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3 other benefits of slope stabilization and erosion control are not always recognized. Those benefits include the following: ⢠Rural employment opportunities involving both skilled and unskilled labor; ⢠Low energy inputs; ⢠Protection of land and water resources; ⢠Preservation of local biodiversity (as native grass and plant species are used in bioremediation applications); and ⢠Aesthetically pleasing road sights. Many slope stabilization solutions being implemented around the world by low-volume road engineers and manag- ers are successful and cost-effective, but relevant information on methods and techniques is not well disseminated or widely used. In this context, a synthesis of effective practices is war- ranted. This work aims to compile available knowledge rele- vant to roadway slope stabilization and erosion control, with the primary audience being public road engineers and managers. WHAT IS ROAD SLOPE STABILIZATION? Road slope stabilization is the practice of stabilizing slopes adjacent to roads. Hundreds of effective road slope stabili- zation methods have been developed and used around the world. Road slope stabilization can range from allowing native grass to re-establish on a disturbed slope to build- ing an engineered wall. The treatment measure depends on the affected area, cost, and feasibility. Royster (1982) found that treatment of one landslide may require extensive and immediate correction, while another slide may only require minimal control with periodic monitoring to achieve a sim- ilar level of service. Slope stabilization or erosion control requires a toolbox approach that considers the level of effec- tiveness and acceptability of the treatment. Site conditions and constraints can vary greatly, and a âone-size-fits-allâ approach is unlikely to work. Instead, the right tools have to be selected for the specific project in light of its unique ero- sion and slope stabilization problems. Although seeding and constructing a rock wall are drastically different in terms of cost and sustainability, they are two tools in the toolbox and each has its place in road slope stabilization. The cost of slope stabilization and erosion control can range from minimal to astronomical. Field studies have shown that the CHAPTER ONE INTRODUCTION This report presents information on cost-effective and sus- tainable road slope stabilization techniques, with a focus on shallow or near-surface slope stabilization and related erosion control methods used on low-volume roads. Many of the identified solutions apply to higher volume roads as well. Specific items discussed in this report include the importance of soil and compost, the importance of having a surface and subsurface water management plan, soil bioen- gineering/biotechnical solutions, reinforced soil solutions, other vegetative and earthwork solutions, and appropriate erosion control measures to maximize the slope stabiliza- tion for a specific treatment. In the United States and internationally, most roads are located in rural areas and have low traffic volumes. World- wide, there are an estimated 21 million miles (33.8 million km) of roads, of which 18.6 million miles (30 million km) are rural, low-volume roads (Faiz 2011). In the United States, there are approximately 4 million miles (6.4 million km) in the road system, of which 3 million miles (4.8 million km) are rural, low-volume roads maintained by some 35,000 fed- eral, state, and local agencies. Low-volume roads often omit surface slope protection. This can lead to slope failure, erosion, and sedimentation, which contribute to water quality degradation and increased road maintenance demands, traffic delays, safety problems, damage to other resources, and, in the long term, reduction in the service life of roads. Soil erosion can cause flood- ing, increased water treatment costs, siltation of harbors and channels, loss of wildlife habitat, disruption of stream ecology, reduced recreational value, and adverse aesthetic impacts (Gray and Sotir 1996). Erosion is the process of separating and transporting sedi- ment by water, wind, or gravity. Removal of vegetation, distur- bance of topsoil, compaction, and creation of steep slopes are among the many causes of erosion (Hayman and Vary 1999). Water erosion is the most damaging type of erosion, especially in developing areas, and erosion control is thus a particular concern for new construction. Erosion and the sedimentation it causes during and after highway construction can result in an unhealthy growing environment for vegetation, have negative impacts on adjacent waterways, and in the long run require additional maintenance (Johnson et al. 2003). In addi- tion to reducing life-cycle repair and road maintenance costs,
4 FIGURE 1 Cross section of typical (idealized) cut and fill construction with sidecast fill technique. FIGURE 2 Cross section of a typical raised road with ditches created on both sides for drainage. The shape of the slope can be a defining factor in its stabil- ity. Natural slopes are generally concave, which is the most stable type of slope and experiences the least erosion (Schor and Gray 2007). Many man-made slopes are linear (Schor and Gray 2007), and research has found that in many cases a linear slope will erode until it becomes concave (Gyasi- Agyei et al. 1996). Linear slopes created with benches are frequently used on larger slopes to reduce erosion poten- tial, but modeling has found that linear slopes with contour benches tend to channel water in concentrated flow paths, causing severe gullying over time (Schor and Gray 2007). HOW ARE ROAD SLOPES STABILIZED? Consideration of surface slope protection and addressing surface water and groundwater issues during road con- struction