The Best Retaining Walls for the Job

March 1, 2011

Even a brief perusal of the history of wall building immediately turns up the Great Wall of China, a miracle of size if nothing else. Or the Berlin Wall, important for its ability to separate one system of government from another. New England is dotted with old rock walls intended to keep livestock in place. And in Manhattan there once existed a wall built in 1644 by the Dutch to protect them from attacks by the British. Long since disappeared, it is memorialized by Wall Street, which seems to some to keep the rich away from the rest of us.

As for retaining walls, a historical search uncovers a plethora of companies who all take credit for inventing if not the first, at least the best, retaining wall. But a wall, after all, is only as good as its performance in the long term.

“I’m Still Standing, Yeah, Yeah, Yeah”
As the owner of Vermont Lumber and Stoneworks in Nashua, NH, Patrick Dupuis has spent the last 12 years building retaining walls. In the summer of 2006, he completed a relatively small project that has stood out as one of his most challenging and surprising work memories.

The project required building a 20,000-square-foot retaining wall for Applebee’s Neighborhood Grill, located on Amherst Street, slightly downhill and just behind another building on SR 101A in Nashua. “When we arrived, the restaurant was sitting low, so we raised the site 25 feet and built up the property in the back,” Dupuis says. The wall was built with an Anchor Wall system from Keystone Retaining Wall Systems.

“We completed the project in two weeks and left the site,” he continues. “Then the parking lot was paved and the covers were put back on the manholes. On the other side of the parking lot, homeowners had an existing concrete wall only 6 or 7 feet away from the restaurant’s retaining wall. Our wall face was facing their wall face.”

Then the rain came. “We had a wicked rain,” Dupuis recalls. “All that water and no drains. The two walls made a riverbed for the stream of water, which then undermined a 25-foot-high section of our retaining wall. Between 3 and 4 feet of wall was undermined at the base.”

Despite the damage, Dupuis says he and his crew were amazed by stability of the Keystone wall above the base. “Those anchor pins held the blocks together, and the wall never moved,” he says. “When we got to the site, the base was stable. We took out some blocks from the face so we could see inside. It was like a cavern. The geogrid was bent but the blocks were intact, just exposed underneath. It never collapsed on itself. We had compacted the soil, backfilled, and put the grid on tight. The pins had held things together; without them, we would have lost base course and all.”

Because the wall was still standing, the crew had time to go beneath the base course and repair the situation. “A 20-foot-long section was undermined, so we put steel plates under the base and then brought in post jacks, which we cut to the right length,” Dupuis says. “We jacked these to the steel plate and secured the base to hold the wall and ensure a secure base course. Then we framed the whole front of the wall bottom with wood. Next, we used a ledge drill to create 25-foot-deep holes at the top of the wall and poured in flowable fill until the void under the wall was filled. During this time, the existing wall never moved even a quarter-inch. When the area was bonded, we removed the forms and concreted all the way down, filling the front and covering the blocks.”

The wall was fortified and never moved again, Dupuis says. “We were impressed that the original wall held like that when the base was undermined. It was the combination of Keystone’s geogrid and pins and our compacted fill that made it hang there and gave us time to fix it.”

It’s Dirty Work, but Someone’s Gotta Do It
Marvin Wyatt, owner of Key West Retaining Systems in Wilsonville, OR, has been in the retaining wall business since 1940. He has been using the Lock+Load Retaining Walls System since the product was introduced in 1988. “It has a lot of advantages,” he says. “These are independent units that can be stacked on compacted fill. The system accepts full compaction at wall face so that there’s the same compaction on the face as there is in all the backfill.”

With the look of quarried stone, the Lock+Load system is engineered with steel-and-fiber-reinforced concrete, resulting in large blocks with a 16- by 32-inch facing area that weighs less than 150 pounds. The installation of Lock+Load walls is a lot like laying bricks in a bed of mortar. The soil backfill acts as mortar, and the panel and counterfort become the brick. As each row is placed, the soil backfill is used to grade and level the panel so that it is plumb, online, and at the correct elevation. The row of panels is then backfilled and compacted, and the process is repeated until finished wall height is reached.

In October 2010, Key West completed a three-month project with Heartland Construction, which involved building a wall and thereby adding commercially usable space for a parking lot and condominiums in Isaquah, WA. The retaining wall, 33,000 square feet in area, ranged from 18 to 43 feet high and was 725 feet long. Complicating the project, the wall was built on a slope in a hilly area. “Using the Lock+Load system gave us maximum space at the top of the wall,” Wyatt says. “The wall face is only 4 inches thick, so they can put the guard fence right at the base; that saves real estate. Each Lock+Load unit, when filled with compacted soil, is independently stable, which is a big advantage in earthquake zones.”

