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Earls Walls J NAMES K. MITCHELL AND JAMES G. COLLIN Soil is our most abundant and least expensive construction material. The range of soil types and possible states for any soil type are almost limitless. When at a suitable density and moisture content, most soils can be strong enough in compression and shear to be structurally useful. On the other hand, like Portland cement concrete, soil is very weak in tension; this limits its use for some applications, such as those requiring slopes steeper than the internal friction angle of the soil, which is about 30 degrees in most cases. But also, as is the case for reinforced concrete, the inclusion of reinforcement that is strong in tension yields a composite material that combines the best features of both components Figure 1 shows a pile of sand with the steepest slope that can be maintained with the sand in its dry state. The same sand with sufficient water added to give capillary stresses in the pores, i.e., fluid pressures less than atmospheric, results in positive contact stresses between sand particles, or an "apparent cohesion" that permits steep slopes to remain stable, as shown in Figure 2, up to some particular height. Because either wetting or drying of the damp sand will lead to collapse of the slope, the use of capillary stresses can hardly be relied upon for long- term stability. On the other hand, the inclusion of reinforcements within the sand can, as a result of stress transfers between the soil grains and the re- inforcements, result in structures that are stable over long time periods. Figure 3 shows the same dry sand as that in Figure 1, but in this case strips of paper are incorporated as reinforcing elements. The paper used 161
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162 TRANSPORTATION TECHNOLOGY as the wall facing is needed to prevent running of the sand from the region between reinforcements; however, it does not assume a major structural or load-carrying function. Over the past 15 years increasing use has been made of earth rein- forced in a manner similar to that shown in Figure 3 to construct new walls or to strengthen existing slopes. There are several reasons for this in addition to the low cost and abundant supply of earth. Construction is simple and rapid. Reinforcements and facing elements can be pre- fabricated. Aesthetically pleasing structures are possible. Earth walls do not require unyielding foundation support as is the case for most rein- forced concrete walls. Keen competition among the developers of dif- ferent reinforcement systems has led to rapid technological development and continued cost reductions relative to both traditional types of rein- forced concrete walls and other types of wall systems such as crib walls and bin walls. EVOLUTION OF EARTH WALLS It has been known for several thousand years that tensile inclusions in soil can provide reinforcement. Large religious towers, called zig- gurats, were built by the Babylonians between 5,000 and 2,500 years ago. These structures (see Figure 4) had walls faced with clay bricks in an asphalt mortar with blocks of sun-dried mud behind. Layers of reed matting were laid as horizontal reinforcing sheets in the mud. In some ziggurats additional reinforcement was included in the form of ropes about 50 millimeters (mm) in diameter placed perpendicular to the wall and regularly spaced in the horizontal and vertical directions. Reference is made in the Old Testament (Exodus 5:6-9) to the use of straw-reinforced bricks by the ancient Egyptians. Many primitive peoples used sticks and branches for reinforcement of mud dwellings. The corduroy road was an early method for construction of roads across very weak ground that was widely used in colonial America. Figure 5 shows a more modern example of a corduroy road. During the seven- teenth and eighteenth centuries French settlers along the Bay of Fundy used sticks for reinforcement of mud dikes. The development of earth reinforcement for walls in its modern form was pioneered by the French architect and inventor Henri Vidal. The results of several years' study and experimentation led to Vidal's patent in 1966 for Terre Armee. The first highway use of a Vidal reinforced earth wall was near Nice, France (Figure 6~. A schematic diagram of this type of wall is shown in Figure 7. Vidal reinforced earth walls were first used in the United States in 1972 to provide support for California State Highway 39 along a steep
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EARTH WALLS 163 slope in the San Gabriel Mountains- north of Los Angeles (Figure 8~. An earth wall was well suited for the site because of the overall instability of the hillside and because of the ability of these walls to withstand substantial deformations without failure. CURRENT REINFORCEMENT SYSTEMS FOR EARTH WALLS Over the past 10 years the Vidal reinforced earth wall has been the most widely used. Galvanized steel reinforcing strips are connected to pr'ecast concrete facing panels, which have largely replaced the metal facings used in early walls. The standard precast concrete facing panel is shown on the walls in Figure 9. For aesthetic reasons special finishes or panel designs can be used, as shown in Figure 10. A number of other reinforcing systems and materials applications have been developed for earth walls as well. In the VSL Retained Earth system, horizontally placed wire or bar mesh systems are used, as shown in Figure 11. The Hilfiker welded wire wall (Figure 12) uses wire mesh reinforcement and a facing consisting of wire mesh covered by shotcrete. Tensar geogrids are high-strength plastic composite grids that can be used in a variety of ways for containing, retaining, and reinforcing soil. Since