Of Beauty and Durability: Retaining Walls in the Urban Infrastructure

Sept. 1, 1999

For thousands of years, humans have retained earth with fiber-reinforced soils and stackable blocks. But for decades, modern engineers relegated gravity block walls to residential backyard landscaping in favor of cantilevered, or steel-anchored, concrete retaining walls.

Then about 30 years ago, Henri Vidal made a discovery while building sandcastles on vacation in France: He found that by layering pine needles in the sand, he could build small, vertical sand walls. Vidal developed and, in 1969, patented his soil-reinforcement methodology, effectively locking up the technology for 17 years. But in 1972, the first soil-reinforced retaining wall in the United States was built over a landslide in the foothills of Los Angeles. When Vidal’s patent expired in 1986, the door to the commercial application of soil reinforcement swung open. There were few early supporters, but the technology eventually caught on, ushering in a new age of geosynthetically stabilized slopes faced with segmental retaining walls (SRWs). Of course, not everyone embraced the revived methodology. Regulated environments, particularly transportation and other governmental agencies, were slow to incorporate SRW processes into their specifications. Nevertheless, SRW systems continued to grow both in volume and scope of application. SRWs are now the leading type of earth retention system used in urban infrastructures.

To Cantilever or Dry Stack: Is That the Question?

Retaining walls fall into one of two categories—gravity or reinforced—and can be constructed of concrete, masonry, steel, or lumber. The most commonly used retaining walls today are steel-reinforced concrete cantilever walls and geosynthetically reinforced segmental block retaining walls. “The easiest way to build a retaining wall is to build-up flat concrete and then backfill behind it or lay it up with mortar. However, you can’t go much over 3 or 4 feet with that kind of wall, because at a greater height, it wouldn’t be able to withstand the lateral load from the soil. To build a higher flat-concrete wall, you need to reinforce it with steel bars and grout, creating a cantilever, which gives the structure integrity,” explains Kevin Callahan, an engineering consultant with the National Concrete Masonry Association (NCMA) in Herndon, VA.

When SRWs exceed 3 or 4 ft. in height, the soil must be reinforced with geosynthetic grids or textiles to make it stable enough to withstand the load. Most segmental block systems range from 3 to 20 ft. high. However, SRWs have been built to heights in excess of 50 ft. “The traditional cantilever poured-in-place technology works fine. But once you get up to 5 or 6 feet, it usually becomes uneconomical because of associated increases in materials and labor costs,” says James Collin, Ph.D., president of the Collin Group in Bethesda, MD, a geotechnical consulting firm specializing in reinforced soil technology. “Segmental gravity systems provide more design and construction flexibility and look better. However, economics is the driving force behind the wide use of segmental retaining walls.”

Determining the best type of retaining wall for a particular application or a specific site depends on myriad factors: soil composition, accessibility, backfill height, degree of slope, aesthetics, lateral pressure, internal and external loads, ground water and surface water, climate, costs, urgency, and regulatory specifications. All of these factors are taken into consideration when designing and constructing a retaining wall.

Sometimes, the decision depends primarily on the standard in effect for the respective region, application, or regulator. According to Ryan Berg, P.E., president of Ryan R. Berg & Associates, geotechnical consultants in Woodbury, MN, the process employed to build retaining walls for private development differs considerably from that used in public projects, such as widening a road. For institutional retaining walls, such as those for highway embankments and bridge abutments, established standards specify which processes and materials can be used in a particular application. The US Department of Transportation (DOT), for example, follows a 155-page guideline for erecting highway retaining walls.

Exceptions are sometimes made. When a government inspector discovered that a recently constructed weather radar tower sat directly below a potential landslide, Robert K. Barrett, a geologist with Yenter Companies in Arvada, CO, was charged with finding a solution fast. With the monsoon season just a month away, the US Department of Commerce, which oversaw the project, determined that traditional tieback anchors would cost about $235,000 and that it would take at least two weeks to mobilize the needed equipment. “The forestry department had instructed the contractor to cut a vertical ridgeline to minimize damage to plants in the pristine rain forest surrounding the tower. Everyone, including the inspector and I, knew that as soon as the rains started, the whole mountainside was going to flow down onto this radar network,” Barrett recalls.