and maintenance activities can reduce erosion and enhance the long-term performance of slopes and embank- ments. A combination of adequate drainage, installation of protective devices and elements, and establishment of desir- able vegetation offers the best means for soil conservation. combined use of structural and vegetative slope protection systems is more cost-effective than the use of either method alone (Gray and Leiser 1982; Xu et al. 2006). WHAT CAUSES INSTABILITIES? Common causes and trigger events for erosion or soil insta- bility include excessive slope angle or height, poor drainage, low-strength foundation, removal of vegetation that anchors soil, increased loading, environmental factors, poor handling of fill materials, high groundwater table, unsuitable geologic features, liquefaction, and wildfires (Shah 2008). Types of slope instabilities that can cause erosion include creep, fall or topple, slides, flow and spread, and settlement (Collin et al. 2008). Although triggers for landslides in transportation projects are often related to water (including intense rain- fall, rapid snowmelt, water level changes, or stream erosion), slides can also be triggered by earthquakes, human activity, or volcanic eruptions (Collin et al. 2008). Improper road construction techniques, including improper selection of equipment, are a common cause of slope instabilities (Shah 2008). One technique often used in mountainous regions is known as cut and cast, cut and fill, or side-cast construction. Side-cast fills are typically not compacted and not draining, and are oversteepened. Picture a road in a mountainous or hilly region where material has to be cut from the uphill side and cast onto the downhill side to create the road benchâthe horizontal plane on which the road will be constructed. Figure 1 shows an idealized cross section of this technique in which the exact volume that was cut is perfectly cast adjacent to the cut. In reality, material is moved around to accommodate the actual shape of the hill or knob. For these roads, the cut-and-fill faces and fill portion, which are now steeper and disturbed, are areas of potential instability that could be treated. On flat ground, a raised road is often built with ditches to improve the drainage of water from the road (Figure 2). The created embankment may be prone to surface erosion if soil is left exposed. Slope failures are the movement of soil, and they occur on both man-made and natural slopes. Potential causes for slope instability range from deep-seated failures (such as with landslides) to surface erosion (such as when steep slopes cause water to travel in concentrated flows, eroding a series of gullies). There are many types of slope failures, includ- ing rockfalls/rockslides, debris avalanches/debris flows, and slumps/earth flows (MSE 1997). Human-induced modifi- cations that may adversely affect external loads to slopes include grading of the existing slope or adjacent slopes, con- struction adjacent to the slope, construction damage caused by blasting, and vibrations of passing vehicles (Turner and Schuster 1996). Slope regrading can create an oversteep toe, or base of the slope, or an accumulation of material at the crest, which can lead to erosion (Turner and Schuster 1996).
5 For instance, seeding disturbed soil as areas of a project are completed can reduce erosion by 90% (Johnson et al. 2003). There are dozens of techniques to stabilize road slopes and prevent surface erosion. Erosion control techniques gener- ally protect the surface from being eroded by water and wind: examples include vegetative cover, crushed stone cover, mats, and blankets. The guiding principles are minimizing the exposed and disturbed areas and exposure time, managing on- site stormwater by reducing velocity and volume, installing erosion and sediment control measures early in the construc- tion phase and during structural maintenance, and keeping sediment on site (Johnson et al. 2003). Temporary erosion con- trol measures should be used during construction, especially when the construction occurs in steep rolling topography, in cases where most of the drainage enters directly into adjacent water bodies or wetlands, or where the subsoils are erosive (Alberta Transportation 2003). After projects are completed and vegetation is established, permanent measures should be implemented. Common devices for permanent erosion control include design elements, ditches and liners, riprap, soil bioen- gineering and biotechnical stabilization, and vegetation estab- lishment. Many erosion problems could be avoided altogether with good design practices (Alberta Transportation 2003). Soil bioengineering techniques utilize plant parts such as roots and stems to serve as structural and mechanical elements in slope protection systems (Gray and Sotir 1996; Sotir and McCaffrey 1997; Grace 2002; Fox et al. 2010). The plants act as soil reinforcement, aid in water drainage, or serve as barri- ers to earth movement (Gray and Sotir 1996). The use of sod, or native grass sod, as a best management practice (BMP) is compatible with highway revegetation prescriptions and is employed in several states (Dollhopf et al. 2008). Similarly, biotechnical stabilization utilizes structures in combination with plants to arrest and prevent slope failures and erosion with biological and mechanical elements functioning together in an integrated and complementary manner (Gray and Sotir 1996). Biotechnical stabilization applies to retaining struc- tures, revetments, and ground cover systems (e.g., sod grass reinforced with netting) (Gray and Sotir 1996). Retaining structures to help hold back the slope include walls of vari- ous shapes and materials. The combined use of structural and vegetative elements (e.g., contour