Retaining walls often go to great heights and lengths to restrain the movement of soil and rocks in ways that are both attractive and economical.
If you were building retaining walls in Chile, even smaller configurations for private property, your decisions and designs would take into account the strong likelihood of seismic activity in the region. Some parts of North America are similar, but most designers of retaining walls do not have to consider earthquakes a top priority. Contractors and public works departments who work with the soil (and know its behavior in theory and through practical experience) would amaze most of the general public if they explained how soils can shift and rocks slide. Readers of this magazine understand soils; your clients and customers might not. An important step in the recommendation of a retaining wall is educating the customer or property owner about the necessity for such a structure. |
Illustrations Courtesy of Keystone |
| These drawings show how retaining walls resist seismic forces. |
The ability to withstand earthquake movements is one feature of the MESA Retaining Wall System from Tensar Earth Technologies that prompted its use for several walls at a high-market condominium complex near Santiago, Chile. The owners selected Emín Ingeniería y Construcción to design and construct the walls. "They not only offered incredible performance," comments Alejandro Palma of Emín, "they gave a savings of 35% over some quoted precast concrete walls." He adds that installation was faster and easier. "The retaining walls at La Dehesa were built in two stages, with heights ranging from 3 to almost 30 feet. We needed 50,000 cubic feet of compacted fill, and the total surface area was about 6,000 square feet." At that seismic-sensitive site, they used MESA high-performance connectors to anchor the Tensar structural geogrid to the blocks. That meant that 200% of the long-term design strength of the geogrid is mobilized at less than three-quarters of an inch of movement; in other words, the structural connection gives a minimum 2.0 Factor of Safety at that low deformation.
The challenges faced by the contractors at the condominium site can be found anywhere; that there is restricted space for construction might be the most common. Such problems as a lack of water and electricity may be less likely in the United States, but stringent deadlines for completion of the retaining walls, difficult terrain, and earthmoving problems are universal. Down in Panama, the MESA Retaining Wall System has been used to support bridge abutments under spans that average 100 ft. Among alternative methods considered were pile-supported bridge abutments, but the configurations of boulders in the rivers crossed (the Chiricagua, Plantacal, and Caimito) would have necessitated extensive, costly drilling. For the MESA system, only a 1-m (about 39-in.) wall embedment was required on each river bottom. Another practical savings at this site was that the MESA blocks for all three bridges took only four days' production, whereas concrete precast panels could have required two months. The installation of the walls for all three projects took 45 days, and contractors estimated that other methods would have taken another two weeks. It was estimated that the savings in machinery and manpower exceeded $20/yd.2 because no crane (or its operator) was required and a five-person crew was enough to set the blocks and compact fill.
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| Retaining walls will save several acres of a beautiful valley from flooding by a proposed dam. |
For retaining walls at Avalon Bay Communities in Fort Lee, NJ, it was the flexibility of the design-build technique from Tensar Earth Technologies that allowed the owner to increase the anticipated number of rental units and profitability at the site. The MESA walls added up to more than 48,000 ft.2, with heights up to 30 ft. The varying elevations of hard rock presented serious challenges, but Tensar engineers designed a custom solution, changing it when necessary as the work progressed and saving construction time by doing so. The MESA wall is an integrated system, says the manufacturer, unlike modular walls, which combine components from different sources. The system combines Tensar structural geogrids, high-strength concrete units, and the manufacturer's locking connector that makes a positive, mechanical, end-bearing connection between the block and the geogrid. Tensar geogrids have been used worldwide for highways, coastal protection, and structural reinforcement.
Design Considerations
"Our philosophy is that shoring system and fill wall design is governed primarily by soil considerations that are best understood and analyzed by experienced geotechnical engineers," notes Jerry Bishop at Geotechnical Design Services (GDS), which has designed many temporary and permanent retaining systems in the last five years. "Project engineers at the I-15 project stated that our designs were typically 10-40% more efficient than those provided by structural engineers. Our experience includes soldier pile tieback walls, soil nail slopes and walls, internally braced sheet pile cofferdams, sinking caissons, and underpinning of existing facilities."
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| This wall demonstrates the height potential of retaining walls. |
At the Deer Crest Development, more than 350,000 ft.2 of mechanically stabilized earth (MSE) walls have been designed, involving more than 70 reinforced soil structures. The walls provide direct support for roads, bridge decks, and ConSpan bridge arch supports. The walls vary in height up to 55 ft. Their modular construction minimizes the inventory of different wire sizes for the contractor but still allows a "cut to fit" approach to the construction. A cobble facing helped blend the walls in with local topography to give a rustic appearance. "Without these retaining systems, onsite roads could not have been built to their present widths in as simple or economical a manner," says Bishop.
For a different industry and type of application, GDS has prepared MSE wall designs for several large mine truck dumps in Wyoming, Colorado, Nevada, and Utah. "For the Kennecott Bingham Mine, we completed the design and monitoring of two companion mine truck dump walls," explains Bishop. "Their total square face footage is approximately 100,000, and they are taller - with a structural height of 126 feet - than any other reinforced soil walls in the world. The truck dump concept with these two walls was chosen over a less expensive conveyor-belt system because of the simplicity and anticipated reliability of the facility. Furthermore, this truck dump design meant there was space for a larger ore stockpile and better access for rail loadout." Bishop goes on to describe how those walls included innovative features never used on such a large scale. A fiber-reinforced concrete fascia was poured concurrently with the wall construction, minimizing abrasion problems at the face, decreasing compression of the face under the high vertical loads, and providing protection for the steel reinforcements from the highly corrosive ore leachate. A variable base design limited the amount of excavation into the hillside behind the wall so that the road along the crest of a hill (with its utility pipelines) was left in place. Bar mat reinforcements were spliced within the structure, allowing the use of smaller mats that were easier to ship and handle. With some face alignment requirements relaxed to permit more rapid construction, the entire project was constructed in about 90 days, using double shifts, six days per week. (Note: If you get a chance to see any of these "retaining walls" for mines, you'll never think the term applies to small structures again!)
Appearance and Costs
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| These retaining walls are often seen at residential locations, but they also have much bigger brothers in the family. |
Everybody wants a retaining wall to be attractive or to fit in with surrounding buildings and landscape, but the owner of the Kennedy Medical Center in Turnersville, NJ, seemed especially concerned about public approbation for the project there. The design phase for the 30-ft. retaining wall took longer than usual, waiting for the most aesthetically pleasing layout. In the final work, the wall was split into upper and lower sections to create a terrace for shrubs and trees. The concrete used included an earth tone pigment that matched the environment. This retaining wall for a 90-unit, partial-care housing center took only three months to complete once the designs were approved. The wall chosen was a Reinforced Earth® design. The consultants (Land Dimensions) had used one before at the same location and liked the results.
A Reinforced Earth® structure is described by its producer (The Reinforced Earth® Company) as a single, coherent gravity mass that can be engineered for specific load requirements. It is a cohesive material of great strength and stability, formed by the association of granular soil and reinforcements. Walls using this technique have been successful along both urban and mountain highway projects for earth-retaining structures and bridge abutments, but they have also proved effective for sea walls, dams, and bulk storage facilities. In Texas, some highway authorities have credited this type of retaining wall with a cost reduction of as much as 40% on applications such as Interstate 35W, where some 300,000 ft.2 of wall was built. The manufacturer points out that a team of five, using standard construction equipment, can install between 750 and 1,000 ft.2 of retaining wall per shift. Some of the speed is attained because installation is basically a repetitive procedure. With so much anxiety expressed about the lack of skilled personnel for construction work, a technique that can be performed well by less skilled workers seems likely to save money. Most Reinforced Earth® panels are 4.92 x 4.92 ft. in the shape of a cross (trademarked by the manufacturer), but larger panels, 5 x 10 ft., have been used to suit particular projects.
After the initial course of panels have been installed, workers spread the first lift of backfill and compact it. They place the steel reinforcements and bolt them to the panels, then spread and compact a lift of backfill over the reinforcing strips. The crew repeats this procedure until the design height is reached. Whatever the height or length of the structure, it remains stable during construction and equipment may operate on any layer of backfill. This simple construction procedure, says the manufacturer, has made the system appropriate for restricted sites or close property lines, because it is done behind the wall face with no scaffolding.
Numerous Applications
Seismic threats played a part in the design of the Reinforced Earth® walls at the San Bernardino Flyover in Los Angeles, CA, where a section of the Metrolink commuter rail line crosses over the existing Union Pacific railyard. Because the site is located near the San Andreas Fault, a horizontal seismic coefficient of 0.7g was used in the design and the walls at the bridge abutments were engineered to carry the lateral displacement loads of the abutment piles in the event of an earthquake. With access limited by the existence of the active rail lines, contractor E.L. Yeager Construction Company was pleasantly surprised to accomplish more than 1,200 ft.2 per day of the 48,000-ft.2 retaining wall project and complete it in two months. Other publicized projects using Reinforced Earth® retaining walls have been: 40,000 ft.2 flanking the two cast-in-place cantilever bridge abutment stubs for the Southwest Corridor light rail system in Denver, CO; more than 100,000 ft.2 to support high access ramps going to Treasure Island, the Mirage, and Caesar's Palace and part of the Spring Mountain interchange in Las Vegas; and a storage slot, holding 50,000 tons of coal, for the North Rochelle Mine near Wright, WY. The latter is 76.5 ft. tall and 360 ft. long, using the Reinforced Earth® technology to form the slot, a V-shaped gravity feed structure from which coal can be conveyed to trains at flows as high as 4,000 tons per hour. And we thought retaining walls were just for the backyard!
