Applications and options for segmental retaining walls.
For decades, commercial site developments have commonly constructed retaining walls using poured-in-place reinforced concrete. Since the mid-1980s, a new approach has been maturing that allows attractive retaining walls to be built faster and for less than half the cost–the mortarless reinforced-soil segmental concrete-block wall. |
| Segmental retaining walls offer flexibility for smaller residential applications. |
Today commercial and institutional site developers find themselves installing more retaining walls than in the past. In many metropolitan regions, the choicest land has now been developed. What’s left is often more hilly, meaning it is more likely that retaining walls will have to be constructed for efficient site use.
Developers seek to maximize the productive use of a piece of commercial real estate. Rather than having the terrain gradually slope up to the border of a property (hilly terrain is not useful for parking), a developer can make better use of a site by grading most of it so it is nearly flat. This practice results in a sudden increase in elevation at the property boundaries and the need for a retaining wall.
Options for Building Retaining Walls
Scott "Sam" Miller, a Little Rock, AR—based civil engineering consultant who has been designing retaining walls for 20 years, observes that until the mid-1980s, there were traditionally several main options for constructing retaining walls on commercial and institutional sites. These were poured-in-place reinforced concrete, stone, conventional mortared concrete block, H-pile walls with wood lagging, and retaining walls constructed of new or used timber railroad ties. By far the most common choice for a retaining wall both in the past and at present, notes Miller, has been the poured-in-place reinforced concrete wall. But owners are increasingly opting for the so-called segmental concrete-block approach.
Anchor Wall Systems’ manager of engineering services, Don Armstrong, claims that cost considerations are a big factor in choosing reinforced-soil segmental walls instead of poured-in-place concrete retaining walls; reinforced-soil segmental wall can be constructed for one-third to one-half the cost of the poured-in-place wall.
Miller estimates that because of cost and aesthetic advantages, use of earth-reinforced segmental retaining walls has grown 20-30% per year over the past several years in commercial and institutional applications. The segmental-wall industry is still in its infancy, only about 15 years old, but Miller expects to this trend continue.
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| A well-compacted surface, rather than a more expensive poured footing, often provides an adequate foundation. |
In some areas such as Minnesota, stone retaining walls are quite common because of the ready availability of stone.
Miller notes that in regions where good field stone is available, a mortared stone retaining wall no higher than 5-10 ft. can be more economical than a segmental concrete-block wall. A 20-ft.-high stone wall, however, would require a wide expanse of stones at the base, making it less economical than the segmental-wall alternative. And in many regions of the United States, the dearth of suitable stone means that stone retaining walls are rare.
Another option, the timber retaining wall, is substantially less expensive than a reinforced-soil segmental retaining wall–an installed cost, Miller estimates, of $7-$10/ft.2 of wall area for the timber wall versus $15-$25/ft.2 for the reinforced-soil segmental-block wall. One problem with the timber-tie approach is that some jurisdictions ban the use of timber ties that have been treated with creosote because of potential water-pollution problems. Another problem, Miller says, is their comparatively short life: They begin to rot out after 10-20 years. By contrast, a reinforced-soil segmental concrete-block wall has a lifetime of at least 75 years.
A Brief History of Reinforced-Soil Structures
Reinforced-soil structures were first used by the Egyptians in ancient times, and later by the Chinese. The Egyptians used reeds from papyrus plants to reinforce soil, and the Chinese used bamboo to reinforce soil in constructing parts of the Great Wall.
During the 1970s and early 1980s, The Reinforced Earth Company of Vienna, VA, improved upon this ancient technology, developing a method of constructing retaining walls using precast concrete panels. The panels were anchored in position using straps of metal (with protuberances on them to "bite" into the soil) that extended horizontally into the soil behind the wall. The Reinforced Earth was the first company in the US to use soil reinforcement for constructing retaining walls.