wattling, willow cuttings, conventional slope planting combined with low gabion walls, bench structures constructed at the toe of a slope, vegetation growth in the voids of structural walls) has been reported to be an attractive and cost-effective method to hold soil and pre- vent slope failures and erosion (Gray and Leiser 1980). Other options for stabilizing weak soils include stabilizing vegeta- tion and structures, erosion control mats and mesh, and earth- work (e.g., terracing, anchoring, effective site drainage, slope modification), as well as the use of lime piles (Rogers et al. 2000), fibers and chemicals (RITA 2011), and electrochemical techniques (Wan and Mitchell 1976; Johnson and Butterfield 1977; Casagrande 1983; Alshahabkeh et al. 2004; Paczkowska 2005). Slope reinforcement can utilize vegetation, concrete, polymers, and other materials. Natural materials such as soil, rock, and timber are more environmentally compatible and are better suited to vegetative treatments or slight modifica- tion than are manufactured materials (USDA 1992). They may also be available on-site at no cost (USDA 1992). Mechanical stabilization techniques utilize nonvegetative or nonliving components such as rock, gabion baskets, con- crete, 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 lat- eral earth pressures and shear stresses than vegetation (USDA 1992). Mechanical stabilization techniques include retaining walls, mechanically stabilized earth, geosynthetically rein- forced soil, and other in-situ reinforcement techniques. For anchoring shallow soils, use of in-situ earth reinforcements and recycled plastic pins has been reported in slope stabiliza- tion (Pearlman et al. 1992; Loehr et al. 2000). Earthwork techniques involve the physical movement of soil, rock, and/or vegetation for the purpose of erosion control and slope stabilization. This involves reshaping the surface slope by methods such as creating terraces or benches, flatten- ing oversteepened slopes, soil roughening, or land forming. Earthwork techniques can be used to control surface runoff and erosion and sedimentation during and after construction (EPA 2008). Land grading can be used at sites with uneven or steep topography or on easily eroded soils to stabilize slopes, and terraces can be used to reduce sediment-laden runoff by slow- ing water flow down the slope, collecting and redistributing surface runoff into designed drainage channels (EPA 2008). To effectively control soil instabilities and erosion, a systematic approach is needed that takes into account gov- ernment regulations and permitting requirements; design, construction, and maintenance issues; various temporary and permanent control methods; and new technologies (Johnson et al. 2003). Although every slope stabilization treatment method can be considered a tool in the toolbox, some treatments may be more appropriate for a site. The current state of the practice has matured in such a way that practitioners no longer view specific slope stabilization treatments as good or bad, working or ineffective. Instead, a multidisciplinary approach that combines knowledge from multiple fields of studyâincluding geology, hydrology, engineering, and landscape architectureâand combines treatment measures to create site-specific slope stabilization treatments is used to solve slope stabilization issues. HOW IS A TREATMENT DETERMINED TO BE COST- EFFECTIVE OR SUSTAINABLE? When considering road slope stabilization techniques for a site, there are generally many options. For example, on an
6 exposed road cut the treatment options may be (1) to build a retaining wall, (2) to build a vegetated crib wall, or (3) to add topsoil or compost to the eroding surface, hand seed the slope, and lay down erosion-control blankets. Any of these options could work well, but which one will be most cost- effective and sustainable? The answer will depend on what is available on site, how much space is available to work with, and how much it costs to bring materials into the site. When selecting a cost-effective, sustainable treatment for road slope stabilization, both the short- and long-term costs need to be considered. One way to ensure that a project is low cost and sustainable is to use local or on-site materials. Reusing on-site soil, rocks, tree stumps, downed trees, live vegetation, leaf litter, and the like can be very cost-effective. Use of on-site materials ensures that the project is sustainable by reducing fuel and transportation costs that would accrue if these materials needed to be brought to the site. Native seed stock present in the local soil is another benefit. In a survey conducted to gain information for this project, survey respon- dents stated that short- and long-term costs are considered important in deciding on a road slope stabilization and/or ero- sion control measure, and are frequently considered together. A sustainable road slope stabilization treatment is one that disturbs the least amount of soil, keeps topsoil on site, reuses on-site vegetation to strengthen the slope, incorporates native plants, and poses minimal disturbance to the ecosystem. In a survey conducted to gain information for this project, 76% of survey respondents stated that they always or frequently consider how environmentally friendly or sustainable a road slope stabilization measure will be. Although many respon- dents stated that a strong sense of environmental stewardship has led them to make sustainable decisions, an equal number of respondents stated that local and state mandates, federal laws, and permit requirements weigh heavily in making a sus- tainable road slope stabilization treatment choice. Aesthetic considerations are also often appropriate when choosing the stabilization technique. It is a common belief