The process at Isaquah began with an excavation that went back 35 feet, to make a “safe” cut that Wyatt said allows for “massive drainage at the bottom.” Next followed a 2-foot-thick blanket of 2-inch crushed rock from the wall face to the back of the geogrid to create a “burrito” drain that would prevent any water flowing under the wall from saturating the geogrid zone.

The 12-foot rolls of geogrid were rolled out by hand on a hard surface, held taut to remove wrinkles, and cut with a utility knife.

“When the backfill is applied, the geogrid looks like a massive membrane,” Wyatt says. Soil then was added in 8-foot lifts all the way to the top.

At the top of the desired grade, a 6-inch pipe was pre-installed so that those who later installed the fence and guardrail would not have to drill through the geogrid and backfill. “We do this all the time,” Wyatt notes.

“When you lay geogrid down and compact the soil, the geogrid “˜laminates’ the soil, like plywood, and provides strength to the soil so it can “˜stand up,'” Wyatt explains. “We use Bobcat-style skid-steers with tracks and forks to bring down the pallets of Lock+Load to the work area. With 14 panels to a pallet, that’s a weight of 2,000 pounds. We used about a thousand pallets of Lock+Load and a full semi-tractor load of geogrid.”

A 2-foot layer of five-eighths-inch crushed stone with no fines was used in the first 2 feet back at the base of the wall. Wyatt says the Seattle area has plentiful glacial till, which is good rock for structural fill. The backfill was brought in on large off-road equipment. Other equipment included 1,000-pound plate compactors, a Bobcat bulldozer, off-road dump trucks and scrapers, a Bobcat tracked skid-steer, an 84-inch, 20,000-pound roller, and a laser mounted on a tripod to keep things level and straight.

“The site balanced well,” Wyatt says. “We added more than 25 to 50 feet to the property, increasing its value.” He estimates the cost of Key West’s part of the project at $18.45 a square foot, which included Lock+Load’s system of blocks and geogrids, in addition to drain pipes, fill, and the labor involved in constructing the foundation and erecting and backfilling the wall.

The main challenge on this project was three full weeks of rain, according to Wyatt. “The last 10 feet of soil placement required cement treatment because the soil was so wet. It was getting toward fall, and increasing rain is standard out here.”

Key West accomplished the task in about 40 working days, at 700 to 1,000 square feet per day for a seven-person crew. “That’s 100 to 150 square feet per man per day,” Wyatt notes.

Even Big Boulders No Match for Lake Erie
Located on the shores of Lake Erie in Painesville, OH, Tom Winfield’s company has more than 40 years in hillside construction and restoration. Aptly named, HillCrafters Inc. offers customers new hope and more land in an area where the mighty lake tries to be the boss of everyone and everything in its path.

According to a 2009 report by the Ohio Department of Natural Resources, of the thousands of Ohio homes on the Lake Erie waterfront, nearly half are within 50 feet of the top of the bluff and a quarter are within 25 feet of the top of the bluff. Many were farther from the bluff when they were built. In some cases, erosion rates have been as high as 110 feet per year.

In the past, shoreline erosion control often employed modules intended to break wave action and build beach, according to Winfield. “These were 10-by-10 blocks of concrete, the smallest of which weighed 16 tons,” he says. “But no matter how stable, they still would collapse into the clay, pulled by their own weight into a hydraulic slide.”

Since Winfield has spent most of his career replacing the massive blocks, he is a fan of newer and more effective products, particularly Stone Strong Systems’ 6,000-pound blocks used widely for large-scale applications. “Each one is built with a cavity, which allows it to be filled with concrete and double its weight,” Winfield says.

There are significant differences in the shoreline composition between the west end, middle basin, and east end of the lake. “My home area, the middle basin of the Lake Erie shore, has nothing but clay,” Winfield says. “The sportsman’s west end and the east end have shale that is close to the surface of the lake, bluffs of shale. The advantage to Stone Strong blocks is that they can be used in any situation. On the west and east ends of the lake, they work using the weight system, and sometimes with core drilling and pinning; but in the middle basin they work best when built on a substantial footer.”

In 2007, Winfield was asked by a homeowner in Willoughby, OH, to stop the movement of a lakeshore bank. The top of the bluff was eroding, and in an attempt to solve the problem and protect what was left of the owner’s backyard, someone had dumped the slabs from a demolished school building over the hill. “But those slabs kept sliding and taking out stable earth from the 50-foot bank. It was digging out as it was sliding,” Winfield says.

Winfield is no fan of throwing chunks of concrete at a problem. “The water just flows through and, as it meets resistance, increases in velocity. This tends to increase the rate of the erosion.”

In the Willoughby case, the homeowners couldn’t walk on their own bank without creating an avalanche. “One time a woman tried, and when it started sliding, she had to jump into the lake,” Winfield says.

