Cost-Effective and Sustainable Road Slope Stabilization and Erosion Control (2012)

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41 CHAPTER FIVE MECHANICAL STABILIZATION TECHNIQUES way) is limited. Low retaining structures at the toe of a slope make it possible to grade the slope back to a more stable angle that can be successfully revegetated without loss of land at the crest (USDA 1992). Such structures can also pro- tect the toe against scour and prevent undermining of the cut slope (Gray and Sotir 1996). Short structures at the top of a fill slope can provide a more stable road bench or extra width to accommodate a road shoulder. Retaining structures can be built external to the slope (such as a concrete or masonry retaining wall), or uti- lize reinforced soil (such as a burrito wall or deep patch). Although some of these techniques can apply to large fail- ures, the focus of this synthesis is on shallow instabilities and appropriate low-cost, sustainable solutions, and so this section will focus on smaller applications. Low Masonry or Concrete Walls (With Slope Planting) Masonry or poured concrete retaining walls are rigid struc- tures that do not tolerate differential settlement or move- ment and are appropriate only at sites where little additional movement is expected. Because of this limitation, their use is more restricted than gabion walls or reinforced soil systems. Masonry or concrete walls can have various cross sections (Figure 33). Gravity walls can be constructed with plain concrete, stone masonry, or concrete with reinforcing bar. Masonry walls that incorporate mortar and stone are easier to construct and stronger than dry stone masonry walls, but they do not drain as well (Hearn and Weeks 1997). Cantile- ver walls use reinforced concrete and have a stem connected to a base slab (Das 2007). FIGURE 33 Cross section of gravity (left) and cantilever (right) retaining walls (Adapted from Das 2007). Figure 34 shows a schematic of a low cantilever retaining wall used to flatten a slope and establish vegetation. Retain- This section summarizes literature and interview results on mechanical stabilization techniques. This chapter provides information about techniques that use nonvegetative or nonliv- ing components such as rock, concrete, geosynthetics, and steel pins to reinforce slopes. These techniques can provide stability to both cut and fill slopes. Structures are generally capable of resisting much higher lateral earth pressures and shear stresses than vegetation (USDA 1992). Similarly, as demonstrated by nonlinear finite element analysis, polymeric reinforcement within a soil slope can alter the probable failure mechanism within the slope, significantly reducing the shearing, horizon- tal, and vertical strains and greatly reducing slope movements (Chalaturnyk et al. 1990). It is important to note that multiple failure mechanisms are possible. When designing a structural wall and determining reinforcement lengths, care must be taken to check other failure surfaces and modes (B. River, personal communication, Nov. 7, 2011). Depending on the soil type, ten- sile strength and aspect ratio of fibers, volumetric fiber content, and so on, the inclusion of fiber reinforcement in soil can induce distributed tension within the soil, and the soil failure can be governed by pullout or breakage of individual fibers (Zornberg 2002). Including anchors in slopes can enhance the safety factor by providing an additional shearing resistance on the slip sur- face, which is a function of the orientation, position, and spacing of anchors (Cai 2003). Depending on the slope to be stabilized, reinforced soil slope techniques would be tailored to address the specific site challenges. To implement reinforced soil slope tech- niques, one can first assess the additional shear force needed for slope stability (indicated by the design safety factor) and then analyze the available forces provided by the reinforcement lay- ers or anchors, followed by the selection of the type, number, location, or spacing of the reinforcement within the slope. The life cycle performance of the reinforcement materials has to be considered at the design stage, as such materials may deteriorate over time in the soil owing to exposure to environmental and mechanical loadings (Jewell and Greenwood 1988). The following sections describe various reinforced soil slope techniques, which may be used individually or in com- bination for slope stabilization. RETAINING WALLS Retaining structures are used to hold back (retain) material at a steep angle and are very useful when space (or right-of-