By far the greatest advantage of The Reinforced Earth’s approach, Miller says, was a dramatic cost savings over the traditional cast-in-place reinforced concrete wall–about one-half to one-third the cost. Because of this, reinforced-earth panel walls have been used extensively in highway applications over the past few decades.
Why is this approach so much cheaper that a cast-in-place wall? Miller highlights several reasons: (1) much less concrete is used in the concrete panels versus the cast-in-place wall (for a traditional wall, the concrete is 50% of the overall cost); (2) there is no need for reinforcing steel; (3) there is far less labor involved; (4) the labor involved in erecting the reinforced-earth panels is far less expensive–no need for pricey carpenters to construct and remove formwork or for iron workers to place reinforcing steel; and (5) the wall can be built much more quickly–no need to wait seven days for concrete curing, then return to the job site to strip off formwork and backfill.
Nonetheless, Miller continues, there were certain logistical issues involved in using this type of concrete panels, which typically measure about 4 x 4 ft. or 6 x 6 ft. and 3-6 in. thick and weigh several hundred pounds. First, few plants in the US manufacture them, so the manufacturing site is likely to be hundreds of miles from the project site, thereby boosting transportation costs. Second, the panels must be transported on a flatbed truck. Third, a crane must be used to load and unload the panels from the truck and place them at the construction site. Such issues, Miller maintains, usually restrict the use of such panels to large retaining wall projects for highway cuts and bridges rather than industrial parks, shopping centers, and other commercial sites.
Mortarless Interlocking Concrete-Block Walls
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| Once in place, hollow-core blocks are filled with aggregate. |
The limitations of reinforced-concrete panels for commercial-type applications are one reason for the emergence of a new retaining wall technology in the mid-1980s–the so-called segmental retaining wall. These are walls made from concrete blocks designed to mechanically interlock with adjacent blocks, eliminating the need for a mason to mortar the blocks together. A typical block is about 6-10 in. high, 16-18 in. wide, and 10-24 in. deep and weighs around 68-120 lb., depending on its depth. Segmental walls more than 3 or 4 ft. high also make use of soil reinforcement using a geogrid or geotextile fabric.
Robert MacDonald of Keystone Retaining Wall Systems in Minneapolis, MN, recalls that the origin of segmental walls goes back to a Minneapolis landscape retaining wall contractor by the name of Paul Forsberg. In the mid-1980s, Forsberg was constructing both commercial and residential retaining walls using rounded boulders. Such stone was readily available in Minnesota, part of the remains left by receding glaciers 10,000 years ago. But such stone is generally rounded rather than angular, making it difficult and a real art to place it in the wall in a way that will form a stable configuration without mortar. Forsberg often had to come to the aid of his crews to place the stone himself. A wide variation in stone sizes and shapes was one reason building such walls was so time consuming and so delicate a task. Further, these stone walls could not be built more than 3 or 4 ft. high.
One day Forsberg asked himself: Why not make the task of building these retaining walls more regular and predictable? Why not make standardized "stones"–concrete blocks that would be readily stackable, have the same standardized shape, readily interlock with one another without mortar, be attractive, and come in a variety of textures and colors? He designed a standardized concrete unit and went on to found Keystone, and his design remains the company’s standard block. At 8 in. high x 18 in. wide, the unit is 22 in. deep to give stability to a nonreinforced-soil wall. The unit weighs 130 lb., in effect mimicking the massiveness of a boulder, while still being light enough for one person to lift into position. The rear of the block has "ears" that protrude laterally, ensuring it remains anchored to the embankment once backfill is placed.
Soil Reinforcement
Such a no-mortar segmental retaining wall was fine at heights up to 4 ft. But to enable construction of higher walls, Keystone developed a system of soil reinforcement based on the general idea used in The Reinforced Earth’s retaining wall system. Soil-reinforced walls (also called mechanically stabilized earth walls) can safely be built to greater heights than walls relying on gravity alone.