Made from high-strength, low-absorption concrete on standard block machines, Versa-Lok retaining wall units resist damage before, during, and after construction in all climates. As wall courses are installed, Versa-Tuff pins (made of noncorrosive nylon and fiberglass) are inserted through holes in the upper course units and received in slots in the adjacent lower course units. On many projects, Versa-Lok retaining walls are gravity systems with the unit weight providing resistance to earth pressures. Gravity walls are generally recommended up to 4 ft. By using horizontal layers of geosynthetics (usually geogrids) to reinforce retained soil, Versa-Lok walls can be built as high as 40 ft. Local site and soil conditions will determine the design of the soil reinforcement and drainage materials.
"For tall walls, our segmental systems typically offer savings of 25-40% compared with cast-in-place, precast panels and crib-type structures," notes Jeff Nelson, marketing and creative manager for Versa-Lok. "No concrete footings or footings below frost are required for Versa-Lok walls, and that eliminates the need for deep excavations. Our walls are placed on shallow, granular leveling pads about 6 inches thick and 24 inches wide." The modular units are dry stacked without mortar and do not require construction forms of any type, with an interlocking mechanism providing quick, uniform alignment during installation.
More than half of the states in the US have approved Versa-Lok for their own applications. Among the successful sites: 5,000 ft.2 for bridge abutment walls in Indianapolis, up to 12 ft. high; 15,200 ft.2 of retaining wall, up to 17 ft. high, for a county road in Minnesota; a three-tiered retaining wall at Alton Memorial Hospital in Illinois, with 7,000 ft.2 of Versa-Lok and some of it as high as 19 ft.; a bridge replacement retaining wall in New Hampshire, up to 20 ft. in height; a 6-million-gal. water tank in Santa Fe, NM; retaining walls under bridges, up to 17 ft. high, in Beaumont, TX; two tiers at 23 and 17 ft. for a county highway in Onalaska, WI; and a retaining wall for a parking garage in Rockville, CT. Installation of Versa-Lok, explains the manufacturer, is fast and easy. An experienced four-person crew will routinely install 400 ft.2 per day. Unlike some segmental systems, this one has no cores to fill, and that should eliminate wastage of materials, unnecessary labor, and the uncertainty of improper construction.
Their flexibility allows Versa-Lok walls to tolerate minor earth movement without damage, which also suits them to those parts of the country where there are freeze/thaw cycles. "Because lateral earth pressures are not transferred to a footing, tall Versa-Lok walls can be built on soils with bearing capacities less than those required for cantilevered retaining walls," explains Nelson. "For a tall wall, each course of Versa-Lok is set back three-quarters of an inch from the front of the course beneath it. This battered configuration causes walls to step back into retained soil at approximately 7 degrees. Walls with batter are more structurally stable than vertical walls because gravitational forces pull the walls into the retained wall."
An interesting paragraph in one of Versa-Lok's technical publications states the following: For the proper stabilization of a segmental retaining wall system, the reinforced soil mass must be large enough to resist loads from outside the wall (external stability) and include enough soil reinforcement to keep the soil mass together (internal stability). For external stability, there must be sufficient width to the reinforced soil mass to resist sliding, bearing failure, and overturning. For internal stability, there must be enough strength and layers in the soil reinforcement to resist tensile overstress and enough length to resist pullout from the stable soil. There is considerable expert engineering, then, in the design of tall retaining walls, but the designs can be implemented accurately and quickly (according to many contractors) by crews who are not immediately skilled in this segment of construction.
Many Sites Require Custom Designs
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| EDICO walls can be installed by unskilled workers. |
The French company EDICO also offers a custom-engineering solution for the construction of dry-build retaining walls up to 66 ft. in height. Its patented block locking system comprises four or eight pairs of concrete blocks that ensure complete linkage between the components for maximum stability. The Beto Concept wall system has four components: Betoflor (20 x 10 x 8 in.) and BetoAtlas (10 x 20 x 8 in.) for retaining walls up to a height of 33 ft.; BetoTitan (10 x 39 x 8 in.) for walls up to 66 ft.; and BetoJard (20 x 10 x 8 in.) for acoustic, sound-absorbing walls. Anything French with "beto" in its name is probably concrete-related, and these blocks are precast concrete cellular blocks that can be cut to form on an internal or external angle. Slow curves are formed without cutting by using the tolerance available between adjacent blocks-about one-third of an inch. After being placed, the blocks are filled with topsoil for greater stability and the rear of the wall is backfilled with low- fines granular material compacted in layers. The excavated face is lined with a geotextile separating layer to prevent particle migration, and the heel of the wall will include a perforated land drain.