42 ing walls with free-draining compacted backfill can be designed and constructed more efficiently than those using poor-quality, cohesive backfill soils. In either case, a drain- age system should be installed behind the wall (Anderson et al. 1997; Das 2007; Shah 2008). The decision to use a dry masonry, mortared masonry, or concrete retaining wall will be greatly influenced by the familiarity and experience of local practitioners. Reinforced concrete is very common in the United States, and there is no shortage of engineers or contractors with adequate expe- rience to design and construct concrete retaining walls. In developing countries with larger labor pools and experi- enced masons, dry masonry or mortared masonry structures are more common (Anderson et al. 1997; Shah 2008). Additional Resources for Retaining Walls Das, B.M., Principles of Foundation Engineering, 6th ed., Cengage Learning, Stamford, Conn., 2007, 750 pp. Hearn, G.J. and R.W. Weeks, Principles of Low Cost Road Engineering in Mountainous Regions, with special ref- erence to Nepal, Himalaya, C.J. Lawrence, Ed., Transporta- tion Research Library Overseas Road Note 16, Berkshire, United Kingdom, 1997. Keller, G. and J. Sherar, Low-Volume Roads Engineer- ing—Best Management Practices Field Guide, Office of International Programs and U.S. Agency for International Development, USDA Forest Service, Washington, D.C., 2003 [Online]. Available: http://www.fs.fed.us/global/topic/ welcome.htm#12. Mohoney, J., et al., Retaining Wall Design Guide, Publica- tion No. FHWA-FLP-94-006, Federal Lands Highway Tech- nology Implementation Program. Washington, D.C., Sep. 1994. Shah, B.H., Field Manual of Slope Stabilization, United Nations Development Program, Pakistan, Sep. 2008 [Online]. Available: http://www.preventionweb.net/english/ professional/publications/v.php?id=13232. Sotir, R.B. and M.A. McCaffrey, “Stabilization of High Soil and Rock Cut Slope by Soil Bioengineering and Con- ventional Engineering,” Transportation Research Record 1589, Transportation Research Board, National Research Council, Washington, D.C., 1997, pp. 92–98. Gabion Walls Gabion baskets are made of heavy wire mesh and assembled on site, set in place, then filled with rock. Once the rock has FIGURE 34 Cross section of a low wall with vegetation planted on the slope for stabilization (USDA 1992).

43 been placed inside the gabion basket, horizontal and vertical wire support ties are used to achieve the reported strength. Gabion walls are composed of stacked gabion baskets and are considered unbound structures. Their strength comes from the mechanical interlock between the stones or rocks (Hearn and Weeks 1997). To achieve the maximum level of strength in the gabion wall, the baskets are to be filled to the greatest possible density, which is generally achieved by hand packing rather than mechanically packing. Packing of the gabion baskets is a skill that is learned through practice. For specific information about how to pack rock, rock types to be used, and wire mess gauges and tying (see Hearn and Weeks 1997, pp. 121–122). Gabion basket manufacturers have a wealth of standard designs for various wall heights and soil types that ensure stability against overturning, sliding, bearing-capacity fail- ure, and deep-seated slope failure (Kandaris 1999). Gabion walls can be used at the toe of a cut slope or the top of a fill slope (Figure 35). The walls can be vertical or stepped and are adaptable to a wide range of slope geometries (Kandaris 1999). Gabion walls can accommodate settlement without rupture and provide free drainage through the wall. They are usually preferred at sites with poor foundations, wet soils, high groundwater, or slope movement caused by creep, slid- ing, and seismicity (Hearn and Weeks 1997). Useful Points • We have had issues with contractors not knowing how to load gabion baskets or not installing gabion basket cross ties. Cross ties should be installed every foot in both directions; otherwise the gabion basket will not achieve the design strength (B. Johnson, personal com- munication, April 18, 2011). FIGURE 35 Low gabion wall and gabion wall stabilization at top of fill slope in Timor (Courtesy: G. Keller and C. Bennett). Additional Resources for Retaining Walls Hearn, G.J. and R.W. Weeks, Principles of Low Cost Road Engineering in Mountainous Regions, with special ref- erence to Nepal, Himalaya, C.J. Lawrence, Ed., Transporta- tion Research Library Overseas Road Note 16, Berkshire, United Kingdom, 1997. Keller, G. and J. Sherar, Low-Volume Roads Engineer- ing—Best Management Practices Field Guide, Office of International Programs and U.S. Agency for International Development, USDA Forest Service, Washington, D.C., 2003 [Online]. Available: http://www.fs.fed.us/global/topic/ welcome.htm#12. Shah, B.H., Field Manual of Slope Stabilization, United Nations Development Program, Pakistan, Sep. 2008 [Online]. Available: http://www.preventionweb.net/english/ professional/publications/v.php?id=13232. MECHANICALLY STABILIZED EARTH/GEOSYNTHETIC REINFORCED SOIL SYSTEMS Retaining walls can also be built with reinforced soil. These are commonly referred to as mechanically stabilized earth (MSE) walls. MSE walls can use different reinforcing ele- ments (e.g., strips of metal, sheets of geosynthetics) and dif- ferent facing systems (e.g., concrete panels, modular blocks, shotcrete). Geosynthetic reinforced slopes can also retain soil to, for example, support a road bench. The technol- ogy has been around long enough to be thoroughly studied with computer models (e.g., Karpurapu and Bathurst 1995, Vulova and Leshchinsky 2003; Hatami and Bathurst 2005); laboratory experiments (e.g., Helwany 1994; Zornberg et al. 1998; Wu and Helwany 2001), and field studies (Tatsuoka et al. 1992; Liang and Almoh’d 2004; Abele 2006). There is still some debate in the geotechnical engineer- ing community about the fundamental