To remediate the bank, HillCrafters began by rough-grading the 20-degree slope, using a crane with a 100-foot boom to reach out over the debris field. “We did all the work above the high-water datum, so we did not require a permit,” Winfield says. “A good, stable wall cannot exist without a good foundation, so we dug a hole 4 feet wide and 100 feet across and drove a sheet pile wall down another 10 feet below the high-water line. We used this as our concrete form. Then we placed a whaler beam behind it with a soldier beam attached to the whaler every 10 feet. All of this was bolted to galvanized sheet pile to create a footer. The wall can’t sink.”

In addition, the crew attached aircraft cable to the rebar within the footer in order to “cable it down” and provide temporary support and stabilization while building the wall. “We were above the high-water datum, but waves were still slapping against us while we worked,” Winfield recalls.

The bottom module of the Stone Strong System was poured with concrete and designed with weep holes for self-draining. “If there is excess water in the tiles, it never drains directly to the lake but instead goes east and west as we designed it. It doesn’t add water to the back of the wall,” Winfield says.

Standard backfill was used behind the wall. “It’s a sealed wall. There is geotextile behind it and geogrid with a formal tieback system. Probably no one accounts for the overflow of the lake, but when the water bridges a module, it can spray upwards 100 feet and that is an inherent problem. Stone Strong’s surface diffuses the water, so it doesn’t shoot straight up.”

Winfield says he usually builds a wall 6 feet high, or two rows of Stone Strong blocks, and pours a 10-foot-wide cap, tying it back with a tieback structure built on 10-foot centers. Ten feet behind the face of Stone Strong blocks is another group of driven H-beams, driven as deadmen and tied with a steel angle iron in a wagon-wheel system of 3-inch angle irons. “I leave the beams 2 to 3 feet above the Stone Strong blocks, and this becomes the secondary wall,” Winfield explains. “When I’m done setting the blocks and tiebacks in place, I pack dirt into the hill. That locks the bottom beams into the cast concrete slab. We build the wall 8 feet above the water level so that any water that bridges it washes back on the concrete and therefore can’t wash into the soil. It hits the secondary wall and is returned back to the sea, which helps stop the flanking and washout from overspraying.”

Winfield says Stone Strong provides computer-drawn plans, showing corners and returns for sculpting the wall. The blocks can be handled with basic equipment, such as holding a 7,000-pound block from a crane. “And you can create the look of a single wall face from separate blocks,” he notes. “Once you concrete them together, they become one.”

But the most important factor, to Winfield, is the footer. “They just built a hospital near here using Stone Strong blocks. It sits on limestone. But if you did that on the lake, it wouldn’t last. It’s like tree roots that go far into the earth: When you’re working on clay, it’s all about the foundation.”

The homeowners with the once-disappearing backyard are delighted, Winfield says. “We did the job three years ago, and since then there have been 12-foot waves, which on Lake Erie have the force of 30-foot ocean waves. They can take out whole piers. But we’ve gone through three winters and there hasn’t been so much as a crack in that wall. I don’t think anything will move it. Winters produce a glacial situation here. We get a mountain of ice moving at a quarter mile an hour and extruding ice 30 feet high. The wall has taken all of that and hasn’t moved even a wiggle.”

Why Maneuver Mismatched Stones
When You’ve Got Legos?
Josh Peterson is field superintendent and project manager for Villa Landscapes, the design/build division of Patio Town, a family owned business in the Minneapolis-Saint Paul area since 1965. A member of the team since 2004, he oversees projects, traveling to various sites with the designers. In October 2010, Josh oversaw the installation of a two-tiered retaining wall for a Minneapolis homeowner.

The current wall standing on the residential corner lot was composed of two tiers of 150 linear feet each and had been constructed of railroad ties. “The soil had settled, and the wall was on borrowed time,” Peterson recalls. “It had pushed out, tipped forward, and eventually would have succumbed to gravity.”

When the installation crew went to demolish the wooden wall, they were surprised to find the remains of a concrete wall behind it. “It was buried in one corner,” Peterson says. “We jackhammered it out. Fortunately we had a skid-steer loader mounted with a jackhammer that we had onsite for another part of the project.”

Fortuitously, the weather cooperated and the job went quickly, taking the crew only a month to complete. Once they had taken out the old walls, they excavated for base material, using a skid-steer loader to install three-quarter-inch and smaller crushed limestone, which they laid in with a small vibratory plate compacter in 3-inch increments. “The industry standard is to use 1 inch of washed sand for leveling purposes,” Peterson says. “But because we had access to a laser level and grade shakes, we did not need the leveling medium. Sand is fluid and mobile, and the laser method is more efficient and more accurate. A laser level can cost up to $1,000, but you will get more accurate elevations.”