Barrett stabilized the soil with Amoco woven fabric, layering the geosynthetic with granular soil every 8 in. He then constructed a 16-ft. facing with standard concrete block. The $64,000 project was completed in less than three weeks. Six months later, Hurricane Hortense remained stationary for three days over the wall, which remained intact.

Because of their reasonable price, design flexibility, and visual appeal, SRWs are frequently used in residential development. In Washington, DC, Allan Blocks were used to transform an ungainly 70-ft. rise to a cliff-top lot into a gorgeous, terraced landscape. Allan Blocks were also used to create a driveway from the road to a house built on the site, which had been deemed too steep for development. Constructed in 38 days, the geogrid-reinforced segmental walls vary in height from 2 to 14 ft. Allan Block retaining walls were also installed to expand the parking lot of a grocery store in British Columbia. During the 11-day installation, 12,000 Allan Block masonry units and 3,300 square meters of geogrid were used to build two terraced walls supporting the parking lot and a third wall providing roadway access to the lot. The owner saved almost 15% in labor and materials costs by selecting the Allan Block SRW over the alternative system.

Unexpected factors can also impact costs. Larry Falls, president of Associated Construction Products Inc. (ACP) in Tampa, FL, cites a recent SRW project in Atlanta, GA, in which site conditions unexpectedly drove up costs. ACP constructed a modular wall reinforced with a geogrid to make land space for an apartment complex. “The site conditions were awful: too much clay and too much rain. We had to remove the poor soil and replace it with more suitable fill, all the while battling pouring rain. As a result, the project took longer and cost more,” Falls remarks.

Sometimes the developer has no choice but to build a more expensive structure, simply because it is mandated by a regulatory agency. “In the City of Los Angeles, developers don’t bother specifying polymer-reinforced retaining structures, even though they’re often less expensive, more attractive, and more durable than the ‘authorized’ methods. The city [currently] prohibits the use of segmental retaining walls,” explains engineer Dean Sandri, western regional sales manager for TC Mirafi, providers of geo-synthetic materials located in Pendergrass, GA.

Shoring Up to Build Out

From the 1950s until the late 1970s, most urban development in America focused on readily available, easily accessible, ready-to-build land. As urban sprawl pushed outward and associated costs pushed upward, developers began to reexamine property that was once considered unusable. The land reclamation quest surged during the 1980s and has continued throughout the 1990s. The desire and need to squeeze more usable land out of existing parcels and to make unsuitable land suitable for building helped invigorate the use of retaining walls in commercial, industrial, residential, and even institutional development.

Retaining wall applications increased considerably after the mid-1980s, when geosynthetic materials, such as geogrids and geotextiles, were coupled with segmental masonry blocks. SRW systems comprise three components: soil, geosynthetic reinforcement, and block facing units. Improvements in the composition and quality of the cement and masonry have played a role in the increased use of modular blocks. “From outward appearances, the facing units, or blocks, don’t look like they’ve changed much, except for variations in size and shape,” says Bruce Baumann, P.E., of Anchor Wall Systems in Minnetonka, MN. “But the physical properties of the cement blocks have improved considerably.” For example, blocks are now treated with chemical additives to enhance freeze-thaw durability and prevent effervescence, a naturally occurring white deposit caused by the migration of soluble salts to the surface of the block.

“This might not pose a problem for plain old concrete, but when people pay for pigmented blocks, they want them to retain their color,” reasons Brent Rymon, director of technical assistance for R&M Chemical Technologies in Washington, NJ, which provides chemical additives to block and paver manufacturers. Guidelines for designing and building SRW systems, published by trade organizations such as NCMA and the Geotechnical Research Institute (GRI), have also helped to ensure the consistent integrity of these retaining walls. “A good deal of the growth in segmental retaining walls is from the design community’s increased comfort level with what had long been perceived as a relatively new technology, even though reinforced soil embankments have been around for a long time,” Baumann says.

In recent years, SRW systems have surpassed the previously predominant cantilever concrete systems. The SRW segment of the industry continues to grow by about 20% a year in relative volume. “Reinforced concrete walls are still and always will be used. Certainly many of the more conservative government agencies—which are driven less by cost than by technology—often feel more comfortable with the cantilever concrete walls,” Baumann says.

Many such agencies, however, including the Colorado, New York, and other state DOTs, now use both concrete cantilever walls and SRWs. Retaining Wall Systems Inc. of Englewood, CO, installed a 30,000-ft. 2 SRW, with walls reaching 20 ft. tall, along a highway embankment in Aspen, CO. It was the first time the Colorado DOT used a geotextile rather than a geogrid soil reinforcement. Cost savings of more than 50% can be realized by using geotextiles. However, geotextiles do not lend themselves to every site condition. Says Winkle, “In projects dealing with a lot of surface or subsurface water, we use either a geogrid or a criblock system.”

Ed Shaback, a technical advisor for Allan Block Corporation in Edina, MN, provider of SRW systems, cites other conditions that might lend themselves to a more traditional cement wall. “Although modular systems are usually more economical and flexible, certain site constraints, such as seawalls or walls in close proximity to property lines, can make it more practical to use a cantilever wall.”

Retaining Wall Systems recently built a criblock retaining wall to stabilize the eroding embankment of a creek that ran below a housing development. Although the creek was not yet endangering homes or lives, it had moved relatively close to property lines. Unchecked, the erosion would have continued and eventually could have posed a safety problem. Because of its soil-binding properties, the 12-ft.-high criblock wall lining the creekbank required no geosynthetic soil reinforcement. “We’ve had criblock walls go up to 60 feet tall at commercial and industrial sites where we had extremely high embankments of soil to retain,” recalls Winkle.

Cribwalls are multiple-bin gravity systems of interlocking, precast, hollow concrete units filled with gravel. The depth at which the wall is sunk into the ground varies according to the height and bearing loads of the wall. The higher the wall and the greater the load bearing, the greater the depth (or the number of bins). The anchoring effect of the multiple bins, coupled with the gravel-filled blocks’ capacity to bond with the soil, typically negates the need for substantial geosynthetic reinforcement. Cribwalls tend to look fairly industrial as well. In certain climates, however, you can fill the wall with dirt and plant the face with vines or other natural vegetation to enhance the aesthetic value of the slope. After a while, the vegetation completely covers the concrete.

Cohesion Equals Stability

Shaback recognizes both advantages and challenges in the expanded use of retaining walls. “We see walls being used to recover lands that were previously unusable. Some of these retaining walls are going up under increasingly adverse site conditions, where the soil isn’t dense enough or water creeps in behind the wall. When retaining walls fail, water management and/or poorly compacted soil are often the culprits.” In terms of durability, concrete cantilever and segmental block systems perform equally well over the long haul, provided they are designed and built properly. Therein lies the challenge—and the single greatest cause of failure in retaining walls. In some regions, freeze-thaw degradation in segmental blocks can pose problems, but improvements in the quality of SRW blocks and the development of retaining wall specifications have greatly reduced the risk of freeze-thaw failures. In winter zones, salt spray from snowmelt can also cause some deterioration in blocks and masonry. The composition of the cement (including the quality of aggregates and beneficial chemical additives) can improve the performance of the material, for example, by increasing the density of the block. Another potential cause of failure occurs during construction—under the wheels of heavy equipment. A sharp-turning bulldozer can severely damage a geotextile, for example. By the same token, an improperly graded and compacted backfill can cause a wall to settle.

Winkle recommends testing backfills as you build the wall. “We build in fairly small lifts, usually backfilling no more than 8 inches of soil at a time. Then we test the soil density before we add another lift to ensure we’ve attained enough compaction.” Excess groundwater and runoff can wreak all kinds of havoc on retaining walls and are reportedly the leading cause of wall failure, but these factors are usually preventable. Most segmental and criblock blocks are designed to allow water drainage and relieve hydrostatic pressure. However, the soil must also be properly compacted, grated, and drained.