that created slope stabilization structures should fit with the natural landscape, and once the project is completed it is important that the site be restored as close to its previ- ous condition as possible (Schiechtl and Stern 1996). Issues include the balance and distribution of cut and fill material, the use of local building materials, the avoidance of deep and steep cuts into slopes wherever possible, and maintenance of the natural landscape. METHODS This synthesis focuses on cost-effective and sustainable shallow (less than 10 ft) or near-surface slope stabilization and related erosion-control treatments used on low-volume roads. An extensive literature review was conducted to gather information on cost-effective and sustainable near- surface slope stabilization techniques used on low-volume roads. Technical documents, government reports, journal publications, conference presentations and proceedings, and textbooks were used initially to identify pertinent information. Information was also sought from local, state, federal, and international governments and organizations; departments of transportation; manuals, field guides, and reports; published specifications; and organizations and companies that work to promote erosion control and slope stabilization. Information from the literature review was used to create the body of the report and the survey ques- tions, and identify individuals and organizations for partici- pation in the survey. Based on information gained from the literature review, a survey was developed to gather additional information from practitioners, scientists, contractors, and vendors on cur- rent practices, effective practices, and emerging solutions that are used regionally, nationally, or internationally. The survey asked participants to provide identifying informa- tion, followed by seven questions requesting information on the respondentsâ direct experience with erosion control and slope stabilization techniques (Appendix A). The sur- vey was distributed by e-mail to individuals identified in the literature review and by project panel members. The survey was available online for 2 months, and 81 responses were received. Survey responses were processed and summa- rized. Information identified in the survey that was incorpo- rated into this report includes resources, references, erosion control and slope stabilization techniques and tools, best management practices, useful points, and photographs. Sur- vey responses aided in focusing the synthesis on the most frequently used road slope stabilization techniques that are cost-effective and sustainable. A list was compiled of survey respondents who indicated they were willing to participate in follow-up interviews. Thirty individuals were selected to be interviewed based on the information they made available in the survey. A total of 25 interviews were conducted, providing an 83% inter- view response rate. Interviews were conducted over the phone with the exception of two responses received through e-mail, owing to intervieweesâ location and language differ- ences. Interviewees were asked 16 questions and instructed to provide responses based only on their direct experience (Appendix B). Interview responses were recorded with a digital recorder and then transcribed or recorded by hand during the interview process. Information gained from the interviews that was incorporated into this report includes additional resources, references, erosion control and slope stabilization techniques and tools, best management prac- tices, useful points, photographs, knowledge gaps, and research needs.
7 The report begins with an introduction to the topic of cost-effective and sustainable road slope stabilization techniques. It defines road slope stabilization, identifies general techniques used to stabilize roads, and provides a discussion of the terms cost-effective and sustainable and how they relate to road slope stabilization treatments. This chapter also includes the methodology section, which outlines how the literature review, survey and interviews were conducted and provides an outline of the report. The following section on the basics provides information on planning and site investigation, soil type and mechanics, and water management including surface and subsur- face drainage options. The next section, Erosion Control Techniques, defines erosion, outlines general causes, and provides examples of erosion control treatments and tech- niques, including seeding, mulching, the use of blankets, mats and geotextiles, check dams, wattles, silt fences, and chemical soil stabilizers. The Soil Bioengineering and Bio- technical Techniques section defines these two techniques and provides a review of treatments on the topic, including the use of live stakes, fascines, crib walls, gabions, and rock walls in a combination of vegetation and structures to stabilize slopes. The next section, Mechanical Stabili- zation Techniques, defines this topic and provides infor- mation on retaining walls, mechanically stabilized earth and geosynthetically reinforced soil systems, geotextile walls, deep patch repairs, and in-situ soil reinforcement techniques. The Earthwork Techniques section defines this topic and provides information on benches, terraces, soil roughening, flattening over-steepened slopes, and land- forming. The report closes with a summary of findings from each section and a discussion of knowledge gaps and areas for future research. The report resembles a guide in structure, but it is more appropriate to use it as a reference document. Each section highlights current cost-effective and sustainable practices in road slope stabilization. Each topic has an Additional Resources section that provides references from which more information can be gathered.