theory of the behav- ior of MSE walls and reinforced soil slopes (VanBuskirk 2010, Adams et al. 2011). In any case, several techniques have been shown over the years to be cost-effective and sus- tainable solutions to slope instabilities. The following spe- cific techniques are presented in more detail: shallow MSE walls, geotextile walls, reinforced soil slopes, and deep patch embankment repair. Shallow Mechanically Stabilized Earth Walls MSE walls are constructed with reinforced soil (Figure 36). The reinforcement can be metal strips (galvanized or epoxy- coated steel), welded wire steel grids, or geogrids. The walls have a vertical or near-vertical face and include a facing system to prevent raveling and erosion. The facing elements could be precast concrete panels, modular concrete blocks, metal sheets, gabions, welded wire mesh, shotcrete, or wood lagging and panels. Hybrid systems are common; for exam- ple, a geogrid-reinforced MSE wall with gabion-basket fac- ing was used at several locations along a new highway in

44 Nantahala National Forest in North Carolina (Simac et al. 1997). A variety of proprietary facing reinforcement sys- tems exist, but because most process patents for MSE walls have expired, many options exist for contractors to purchase and erect them. MSE walls can be designed and built to accommodate complex geometries and to heights greater than 80 ft. They offer several advantages over gravity and cantilever concrete retaining walls: simpler and faster con- struction, less site preparation, lower cost, more tolerance for differential settlement, and reduced right-of-way acquisition (Elias et al. 2001). FIGURE 36 Schematic of a generic mechanically stabilized earth wall (Berg et al. 2009). Although the economic savings of MSE walls compared with traditional concrete retaining walls are significantly better at heights greater than 10 ft, even short MSE walls can be constructed economically. For shallow walls, the less expensive option is usually modular block facing, as opposed to precast concrete or metal sheet (Elias et al. 2001). Consider using good-quality backfill material, especially for high walls and bridge abutments, although shorter walls can more easily tolerate poorer quality soils. “[The most cost-effective road slope stabilization tech- nique is] shallow MSE walls, by far and away, because you use native materials and you don’t need specialized con- tractors to do it. We got away from steel strip reinforce- ment a long time ago. Now we mostly use welded wire because it’s easy to install” (S. Romero, personal commu- nication, May 11, 2011). Additional Resources for MSE Walls Berg, R., B. Christopher, and N. Samtani, Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Volumes 1 and 2, FHWA-NHI-10-024 and 025, Fed- eral Highway Administration, U.S. Department of Trans- portation, Washington, D.C., 2009. Elias, V., B. Christopher, and R. Berg, Mechanically Sta- bilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, Report FHWA-NHI-00-043, Fed- eral Highway Administration, Washington, D.C., 2001. Geotextile Walls Geotextile-wrapped walls, sometimes called burrito walls, were developed by the U.S. Forest Service in the Pacific Northwest as a low-cost alternative to walls requiring facing elements. Geotextile walls are used to stabilize the fill slope by placing sheets of geotextile between layers of soil (pit run or road base) that are usually 6–18 in. (15–50 cm) thick (Figure 37). The geotextile is wrapped at the face; temporary forms or careful compaction can be used before flipping the geotextile over the soil (Powell et al. 1999). Figure 37 shows two examples of geotextile walls. Because the geotextile face can degrade from sunlight and ultraviolet radiation, consider protecting the geotextile unless the wall is constructed as a temporary structure (service life of about 3 years or less). A layer of gunite (cement, sand, and water mixture) or asphalt emulsion can provide adequate protection (Powell et al. 1999). Vegetation can also shade the geotextile sufficiently. To vegetate a geotextile wall, seeds are sown on the outer face of the soil before wrapping the front with the geotextile; cuttings are also placed in the thin soil layer between sheets of reinforcement (Shah 2008). Useful Points • There is a learning curve to creating these the first time and without forms. Using a contractor who has experi- ence in the technique of building a geotextile wall will help (J. Currey, personal communication, April, 15, 2011). Additional Resources for Geotextile Walls Elias, V., B. Christopher, and R. Berg, Mechanically Sta- bilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, Report FHWA-NHI-00-043, Fed- eral Highway Administration, Washington, D.C., 2001. Keller, G. and J. Sherar, Low-Volume Roads Engineer- ing—Best Management Practices Field Guide, Office of International Programs and U.S. Agency for International Development, USDA Forest Service, Washington, D.C., 2003 [Online]. Available: http://www.fs.fed.us/global/topic/ welcome.htm#12. Powell, W., G.R. Keller, and B. Brunette, “Applications for Geosynthetics on Forest Service Low-Volume Roads,” Trans- portation Research Record 1652, Transportation Research Board of the National Academies, 1999, pp. 113–120.