One of the disadvantages of using metal strips for soil reinforcement, points out MacDonald, is that they are susceptible to corrosion. To reduce that corrosion, water in the backfilled soil behind the retaining wall must be held to a minimum. This usually involves the costly importing of appropriate backfilling materials–sand or gravel (which readily allow the water to drain out of the soil through the unmortared joints between adjacent concrete blocks) rather than silts or clays (which inhibit the free drainage of the backfilled area).
To avoid the substantial expense of having to import appropriate backfill materials, Keystone searched for something other than metal strips, something that would not be susceptible to corrosion by high-water-content soils.
Specifically, for soil reinforcement of its segmental wall system, Keystone decided to use a geogrid system developed in Japan in the late 1980s. In the Keystone system, the geogrid is anchored securely to the segmental wall using the same dowels that hold adjacent concrete blocks in position. Some other systems hold the geogrid in position by inserting the end of the geogrid between two lifts of concrete blocks and relying on the weight of the upper lift to hold the geogrid in position.
The geogrid is usually fabricated from either a polyester or a high-density polyethylene. Since polyester is susceptible to degradation from gasoline, other chemicals, low-pH soils, and microorganisms, the polyester base is usually coated with chemically resistant PVC or high-density polyethylene. By using a corrosion- and microorganism-resistant geogrid, Miller notes, the designer no longer needs to be so fussy about the corrosive character of the backfill material and can often use backfill materials readily available on the construction site without worrying about the degradation of the soil-reinforcement material.
How long will a typical geogrid last? The design life is 75 years, a number based on accelerated chemical and durability tests. Miller believes this soil reinforcement will last much longer.
Evolution of Concrete-Block Walls
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| Lightweight blocks can be placed by hand. |
In the past, Miller explains, it was commonplace for contractors to construct retaining walls not only of poured-in-place reinforced concrete, but also of conventional concrete blocks, mortaring one course on top of the previously placed course. In constructing such walls, rebars are usually placed vertically down through the holes in the block and then mortared into the holes, a procedure that gives the wall some tensile strength.
Yet such walls were sensitive to soil movement, frequently settling and developing stair-step cracking. Aesthetically, they left much to be desired. And labor costs were high: Masons had to be employed to mortar the blocks together, and steel workers were needed to place the reinforcing bars.
As Miller sees it, developing a way to link concrete blocks together without using mortar was a major innovation. The system was quite simple, enough so that a single low-skilled worker could construct such a retaining wall, lifting a block (weighing anywhere from 60 to 130 lb.) into position unaided. It was merely a matter of placing one block on top of the previously laid course of blocks, then inserting a sturdy pin (dowel) into prefabricated holes on the top and bottom surfaces of interfacing blocks. These segmental concrete blocks could be purchased from a local concrete-block manufacturer, keeping transportation costs to a minimum.
These innovations back in the mid-1980s, Miller recalls, caused the use of such segmental concrete blocks to take off, especially for constructing landscaping gravity walls (typically up to 4 ft. high, with no soil reinforcement) in residential yards, office and industrial parks, shopping centers, and other commercial and institutional settings. In most cases there is no need to pour costly footings to provide a foundation on which to place the concrete blocks: a compacted gravel bed, Miller says, is sufficient.
According to Miller, all segmental blocks are made of concrete rather than stone or other material. Further, as the 1980s wore on, segmental-block manufacturers began to make the blocks available in several textures and colors. As mentioned, some block systems use pins to link adjacent blocks together, while other manufacturers rely on a lip or on the weight of the blocks themselves to hold each block in position.
Gravity walls are often designed and built by a local subcontractor. A typical wall-construction crew, Miller explains, consists of three laborers for lifting blocks and shoveling earth and one skilled supervisor. Large walls (3,000 ft.2 or more) are typically constructed by nationally oriented wall companies.
Selecting Appropriate Blocks
Which of the numerous segmental concrete block systems on the market to choose? The first step for the owner or contractor is to identify the concrete-block manufacturers within a 200- to 300-mi. radius of the construction site, then to contact them to see what segmental-block systems they have available. There might only be two or three different block systems available in the region, and one would have to choose from them; transporting blocks more than 300 mi. would be cost-prohibitive. Miller believes either geogrid or geotextile fabric can be equally effective for soil reinforcement.
A recent trend, Miller says, has been for manufacturers to prefabricate several blocks together to make a panel that can be maneuvered into position at the job site. Among those making such multiblock panels is Versa-Lok.
Finding a Wall Installer or Subcontractor
Unfortunately, there is no comprehensive directory of segmental concrete wall designers or subcontractors (also called wall installers). The easiest way to come up with a list of wall designers and installers, Miller suggests, is to contact concrete-block manufacturers or geogrid distributors in your region. They will be able to provide a list of companies with which they regularly do business.
Some materials suppliers and wall installers actively pursue projects themselves by, for instance, using Dodge Reports to search for requests for bids for forthcoming retaining wall projects. A subcontractor will often obtain a copy of the plans and specs for an upcoming project he intends to bid on, then contact a wall engineer to sketch out a preliminary design. Based on this design, the subcontractor will prepare a bid and submit it to the appropriate general contractor.
In more than 80% of site-development plans, the site-development civil engineer indicates where retaining walls are to be constructed on a site with the notation, "Wall design to be submitted by contractor for review." Contractors without qualified civil engineer wall designers on staff usually retain a qualified designer to design an appropriate and aesthetic retaining wall, an important step toward minimizing the possibility of a wall failure in the future. Soil parameters, wall height, slope length and angle, drainage conditions, and space available behind the wall for adequate reinforcement all must be taken into account.
Regional Manufacturing
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| Segmental walls provide an attractive alternative for large highway applications. |
Companies marketing blocks for segmental retaining walls include Keystone; Versa-Lok; Allan Block; Anchor Wall Systems; Rockwood Retaining Walls; Hydropave Erosion Control Systems; Lock + Load Retaining Wall Systems; Risi Stone Systems, which manufactures Dura-Hold units; and Tensar Earth Technologies with the Mesa Retaining Wall System.
Some, such as Keystone, have no concrete-block manufacturing facilities of their own; Keystone’s Minneapolis headquarters, for example, includes engineering and marketing staffs on which licensees can draw for technical help and for product promotion. Instead, companies may license out the product design to concrete-block manufacturers across the US and in other countries, receiving a royalty on every block sold. Such a licensing approach through an extensive network of long-established local concrete-block manufacturers makes sense, because it is important that concrete-block manufacturing plants be located within a few hundred miles of a construction site to keep block transportation costs reasonable.
Many local concrete-block manufacturers, Miller explains, jumped at the opportunity to get into the business of manufacturing, under license, the new segmental concrete blocks. For decades they had been selling the usual standard concrete block, a commodity item, at $0.50—$1.00 per block. But the new segmental block was a premium item. It could be interlocked with adjacent blocks with no need to use mortar, making it possible for contractors to erect a segmental wall for half the cost of a poured-in-place concrete retaining wall-- even while paying block manufacturers a premium price for the new segmental blocks. These segmental blocks typically sell for anywhere from $3.50 to $7.00 per block.
Cost of Segmental Retaining Walls
Miller gives the following cost breakdown for a typical reinforced-soil segmental concrete-block retaining wall.
- Cost of segmental concrete blocks and soil reinforcing (geogrid or geotextile fabric): 30-40%
- Wall designer’s engineering cost: 5%
- Labor: 55-65%
The blocks and soil-reinforcing material are sometimes sold as a package. The amount of soil reinforcement required does not vary linearly with wall height. For instance, a 20-ft.-high reinforced-soil segmental wall uses three times the amount of soil reinforcement as does a 10-ft.-high wall. Generally the total installed cost for a reinforced-soil segmental retaining wall, Miller estimates, is $15-$25/ft.2 of exposed vertical wall surface.