"With Beto Concept, the construction area is optimized and takes up little space," comments J.D. Rossi from the Nice headquarters of EDICO. "What is also appealing to contractors is that the assembly of the retaining wall is simple and quick and can be carried out dry. Unskilled staff can fit the components together without the use of cement. Three workers can lay 650 square feet of wall per day. Our walls are flexible and are classified as earthquake resistant because they have been tested in earthquake zones." Rossi also says the system has been credited with savings of as much as 60% over other retaining wall systems.
EDICO uses software at their design office for structural design studies and computations for specific applications. Its retaining walls have replaced concrete walls to steepen embankments for bridge abutments and approach fills to widen existing highways and for both residential and commercial site developments, with the latter tending to require the high walls. EDICO has the blocks manufactured worldwide in 30 factories from North Africa to Poland to Switzerland and is seeking further global expansion.
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| Retaining walls can support walkways and roads that lead to raised entrances for buildings. |
There are several systems offered by Hilfiker Retaining Walls. Its Welded Wire Wall is the most economical and adaptable to several different site conditions, with the lightweight components requiring a minimum of equipment and labor for installation. Workers can install this kind of wall in a battered configuration or make it vertical. It can be left exposed, completed with vegetation suitable to the local environment, or covered with a concrete skin or gunite. Similar in concept and construction is the Eureka Reinforced Soil (ERS) solution. As the wall is erected, workers install form anchors of the correct size and pattern to accommodate the desired form system and pour rates. When the wall is complete and any specified settlement achieved, a reinforcing steel and forming system is erected, using the anchors already installed. Then the fascia (6-8 in. thick) is poured and stripped. When finished, the wall resembles a nonproprietary cast-in-place wall, using almost any architectural treatment requested. This ERS system from Hilfiker is used especially in applications where poor foundations can cause differential settlement within the wall.
Hilfiker also makes segmental blocks for retaining walls that comprise dr-stacked concrete blocks. The T-Block is a shaped concrete block with a series of channels cast into it. To make the system adaptable to different situations, the block channel can hold a variety of connections. This system has been used successfully in applications with cut and fill techniques and in veneer conditions. It has also served for free-standing structures, such as parapet walls or sound walls. The soil mass of a structure can be strengthened with a reinforcement of galvanized steel, and the T-Block can function as a gabion structure. With no grout or mortar, this Hilfiker structure is most flexible, while the vertical alignment of the cells means that a retaining wall system can be designed on standard dry-stacked masonry principles (with the interior cells reinforced or grouted, if desired).
"Our T-Block segmental retaining wall structure has developed the first true mechanical connection for block retaining wall systems," asserts a Hilfiker spokesperson. "The interaction of the soil reinforcement with the channel and the core medium and alignment pin make this connection the strongest of all systems." One of Hilfiker's systems that has been especially popular in highway construction, with heights far beyond those of garden walls, is the Gabion Faced MSE wall. Developed in cooperation with the Federal Highway Administration 10 years ago, this system comprises the company's ArtWeld gabions (used as the facing) with welded-wire soil reinforcement mats strengthening the fill every 3 ft. as the wall goes upward.
Multiple Benefits
Retaining walls, then, are as much commercial as residential solutions. This can be seen in the variety of applications for which Keystone Retaining Wall Systems have been used. Just south of Birmingham, AL, is the planned city of Mt. Laurel. A large dam was going to be built as one of the centerpieces of the city and would be situated in a valley with hardwood growth, mountain laurels, rock outcroppings, and a mountain stream. The proposed dam slope, 52 ft. in height, would have run several hundred feet down this valley, inundating several acres of highly prized terrain. To save the valley, a 51-ft.-high KeySystem wall was built on the downstream side. A centerpiece of the dam is a walkway running its length. The walk turns into a bridge near the center of the dam, where the wall is slightly recessed at the top to create a waterfall over the face.
For Morris Corporate Park IV in Parsippany, NJ, the owner and architect reviewed eight different concepts before choosing the Keystone solution. Structural integrity was an important factor in their decision, notes Keystone's Bob MacDonald. "Our system complements the intense grade changes at the site and gives a prestigious look to the structures." The tallest wall is 30 ft. and supports the parking lot. It elevates pedestrian paths and allows the parking lot to be located closer to the main entrance, an advantage appreciated in inclement weather. Anchor Concrete Products (Keystone's licensed manufacturer/representative in Manasquan, NJ) applied 44,000 ft.2 of Compac Classic Straightface Units to create the ramps, terraces, and parking lot walls. During Hurricane Floyd, the business park received 16 in. of rain in 12 hours, and levels came within inches of the 100-year breach mark. All of the retaining walls held firm.