45 Shah, B.H., Field Manual of Slope Stabilization, United Nations Development Program, Pakistan, Sep. 2008 [Online]. Available: http://www.preventionweb.net/english/ professional/publications/v.php?id=13232. Reinforced Soil Slopes Reinforced soil slopes (RSS) can generally be steeper than conventional unreinforced slopes because geosynthetics (geogrids and geotextiles are both common) provide tensile reinforcement that allows slopes to be stable at steeper incli- nations. The design methods for RSSs are conservative, so they are more stable than flatter slopes designed to the same safety factor (Elias et al. 2001). RSSs offer several advan- tages over MSE walls: backfill soil requirements are usually less restrictive, the structure is more tolerant of differential settlement, no facing element is required so they are typi- cally less costly, and erosion protection vegetation can be incorporated into the face of the slope. Useful Points • When using geosynthetics for reinforced soil slopes you need to match the type with the site-specific parameters (K. Mohamed, personal communication, April 26, 2011). Additional Resources for Reinforced Soil Slopes Berg, R.R., B.R. Christopher, and N.C. Samtani, Design of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Volume II, Report FHWA-NHI-10-025, Federal Highway Administration, Washington, D.C., 2009. Berg, R., B. Christopher, and N. Samtani, FHWA Geotech- nical Engineering Circular No. 11: Design and Construc- tion of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Volumes I and II, Reports FHWA-NHI-10-024 and FHWA-NHI-10-025, Federal Highway Administration, Washington, D.C., 2010. Elias, V., B. Christopher, and R. Berg, Mechanically Sta- bilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines, Report FHWA-NHI-00-043, Fed- eral Highway Administration, Washington, D.C., 2001. Deep Patch Embankment Repair The deep patch embankment repair is similar to a reinforced soil slope, except the repair is limited to the top of the fill slope instead of reinforcing the entire slope. It is commonly used on paved forest roads with recurring cracks and settlement in the outer portion of road. A deep patch repair involves excavating 3–8 ft (1–2.5 m) deep and reconstructing with compacted, granular soil and geogrids. Drainage is usually incorporated in the repair. Vertical spacing for the geogrid is 1 ft (30 cm), so a 6-ft-deep (2 m) repair would need six layers of geogrid. The depth, width, and length of the deep patch depend on the location of the cracks. For cracks near the outer edge of the road, a 3-ft-deep (1 m) repair is usually fine. For cracks near the centerline, especially with greater settlement (verti- cal displacement), a deeper repair is needed. The length of the repair should extend at least 5 ft (1.5 m) beyond the ends of the crack. Deep patches have been as short as 20 ft (6 m) and as long as 800 ft (250 m), although repairs 50–150 ft (15–45 m) long are more common. The width of the deep patch needs to extend beyond the crack so the repair is “anchored” into the stable portion of the slope. A good rule of thumb is to extend the patch 5 ft (1.5 m) behind the crack, although an analysis of pullout failure could be performed. Figure 38 shows photos of a deep patch in Gifford Pinchot National Forest in Washing- ton during and after construction. Additional Resources for Deep Patch Embankment Repair Cuelho, E.V., S. Perkins, and M.R. Akin, Evaluation and Revision of Deep Patch Design Method, Western Federal Lands Highway Division Report FHWA-WFL, Vancouver, Wash., 2011. FIGURE 37 Early work by a contractor creating a geotextile wall in Alaska, and geotextile wall on fill slope of road in Klamath National Forest in California (Courtesy: J. Currey and G. Keller).