the grids can easily be interconnected, a variety of combinations is possible. Large (l m x 1 m x 1 m) earth- or rock-filled baskets similar to gabions can be stacked on top of each other to form walls or barriers. The plastic grids can be used during reconstruction of failed earth slopes to improve stability. Figure 13 shows an example of a brick-faced re- taining wall using tenser geogrids. Synthetic fabrics for geotechnical use, called- geotextiles, have been used as earth reinforcement. They are particularly suited for relatively low walls along remote or relatively lightly traveled roads. An example is shown in Figure 14. A schematic diagram of the internal structure is shown in Figure 15. The "Kabil Stack-Sack Wall" shown schematically in Figure 16 is also a simply constructed system that is adaptable to remote sites. The facing consists of vertical reinforcing bars over which sacks full of premixed but just wetted concrete are placed by dropping the sack over the top of the bar. Wire mesh or chain link fencing is used for horizontal re- inforcement, as shown. Reinforcing systems are also used to strengthen existing ground in order to improve slope stability or to enable slope steepening on ex- cavations without internal bracing or active anchor systems. Soil nailing (Figure 17) consists of the insertion, by driving or drilling, of reinforce- ments into a natural or cut slope. Grouting around the reinforcements is often used to assure good bond between the soil and the reinforce-
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164 TRANSPORTATION TECHNOLOGY meets. "Root piles," consisting of small-diameter, cast-in-place concrete piles containing a single reinforcing bar down the center are also used. Root piles are commonly installed at different inclinations, as shown in Figure 18. APPLICATIONS IN TRANSPORTATION SYSTEMS Earth reinforcement systems of various types have become extensively used in transportation projects. Probably the two greatest uses are for retaining walls and bridge abutments, where they compete very favor- ably, economically and aesthetically, with reinforced concrete. A bridge abutment application is shown in Figure 19. They are equally useful for rail networks. The need to rehabilitate and expand existing road and rail systems in urban areas portends increased use in rights-of-way pre- sently containing earth slopes that will have to be steepened or removed in order to provide the needed space. Any situation requiring an ele- vation change of more than a few feet is potentially suitable for use of an earth wall. Earth walls have also been used for waterfront structures, e.g., quay walls. Here the facing elements play an additional role, namely, erosion protection for the soil behind them. DESIGN CONSIDERATIONS Earth walls are subject to the same external design criteria as those for a conventional retaining wall. That is, they must be stable against sliding due to the lateral pressure of the soil retained by the wall, they must resist overturning, and there must be safety against foundation failure. Classical methods of soil mechanics have been used for the analyses necessary for this part of the design, and they have been sat- isfactory. The internal design of the wall itself must ensure against (1) failure of reinforcements in tension, (2) pullout of reinforcements, and (3) loss of reinforcements by corrosion or other forms of deterioration. To ensure against the first two failure modes requires knowledge of soil-reinforcement interactions. These depend, in turn, upon soil type, reinforcement type and geometry, and the stress state of the soil. The effective friction coefficient between the soil and reinforcements controls the stress transfer between the materials. Many analytical, numerical, model, and full-scale field experiments have been done to provide information on which the selection of needed parameters for design can be based. Sufficient information has been
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EARTH WALLS 165 obtained from these studies and from experience with existing walls that safe designs are possible for normal loading conditions. The soil used in the reinforced zone of permanent reinforced earth. walls is required to be cohesionless so that it will be freely draining, there will be a reasonable frictional strength, and there will not be problems owing to large creep deformations, as might be the case if a clay were used. For the Vidal type of reinforced earth wall the distribution of tensile stresses along reinforcements has been found to be about as shown in Figure 20. The locus of maximum tensile stresses as a function of depth is also shown. This geometry can be used with soil property and rein- forcement data and suitable factors of safety to select the length, cross- sectional area, and vertical and horizontal spacings of reinforcements. A similar methodology can be used for other reinforcing systems. A generally accepted method for the seismic design of earth walls has not yet appeared. High-magnitude earthquakes were considered in the design of the walls at Valdez (Figure 9) and for other walls in areas of known seismicity. The design methodology led to a somewhat increased length of reinforcements and a higher density of reinforcements in the upper part of the wall. To the best of our knowledge, there have been no failures or significant distress of earth walls due to earthquakes. Reinforcement durability is probably the area of greatest concern at present. The rate of corrosion of metal reinforcements depends on many factors, most of which cannot be controlled over the long term in the ground, e.g., local chemical concentrations and stray electrical currents. Galvanized steel has been extensively used. The by-products of zinc corrosion cover the base metal and tend to seal affected areas. A cor- rosion loss over the design life of the wall is taken into account in the specification of metal reinforcement cross sections. Epoxy-coated steel reinforcements now being developed offer the potential for high dura- bility. Nonmetallic reinforcing materials such as geotextiles, fiberglass, plas- tics, and composites, while not susceptible to corrosion, may undergo other chemical and/or biological forms of deterioration. Unfortunately, many of the materials are new, and the effects of long-term burial and exposure to the elements are not known. Hence, durability emerges as an area of major concern, and further studies are likely. CONSTRUCTION Generally the construction of earth walls is relatively simple, rapid, and straightforward. Equipment for placing and spreading the backfill