Sizes, Colors, Textures, and Fasteners
Appearance is a great advantage of segmental retaining walls. Most poured-in-place walls are uniform and bland unless they’re covered with costly stone or masonry veneer. In designing a segmental retaining wall, on the other hand, the architect or engineer can choose from a variety of block sizes, shapes, colors, and surface textures. In addition, because of their rough-textured surfaces, segmental retaining walls are less susceptible to "attack" by graffiti vandals than smoother surfaces.
The variety of surface textures is increasing. The so-called straight-face block has a rough, attractive texture; the triface or beveled-face block has part of the block beveled off at each corner of the face, making for a deeply textured, shadowed wall. Finally, there is striated segmental block, which has a series of deep, vertical parallel lines on its surface.
Manufacturers can add dye to the cement used in making blocks; color intensity depends on how much dye is added. The most common colors, Miller notes, are standard gray (the cheapest); buff or earth-tone brown; various reds, including rose; and charcoal. Other colors, such as greens or blues, are sometimes available but less common. Colors add 10-20% to the cost of a block.
According to Don Armstrong of Anchor Wall Systems, most of the segmental retaining wall systems on the market have some similarities, and the appearance of the blocks tends to be similar. An important area where they differ, he says, is in how the geogrid fastens to the segmental wall. Usually the geogrid is anchored to the wall by the weight of the upper block on a lower block (the geogrid is sandwiched between the two blocks). Some systems provide pins or connectors that lock to the block so that the geogrid cannot be pulled out. A move in the direction of providing such a positive connection between block and geogrid, Armstrong says, is a major trend right now in the segmental concrete-block industry. There is also a trend, he believes, for manufacturers to create blocks with more aesthetic, better-looking faces.
Miller believes a major need in the industry is for a lighter-weight block that can perform the same functions as the current heavier blocks. A lighter block would make it much easier for the workers constructing the block and would also reduce transportation costs. To that end, Miller believes some companies might start marketing lighter blocks made of recycled plastics or wooden products.
Avoiding Retaining Wall Failures
According to Miller, failures of reinforced-soil segmental concrete-block retaining walls are not all that common–probably less than 1% of all such walls constructed. Nonetheless, from time to time walls do fail. And those failures, he contends, are usually the result of the following factors.
Inadequate Design. The owner or site-development engineer should contact local concrete-block manufacturers to get a list of qualified designers.
Inadequate Soil Compaction Behind the Wall. The "real" retaining wall is not the facing block itself (which is merely a veneer) but rather the structure of geogrid and compacted soil behind it.
The geogrid at a particular elevation must be laid down in the proper direction and pulled tight, and a layer of backfill must then be placed and properly compacted before laying down the next plane of geogrid material. Said another way, the soil must be compacted in stages, as one progresses upward–not be done all at once after all the backfill has been placed.
Unanticipated or Poorly Managed Water Near the Wall. Appropriate drainage of the soil behind the wall is critical to the design of any segmental-wall system so the soil won’t become too heavy, placing excessive forces on the wall. One way to achieve drainage is to fill the region immediately behind the wall (from the wall back into the embankment for 1 ft.) with _- to _-in.-diameter crushed stone for the full height of the wall. Onsite soils can then usually be used for the remainder of the backfill behind this drainage zone. Such an arrangement allows water to drain out of an extended volume of soil behind the wall. Water from surrounding soil flows into the crushed-rock zone immediately behind the wall, then through the unmortared joints between adjacent concrete blocks. A segmental concrete wall, unlike a cast-in-place concrete wall, is highly porous, thereby preventing the buildup of hydrostatic pressure in the retained soil embankment.
Poor Construction Practices. This category includes a number of possibilities: the segmental-block foundation not being level, the lifts of the blocks themselves not being level or sufficiently squared or being excessively "shimmed," inadequate filling of the hollow block cores with aggregate, geogrid installed in the wrong direction or not properly tensioned before backfill is placed over it, or not following the manufacturer’s recommendations for corners and curves.