46 Wilson-Musser, S. and C. Denning, Deep Patch Road Embankment Repair Application Guide, USDA Forest Service, Washington, D.C., Oct. 2005 [Online]. Available: http://www.fs.fed.us/t-d/pubs/pdf/05771204.pdf. Tire Walls Tire walls have been used as retaining structures, for erosion control, and to stabilize slopes (Steward 1992; Retterer 2000) (Figure 39). The tires can be used as facing or to reinforce backfill soil. Tire reinforced walls can made from whole tires or bales of compressed tires (Retterer 2000). Tire walls can be constructed up to 10 ft in height. There are many ways to con- struct tire walls using varying soil and rock fill types and geo- synthetics (Garga and O’Shaughnessy 2000a; Retterer 2000). To ensure tire wall strength and stability, connect the tires together appropriately (Garga and O’Shaughnessy 2000b). Significant settlement of tire walls has been observed in field applications (Steward 1992). Some users consider tire walls to be visually unappealing. Vegetation, geotextile, shotcrete, concrete blocks, and the like can be used to cover the tire wall surface. Tire walls can be less costly than other retaining wall structures, but cost savings will vary depend- ing on location and availability of materials. In general, tire walls can be constructed without skilled labor or special equipment (Retterer 2000). Additional Resources for Tire Walls Garga, V.K. and V. O’Shaughnessy, “Tire-reinforced Earthfill. Part 1: Construction of a Test Fill, Performance, and Retaining Wall Design,” Canadian Geotechnical Jour- nal, Vol. 37, No. 1, 2000a, pp. 75–79. Garga, V.K. and V. O’Shaughnessy, “Tire-reinforced Earthfill. Part 2: Pull-out Behavior and Reinforced Slope Design,” Canadian Geotechnical Journal, Vol. 37, No. 1, 2000b, pp. 97–116. Garga, V.K. and V. O’Shaughnessy, “Tire-reinforced Earthfill. Part 3: Environmental Assessment,” Canadian Geotechnical Journal, Vol. 37, No. 1, 2000c, pp. 117–131. Keller, G. and O. Cummins, “Tire Retaining Structures,” Engineering Field Notes, Vol. 22, Mar./Apr., pp. 15–24, For- est Service, U.S. Department of Agriculture, Washington, D.C., 1990. Setterer, T.A., “Gravity and Mechanically Stabilized Earth Walls Using Whole Scrap Tires,” Master’s thesis, Texas Tech University, Dallas, May 2000. Steward, J.E., “History of Reinforced Walls in the USDA Forest Service Engineering Field Notes,” Engineering Tech- nical Information System, Vol. 24, Washington, D.C., Sep.– Oct. 1992. IN-SITU SOIL REINFORCEMENT In-situ soil reinforcement involves repairing instabili- ties with minimal to no excavation by inserting reinforc- ing elements into the soil. Although fibers can be used as soil reinforcement (Park and Tan 2005), they are currently considered too expensive unless more low-cost fibers (e.g., recycled fibers) of high quality become available for slope stabilization applications. Similarly, the use of lime piles (“holes in the ground filled with lime”) has been reported to be successful for “in situ treatment of failing clay slopes” (Rogers and Glendinning 1996) but not widely implemented for roadside slope stabilization, likely for cost reasons. Three cost-effective techniques were identified: launched soil nails, pin piles, and plate piles. Launched Soil Nails Shallow instabilities can be repaired by launching an array of soil nails (also referred to as ballistic soil nailing) through FIGURE 38 Deep patch during and after construction in Gifford Pinchot National Forest in Washington (Courtesy: B. Collins).