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166 TRANSPORTATION TECHNOLOGY soil and a roller for compacting it are required. Reinforcements can usually be carried and placed by hand, as shown in Figure 21. If precast facing panels are used, a small crane is required for handling them (Figure 22~. Other facing types, e.g., geotextiles, do not require equip- ment for installation. A significant advantage of reinforced earth walls over reinforced con- crete walls is that no formwork is required. On the other hand, con- struction of a reinforced earth wall requires considerably greater space behind the wall face, as the reinforcement lengths are usually at least 0.7 times the wall height, whereas the base width of a concrete wall is only about 0.3 times the wall height. COSTS The costs for reinforced earth walls today are less, in constant dollars, than they were 8 to 10 years ago. This is because the technology is maturing and because there is competition among earth reinforcement systems. In spite of the widely different technologies, a parallel is evident with the electronics industry. The total cost in any case will be composed of the costs of materials, erection, backfill soil (if the onsite soil is unsuitable), and any special aesthetic treatment of the facing that may be required. A reasonable figure for the cost of materials and erection for walls in the height range of 10 to 15 ft is about $15/ft2. For walls of 15 to 30 ft in height the cost increases to $17 or $18/ft2. In urban areas reinforced concrete is likely to be more economical than reinforced earth for low walls up to 10 ft in height. The two types are competitive for wall heights of 10 to 30 ft. and the earth wall is less expensive for heights greater than 30 ft. THE FUTURE A number of reinforcing systems have been developed for construc- tion of earth walls. Some are best suited for permanent high walls, some are best for low walls, some have their best application in remote areas, and others must withstand the rigors of high-traffic-volume urban areas. As experience broadens and as uncertainties concerning the durability question disappear, it can be anticipated that earth reinforcement will see even greater use in surface transportation networks. The current competition between the manufacturers of different reinforcing mate- rials and systems can lead to more efficient designs. The rapid development and importance of the field has been recog-
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EARTH WALLS 167 nized at the federal level. Work is in progress on National Cooperative Highway Research Project 24-2, Reinforcement of Earth Slopes and Embankments, administered by the National Research Council's Trans- portation Research Board. An international team of experts has been assembled to evaluate and compare the major systems and methods. The results of this study, due in late 1984, will be in the form of a report providing the user with the information required to make the analyses, designs, and estimates needed for the rational choice of an earth wall system. BIBLIOGRAPHY Bassett, R.H., and N.C. Last, "Reinforcing Earth Below Footings and Embankments," in Proceedings of the Symposium on Reinforced Earth, American Society of Civil En- gineers, Pittsburgh, Pa., April 1978, pp. 202-231. Binquet, J., and K.L. Lee, Bearing Capacity of Strip Footings on Reinforced Earth Slabs, G.I. 38983, Report to the National Science Foundation, Washington, D.C., May 1975. Elias, V., and D.P. McKittrick, "Special Uses of Reinforced Earth in the United States," in Proceedings of the International Conference on Soil Reinforcement: Reinforced Earth and Other Techniques, Paris, Vol. 1, March 1979, pp. 255-259. Goughnour, R.D., and J.A. Dimaggio, "Application of Reinforced Earth in Highways Throughout the United States," in Proceedings of the International Conference on Soil Reinforcement: Reinforced Earth and Other Techniques, Paris, Vol. 1, March 1979, pp. 301-313. Maluche, E., "Reinforced Earth Used as Supporting Structures in Hydraulic Engineer- ing," in Proceedings of the International Conference on Soil Reinforcement: Reinforced Earth and Other Techniques, Paris, Vol. 1, March 1979, pp. 335-339. McKittrick, D.P., "Reinforced Earth: Application of Theory and Research to Practice," in Proceedings of the Symposium on Reinforcing and Stabilizing Techniques sponsored by the New South Wales Institute of Technology, October 1978. (Available from the Reinforced Earth Company, Rosslyn, Va.) Mitchell, J.K., "Soil Improvement State-of-the-Art Report," in Proceedings of the Tenth International Conference on Soil Mechanics and Foundation Engineering, Stockholm, Vol. 4, June 1981, pp. 509-565. Schlosser, F., Reinforced Earth, ISSN 0337-1565, Ministere de l'Equipement, Laboratoire Central des Ponts et Chaussees, Paris, France, April 1976. Vidal, H., "The Principle of Reinforced Earth," High w. Res. Rec., 282:1-16, 1969.