To build the walls, the crew used Versa-Lok’s Retaining Wall System. Each of the two walls was 4 feet high. At that height, the lower wall did not need soil reinforcement. “So that it wouldn’t add a load to the lower wall, the second tier was placed 8 feet back in order to be out of the failure plane or shear angle of soil,” Peterson says. “The sloped grade at the top of the second tier added a load to that wall, so geogrid was used for soil reinforcement.”

The house on the property was 12 feet above the street at the highest level, and the deteriorating wall was only 4 feet from the pedestrian sidewalk. To build the new wall, a combination of Versa-Lok’s solid, segmental units fit the job nicely. “We were able to get columns, steps, walls, and a double-faced wall all from the same product,” Peterson says.

Because Versa-Lok is a manufactured product composed of Portland cement, aggregate, and iron oxide pigment, the result can be a natural look with the efficiency of an engineered product. “Manufacturers can vary the color of the product for their local market,” Peterson says. “People typically want colors that complement features of the environment.”

For this particular project, the lower wall was built with Versa-Lok’s weathered Mosaic. “It has a random look but with rhythm and form,” Peterson says. “The randomness breaks it up, and the aged look matches the older bricks of the couple’s home. It gives it history.”

For the second tier, a mix of Standard and Cobble units was chosen. “It was cheaper than the mosaic,” Peterson notes, “and eventually plantings will cover much of that part of the wall.”

Ten columns and 20 steps were added to the mix, breaking up the slope and adding aesthetic qualities much admired by passersby. “The crew was interrupted 10 times a day,” Peterson says. “People were telling us, “˜It looks fantastic,’ and asking us how they could do the same thing.”

Peterson is most impressed with the ease of Versa-Lok’s application. “It makes a wall easy to build-like using Legos instead of trying to fit together a bunch of rocks. The pin attachment is quick and well ordered. And in a cold climate, you can put the wall on a gravel base and not worry about putting in a frost footing as you would with a rigid system.”

New Design Gives Bigger Bang for the Buck
About 5 miles east of Tacoma is the fast-growing town of Puyallup, WA, so named for the Native American tribe who called themselves “the generous people.” In May 2010, Sound Retaining Walls of Puyallup constructed four retaining walls for an apartment complex to be built on Linden Lane.

Cory Bales, co-owner of the company, says the purpose of the month-long project was to maximize space on the property, which abuts residences and a golf course in the fairly flat area. During that month, the weather cooperated and the only real challenge for the company was the owner’s desire that cost be the main priority in constructing the four walls, three of which were onsite with the fourth being part of a project to widen an access road.

“We were contacted by RPD [Rickabaugh Pentecost Development Co.] to work with them,” Bales says. “We gave them a budget of approximately $135,000, which included design, engineering, materials, and installation.”

Sound Retaining Walls then contacted Jim Hammer of WestBlock Systems in Tacoma. “We’ve had a long relationship with that company and product,” Bales says. “It’s a local company. Jim lives just down the street and has been a supporter of our ventures throughout the years.”

Hammer had been talking for a while about a new product, GravityStone Edge, which his company had produced but not yet used in Washington. “I took a trip to Spokane, which is on the other side of the mountains from us,” Bales says. “I went to White Block Co., a concrete block manufacturer there. Jim had shipped a mold of GravityStone Edge to them, and I wanted to see the product physically. I decided it would be the favored product for the Linden Lane project.”

GravityStone Edge is unusual, Bales says, because it has a 1.33-square-foot face area as opposed to the standard 1-square-foot face. “There is a twofold advantage: It cuts cost because there is more square foot per block, and you can install one-third more of the product for every block. It enabled us to give the customer a better price. We install the walls by hand and labor adds up.”

RPD contracted all of the mass excavation and wall prep in four days, Bales says. “Then we came in and put in a 6-inch leveling pad of five-eighths-minus gravel, 6 inches in depth by two-and-a-half feet wide, compacted at 95%. We used a laser to level the pad. Then a geotech came in and tested the soils for compaction.”

The GravityStone Edge blocks were brought in 18 pallets at a time on a semi, with 40 blocks to the pallet. A skid-steer loader with pallet forks brought the 85-pound blocks as close as possible to the wall sites. “We got them as near as we could; they’re heavy,” Bales notes, adding that the crew did all the work manually.

“We set up a string line with stakes at either end of the area, and a crew of four set the blocks, using dead-blow hammers and a 2-foot level.” The labor hours varied by wall, but Bales estimated about 100 square feet of wall per person each day.

The GravityStone Edge blocks used in this project had a concrete nub system interlocking the blocks, Bales says. “The top of the bottom block has two concrete nubs, and there are holes in the bottom of the next block that fit over the nubs. For walls over 4 feet high, we used geogrid fabric to tie them together.”

Bales said the customer was satisfied with the choice of block. “We got the price point where it needed to be, in addition to ease of installation and durability.”
About the Author

Mary Ellen Hare

Mary Ellen Hare is a frequent contributor to Forester Media publications.