47 the ground surface deep enough to penetrate into a stable region. The technique was developed in the United Kingdom to avoid the need to excavate and construct a working plat- form from which traditional soil nails could be drilled and grouted in place. As illustrated in Figure 40, an excavator with a hydraulic boom is used to install soil nails between 5 and 35 ft (1.5 and 10 m) above and below its position on the road. This technique can be used for instabilities as deep as 15 ft (4.5 m) from the surface, in which case 20-ft-long (6 m) nails with a diameter of 1.5 in. (3.8 cm) would be used. For shal- lower instabilities, shorter nails are used and/or the portion of the nail protruding from the ground is cut off at the ground surface (USDA Forest Service 1994). Originally solid nails were used, but now hollow galvanized steel or fiberglass tubes are much more common (Barrett and Devin 2011). This tech- nique can be used in sands, gravels, silts, clays, and soils with only a few cobbles and boulders. Too many cobbles or boul- ders would reduce the penetration depth of the nails. Design charts and design examples are available in a USDA Forest Service application guide (1994). Additional Resources for Launched Soil Nails Application Guide for Launched Soil Nails, USDA Forest Ser- vice. Report EM 7170-12A, Washington, D.C., 1994 [Online]. Available: www.fs.fed.us/eng/pubs/pdf/em7170_12a.pdf. Barrett, C.E. and S.C. Devin, “Shallow Landslide Repair Analysis Using Ballistic Soil Nails: Translating Simple Sliding Wedge Analysis into PC-Based Limit Equilibrium Models.” In The Proceedings Geo-Frontiers 2011 Conference, 2011. Steward, J.E. and J.M. Ribera, “Launched Soil Nails: New Method for Rapid Low-impact Slope Repairs,” In Proceed- ing of the Sixth International Conference on Low-Volume Roads, Minneapolis, Minn., June 25–29, 1995. Pin Piles (Micropiles) Pin piles (also known as micropiles) are more commonly used for foundations than slope stabilization (Tarquinio and Pearlman 1999; Pearlman 2001). In 2000, when FHWA published design and construction guidelines for micropiles, the chapter devoted to applications for slope stabilization was left out because of a lack of consensus on design procedures. Even in 2008, use of the technique was noted to be limited (Loehr and Brown 2008). Most refer- ences to pin piles or micropiles for slope stabilization are for repairs to deep-seated failures and involve driving (or drilling and casting) long piles at various angles to form “a monolithic block of reinforced soil” (Holtz and Schuster 1996). However, anecdotal evidence of shallow repair fail- ures 5–10 ft deep (1.5 to 3 m) using recycled railroad rails was found during the interviews, although performance and design information was not identified. When asked to provide an example of an underutilized tool, technique, or method (P. Bolander, personal communication, May 2, 2011) replied, Possibly the use of pin piles to stabilize shallow fill slope failures, some forests in Idaho and Montana have been using railroad rails (steel, long rectangular cross section) as pin piles and have had some success. There are a couple (of) techniques. In the east coast they’ve taken these piles (steel or wood) and driven them in the top of the fill slope to reduce the fill slope settlement— intended to be a shallow repair (maybe anywhere from 5 to 10 ft deep). FIGURE 39 Tire wall and construction (Courtesy: G. Keller).

48 FIGURE 40 A soil nail launcher mounted on an excavator installing nails on an unstable fill slope below a road (USDA Forest Service 1994). Plate Piles Plate piles are a relatively new slope stabilization tech- nology; the method and device were patented in 2006. As illustrated in Figure 41, an array of plate piles is driven into the soil to prevent shallow slope creep or landslides. A typical galvanized steel plate pile consists of a 2.5-in. (6 cm) L-shaped stem that is 6 ft (1.8 m) long with a 2-ft- by-1-ft (30 by 60 cm) rectangular plate attached near the top. Typical spacing is 4 ft (1.2 m) between piles within a row and 10 ft (3 m) between rows (Figure 41). Other sizes are available depending on site requirements. Successful full-scale field tests and demonstration projects have been reported (McCormick and Short 2006; Short and Collins 2006; Y. Prashar, personal communication). Ideally, the plate piles would be driven through shallow, unstable fill 2–3 ft thick (0.6–1 m) and into a more competent stratum (e.g., claystone, weak sandstone). Additional Resources for Plate Piles McCormick, W., “Platepiles: Caltrans Experiments with the Next Generation Slope Repair Alternative,” AEG News, Vol. 54, No. 1, Mar. 2011. McCormick, W. and R. Short, “Cost-Effective Stabi- lization of Clay Slopes and Failures Using Plate Piles,” In Proceedings of the 10th IAEG International Congress, Not- tingham, United Kingdom, Sep. 6–10, 2006. Platepile Slope Stabilization Design Guidelines, 2nd ed., 2011. Slope Reinforcement Technology, LLC, Danille, Calif., 2011. Short, R.D. and Y. Prashar, “Modeling a Full Scale Slide Test,” In Proceedings of Geo-Frontiers 2011 Conference, Dallas, Tex., Mar. 13–16, 2011. Titi, H. and S. Helwany, Investigation of Vertical Mem- bers to Resist Surficial Slope Instabilities, SPR# 0092- 05-09, Wisconsin Highway Research Program, Madison, 2007 [Online]. Available: http://minds.wisconsin.edu/ handle/1793/53953. FIGURE 41 Six-foot plate piles (left) and plate pile installation (right) using an excavator with a hydraulic hammer (Courtesy: Y. Prashar).