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FIGURE 1 Dry sand with maximum stable slope. TRANSPORTATION TECHNOLOGY FIGURE 2 Moist sand can develop stable near-vertical slopes be- cause of internal capillary water pressures. k ~~ ~ ~~ it. ~ ~~ ~~ ~~ ~~ ~ ~~ ~ ~~ ~~ ~ ~ ~~.~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~ . ~ ~ _ : ~~ ~~:~:~'~
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EARTH WALLS 169 FIGURE 3 Dry sand reinforced with strips of paper.
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TRANSPORTATION TECHNOLOGY FIGURE 4 (Top) Ziggurat of Ur in Mesopotamia, circa 2500 B.C. Reinforcement is considered a major factor in its longevity. Photo Max Hirmer. (Bottom) Reconstructed view of the Ziggurat. Photo John Freeman. Reprinted from M. E. L. Mallowan, Early Mesopotamia and Iran, McGraw-Hill, New York, 1965, with permission.
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EARTH WALLS 171 FIGURE 5 Corduroy road for crossing swampy areas.
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172 ,. ~ . . TRANSPORTATION TECHNOLOGY F~GuRE 6 First highway use of modern reinforced earth wall in France between Nice and the Italian border. Dent Thickness = t ' `~: F~GuRE 7 Concept of the Vidal reinforced earth wall.
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EARTH WALLS 173 F~GuRE 8 First reinforced earth wall constructed in the United States, along Highway 39 in the San Gabriel Mountains of Southern California. F~GuRE 9 Reinforced earth walls used at Valdez, Alaska, in connection with the Alaska Oil Pipeline Terminal.
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74 TRANSPORTATION TECHNOLOGY FIGURE 10 Special facing panels for reinforced earth wall along Interstate Highway 70 through Vail Pass, Colorado. a i.~,..~. FIGURE 11 VSL Retained Earth system. in
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EARTH WALLS ~ ~ q Roadway 175 \ Li~ ' ~ Backing Mat Behind 9 Ga. 2" X 6", Typ. FIGURE 12 Schematic diagram of Hilfiker welded wire wall. comic d fit 11 Tensar Geogrid reinforcement - Geogrids cemented through brick faces to act as ties. /N FIGURE 13 Tensar geogrids used to reinforce a brick retaining wall.
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176 TRANSPORTATION TECHNOLOGY FIGURE 14 Fabric-reinforced earth wall. Typical overlap 12-36 in. ~ _ I , _ , Paved surface ~ Fabric laver n Wall height ( Fabric lever 2 <_ ~ Fabric layer 1 L Typically _' ~:;~ F~GuRE 15 Schematic diagram of fabric-reinforced earth wall.
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EARTH WALLS 177 FIGURE 16 Schematic diagram of Kabil Stack-Sack Wall. FIGURE 17 Soil nailing for improving cut slope stability. . ~ r-,i
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178 TRANSPORTATION TECHNOLOGY ~T~ "Reticulated Pall /~ ~' ~ ~ I Radice" Structure _ ~ surface hi Hi: ,; . ',,_ ~ . . FIGURE 18 Root pile system for strengthening steep slope FIGURE 19 Reinforced earth bridge abutment. _ .
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EARTH WALLS H/2 To H 1' ~ H/2 ~~ I 7777~73 7~///// 7777777 ///// ~ / //~/ 1. ~ ~ = 0.3 H 1 Resistant Zone ///~ }it_ FIGURE 20 Stress distribution along reinforcement and locus of maximum tensile stresses in a reinforced earth wall. 179 FIGURE 21 Reinforcement placement.
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180 TRANSPORTATION TECHNOLOGY FIGURE 22 Facing-panel installation.
Representative terms from entire chapter: