. "On a US Army Corps of Engineers's water line, they excavated too much for the core line, so it needed a 20-foot retaining wall, 300 feet long, which would also handle a street on top of it. In their overflow line for a dam, there wasn't another product available for what they wanted to do. The engineers for the project considered doing something else, but they weren't happy with the height of the wall. We did put in some geogrid for safety and reinforced the wall up to 9 feet, but not past there because it was considered a ‘grabby' wall. The finished wall cants back about 1.5 inches every layer, and it's 10 layers high."

Stone Strong Systems produces Redi-Rock for its service area, but it's also developing a new product. "We have our own block, which is four times bigger than Redi-Rock [8 feet wide and 3 feet high], yet it weighs only half as much. We've named it Stone Strong block. The block contains a noncorrosive rebar retainer and voids–we can put infill in it. We have the forms on the way and expect it to be on the market soon."

So Big Yet So Fast

John Peterman of Carroll Concrete in Newport, NH, finds that Redi-Rock allows him to put up retaining walls fast. "We just finished a 1,000-plus-block job on a road washed out a few years ago. This road ran down a hill, so there was a slope above and one below; the lower one washed out. The community secured FEMA dollars to fix it, and the project was awarded during the first week of June with the stipulation that within 30 days it had to be complete! We were able to do it," Peterman states.

Each course of block was secured.

The finished wall

"Someone [else] might have used smaller blocks, but with the labor shortage in this area, they could never have gotten it done," he continues. "This project entailed 400 blocks on the uphill side, 600 on the downslope. We used a three- or four-man crew and got it done on schedule." 

Peterman explains how he uses Redi-Rock: "It's designed as a gravity wall retaining system. We usually set the blocks back 2 inches for every 18-inch rise and can go as high as 20-odd feet without having to use geogrid–with no soil nailing. We have a planter block, which we stair step; in some cases, we might be able to eliminate the use of geogrid."

Although his area doesn't experience severe seismic activity, New Hampshire experiences freeze/thaw cycles that can jostle Peterman's walls "a minor amount," he relates. "To minimize the effect, we ensure that the wall has proper drainage. We prefer to put piping in it, to make sure the water is taken away. For stability below and behind, we make sure there's full compaction of the dirt. And because the wall is so stable, most times we can go right up to the property line."

For the past three years, Carroll Concrete has produced the blocks in five locations in New Hampshire and Vermont, and it distributes the product throughout New England.

"We do retaining walls from one block–18 inches–to 20 feet high," Peterman maintains. "Some commercial, industrial, some architectural. We're looking into doing bridge abutments because they don't take much time. We're currently looking at a job in which the client thinks there's ‘too much concrete exposed.' They will use Redi-Rock, which has a limestone appearance, to cover it–a pure architectural preference. This project will rise to about 22 feet high."

This isn't the first time Carroll Concrete has used the blocks to "pretty up" a site. "On one job, there was an existing 14-foot railroad-tie wall, which was starting to fall apart. We didn't demolish the old wall, which also saved money. We left it intact and built a Redi-Rock wall in front of it."

Janis Keating is a frequent contributor to Erosion Control.

EC - November/December 2002