A tri-level retaining wall before vegetation is established to create one continuous slope.

Two walls—one before planting (foreground) and one after planting (background).

Stan Johnson, co-owner of J&B Concrete Construction in Tacoma, WA, struggled with a water problem when he was subcontracted to build a retaining wall for a parking lot of an Alzheimer’s center in nearby Kent. The clinic was built on the top of the property, which descended at a considerable angle to the parking lot below. The contractor built the slope and installed geosynthetic soil-retention material. J&B then came in and installed a poured-in-place concrete retaining wall with an 8- x 18-ft. steel footing and a 14- x 240-ft. concrete slab. Johnson used snap ties to taper the thickness of the wall from 12 in. at the base to 6 in. at the top, with one edge plum and the backside canted. “We had a serious seepage problem throughout construction and constantly had to pump out water,” Johnson explains. “The grade extended beyond the wall and the parking lot to a retention pond and wetlands at the bottom of the slope. So water was always going to be a problem.” The contractor later had to put in drains behind and on either side of the wall to correct the runoff problem. “Contractors sometimes channel water to a localized area right behind a retaining wall. That’s trouble waiting to happen,” Winkle points out. Many of these types of problems could be eliminated if the general contractor works in tandem with the wall designer and builder to address erosion and water drainage issues upfront. “Modular wall systems are so strong and have so many built-in safety factors that some people become complacent and deviate from the design plans,” says James Collin. “When that happens, the wall might perform well for a period of time, but then a storm comes, water gets behind the wall, and failures happen. That’s why it’s important to hire the right designer and contractor.” Winkle concurs: “We tend to see more installation than design failures. When you do see design failures, often it’s because the contractor doesn’t have adequate experience and works without a design. They just put up a significant wall with poor soils and no reinforcement, which is a recipe for failure. Inexperienced wall builders mistakenly think the blocks retain the earth when, in fact, the geosynthetic material stabilizes the soil, with the blocks functioning primarily as a veneer.”

Too often retaining walls are treated as a minor feature in the construction plan, and the wall designer is separated from the rest of the team. Ryan Berg believes that to minimize the risk of failure, the retaining wall designer should be part of the overall project team. Sandri agrees: “One of the biggest problems in the retaining wall industry is that there is little continuity between the guy who makes the geotechnical recommendations, the gal who designs the wall, the guy who builds it, and the person who inspects it.” If one of those key players is operating off a different page, it can spell disaster.

Back to the Future

Masonry and block materials are constantly being improved and new ones introduced to add quality, functionality, and fun to SRW construction. New and broader applications also continue to appear. When a drainage canal was rerouted through a residential development in Jacksonville, FL, ACP used geogrid to reinforce the embankment and constructed a 6-ft.-high SRW, using Anchor Vertica Pro units, along the 1,200-ft. stretch of canal that was moved. Over the past two years, ACP also has used SRWs to construct several head walls for bridges in Florida, Georgia, and Texas. These structures feature arched-block culverts, similar to those used in Roman aqueducts. “We built the arched culverts by dry-stacking blocks on top of one another to form a ring, then we kept building rings until we’d created the arch,” Falls explains. As retaining walls have become more specialized, the technology has greatly expanded the amount of buildable property as well as the functionality and beauty of the walls themselves. One of the most outstanding retaining wall systems in the country—and perhaps the wave of the future—is a unique turn-key system developed by Jan Erik Jansson, president of Soil Retention Systems (SRS) in Oceanside, CA. For the past several years, the SRS team has been engineering and building a network of plantable SRWs to support the development of residential and commercial properties on the hilly coastal headlands of Newport Beach, CA. The Irvine Company, one of the largest and most prestigious developers in southern California, is developing this prime real estate, where homes costing $5 million or more snuggle into hillsides with million-dollar views. The cost of grading four lots in one recent project reached $20 million. “Jansson has built several hundred thousand square feet of retaining walls at Irvine Ranch-up to 50 feet high, all of them planted-and they continue to go up year after year,” says Sandri.

Not only must Jansson’s SRW system meet southern California’s strict retaining wall standards, it must also meet the regulatory requirements of the Orange County Environmental Management Agency and the Irvine Company’s stringent quality standards. “On the southern California coast, where there is little raw land for development, sites located high on hills and in steep canyons provide better views and more privacy and, therefore, demand more money and appreciate faster. But to make the land usable, you have to develop it in such a way that it maximizes land use, adds commercial value, and protects wildlife while preserving the land’s aesthetic value,” Jansson explains. Most developers would cut into the hillside, create a flat surface, build a retaining wall, put up a house—and the homeowner gets to stare out at a big, ugly concrete retaining wall. The Irvine Company wanted to give property owners more natural and attractive slopes in their backyards. Jansson’s Verdura system meets those critical requirements and then some. With Jansson’s unusual SRW methodology, gradual slopes are created directly above and directly below the oversteepened SRW, which is battered and reinforced with geogrids. All of the slopes, including the SRW midsection, are vegetated, blending into one continuous, natural-looking slope. Because the upper and lower slopes are more gradual, they typically require no geosynthetic reinforcement. “With Jansson’s walls, when a homeowner walks to the edge of his yard, he sees a long, vegetated slope and a beautiful view, rather than a drop-off concrete wall,” Sandri points out. Is anything compromised?

Certainly. It would be cheaper to cut a lot and build a vertical retaining wall and then build another lot and another vertical retaining wall, creating a stair-step pattern. “But when you’re asking a million dollars a pop for a lot, you don’t want to pack people on top of one another like that. You want to create a livable, visually attractive habitat,” Sandri asserts. The Verdura system is a geosynthetic-reinforced SRW that is battered (angled back) at about 20? from vertical (4V:1H). Each uniquely shaped, interlocking Verdura block features a base (lip) and a planting pocket. As each course of blocks is stacked, the lip of each block straddles the two blocks below it, stepping it back slightly from the one beneath it. This gradual battering, coupled with the geogrid reinforcement and the vegetated pockets, produces an exceptionally strong, flexible, and attractive wall. The open nature of the block prevents the possibility of hydrostatic pressure building from trapped water.

Conventional retaining walls, on the other hand, must be fitted with extensive drainage systems to prevent hydrostatic pressure and excessive salt deposition on the wall face. Because the wall fa?ade is 100% plantable, a wall planted with natural vegetation soon blends into the slope, completely concealing the structure—and graffiti-proofing the wall. Another major advantage of the Verdura system is that it can support heavy equipment directly behind the wall—during construction, which is impossible with cantilevered systems and even with other SRW systems. “With cantilever systems, you build a wall with concrete footings and blocks, then you wait two to four weeks for the concrete to cure before you can backfill,” Jansson explains. “We construct our walls in such a way that an earthmover with a load of dirt can come in behind the wall and dump it to fill the wall. Then a grader can come in right behind it to distribute the soil evenly.

Then compactors come in behind that to compress the earth behind the wall. The only hand-compression required is for the soil 2 feet behind the back of the block. So we can move more earth more quickly than with most other retaining wall systems.” SRS is also unique from other retaining wall contractors and providers in that the company does it all. Jansson developed the Verdura blocks; the company manufactures and licenses the blocks. The company designs and installs the complete retaining systems, including completing permit drawings, heavy-equipment work, installation, and planting—all very different from what other retaining wall businesses are doing.

During the 12 years that SRS has been in business, the company has installed close to 2 million square feet of retaining walls. To date, there have been no failures. States Sandri, “What’s amazing is that Jansson is a small, albeit high-end, builder who is doing this single-handedly in one small part of the country. It’s absolutely fabulous!”
About the Author

Colleen Sell

Colleen Sell is a writer and sustainable-living advocate residing in Oregon.