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

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54 CHAPTER SEVEN CONCLUSIONS AND KNOWLEDGE GAPS CONCLUSIONS This report presented information on cost-effective and sus- tainable road slope stabilization techniques, with a focus on shallow or near-surface slope stabilization and related ero- sion control methods used on low-volume roads. To docu- ment the state of the practice, a comprehensive literature review was conducted, followed by a survey and interviews. Information was presented on how to plan for success, including the importance of creating a work plan, project timing, identifying necessary preliminary work, using a multidisciplinary approach, and, once completed, how to perform a site assessment and/or necessary maintenance. The role of soil type and soil mechanics in slope stabil- ity was reviewed, including the importance of understand- ing what soil types are present, how they will behave under stress or saturated with water, and the mechanism by which they could fail. Understanding these concepts will aid in the selection of appropriate slope stabilization techniques and vegetation to stabilize the slope. TABLE 6 SUMMARY OF EROSION CONTROL TECHNIQUES Treatment Component Stabilization Method or Depth Pros Cons Grass Hand seeding Shallow No equipment required Hydroseeding Shallow High success rate Lack of available of equipment, limited application distance Sod Shallow High success rate Slips Shallow Can be used to create drainage channels Hand planting takes time Mulching Wood, leaf litter, straw, bark, stone Surface Keeps soil moist and cool, protects surface from erosion If mulching with wood chips, nutrients may be removed from soil Blankets and Mats Jute, geosynthetics, rock Surface Keep soil moist and cool, protect surface from erosion. Aid in revegetation of steep slopes and where revegetation may be difficult Nonbiodegradable products should be cleaned from the site Check Dams Inert (stone, wood, concrete) Concentrate and con- trol surface water flow Reduce suspended solids in runoff Maintenance may be required to clean out deposited sediment Live (vegetated) Concentrate and con- trol surface water flow Reduce suspended solids in runoff; roots increase slope stabilization; modify shallow slope hydrology Maintenance may be required to clean out deposited sediment Wattles and Rolls Inert (geosyn- thetic, straw, coir, pine needle) Protect against sheet flow, reduce surface water velocity by breaking up the slope Reduce suspended solids in runoff Maintenance may be required to clean out deposited sediment, restake and replacement may be necessary; nonbiodegradable prod- ucts need to be cleaned from the site Live (vegetated) Protect against sheet flow, reduce surface water velocity by breaking up the slope Reduce suspended solids in runoff; modify shal- low slope hydrology Maintenance may be required to clean out deposited sediment, restaking and replace- ment may be necessary Straw Bale Barriers Slow surface flow Reduce suspended solids in runoff; can be used at base of slopes and around drains Wet bales can be heavy and difficult to move; baling material may need to be removed from site if nonbiodegradable Silt Fences Reduce surface flow Reduce suspended solids in runoff; can be used at base of slopes and around drains Difficult to construct and maintain; need to be removed from the site

55 Appropriate water management may be the key to pre- venting slope failures. It is critical to develop a water man- agement plan that identifies where the water is coming from, how the water interacts with the soil and topography of the site, where the water will go, and how much water is on site. When designing and building roads, using well-draining materials and incorporating surface and subsurface drain- age where appropriate is critical. Use of mulch and soil amendments (e.g., compost) can help on-site vegetation to stabilize a slope. In many cases, the soil exposed when cut-and-fill slopes are created along roadways is not suitable for plant growth. Soil amendments with fertilizer, compost, mulch, or additional topsoil may be necessary. Sustainable practices include saving and reusing topsoil and mulching with on-site materials. Erosion control is the proactive use of products and tech- niques to prevent soil from eroding from slopes. Table 6 summa- rizes cost-effective and sustainable erosion control products and techniques. Erosion control products should be considered for use at every site on any disturbed soil surface. It is much easier to prevent erosion than to fix a slope that has eroded. Methods used to control surface erosion can be used alone or as compo- nents of a system. This is also true of the other slope stabilization techniques presented in this synthesis. Users of these techniques should pay close attention to ecological issues in order to mini- mize any possible disturbance to the local ecosystem. Soil bioengineering and biotechnical slope stabilization is the use of vegetation and structural elements to stabilize slopes and can be both cost-effective and sustainable. Table 7 summarizes soil bioengineering and biotechnical slope stabilization techniques. TABLE 7 SUMMARY OF SOIL BIOENGINEERING AND BIOTECHNICAL STABILIZATION TECHNIQUES Treatment Component Stabilization Method or Depth Pros Cons Crib Walls Inert (wood, concrete) Shallow; used at base of slope Reduce steepness, prevent shallow slope failures, work where space is limited Do not work for large lateral stresses; maximum height 6–10 ft (2–3 m) Live (vegetated) Shallow; used at base of slope Modify shallow slope hydrology; reduce steepness, prevent shallow slope failures, work where space is limited; vegetation provides flexible binding Do not work for large lateral stresses; maximum height 6–10 ft (2–3 m) Stakes Shallow, slump, or slips Work well for projects with limited con- struction time; can be used to pin or anchor erosion control materials; modify shallow slope hydrology Cuttings should be harvested within a day of planting Fascines Shallow; protect against sheet flow, reduce surface water velocity by breaking up the slope Modify shallow slope hydrology; reduce suspended solids in runoff; well suited for steep, rocky slopes; can be use to create drainage channels Maintenance may include thinning vegetation Brush Layering and Palisades Shallow; protect against sheet flow, reduce surface water velocity by breaking up the slope; armor the slope Modify shallow slope hydrology; reduce suspended solids in runoff; can be used to create drainage channels Maintenance may include thinning of vegetation Branch Packing Shallow; used for small localized slumps, embank- ments, or holes. Modifies shallow slope hydrology Does not work on slumps greater than 4 ft (1.2 m) deep or 5 ft (1.5 m) wide; maintenance may include thinning of vegetation Rock Joint Planting Shallow Modifies shallow slope hydrology Maintenance may include thinning of vegetation Gabion Rock or earth filled, vegetated Shallow; used at base of slope Modifies shallow slope hydrology; reduces steepness, prevents shallow slope failures, works where space is limited; vegetation provides flexible binding Maintenance may include thinning of vegetation Soft, vegetated Shallow; used at base of slope Modifies shallow slope hydrology; reduces steepness, prevents shallow slope failures, works where space is limited; vegetation provides flexible binding; can be used when rock is not available Maintenance may include thinning of vegetation Rock Wall Vegetated Shallow; used at base of slope Modifies shallow slope hydrology; reduces steepness, prevents shallow slope failures, works where space is limited; vegetation provides flexible binding; can be built against undisturbed slopes Maintenance may include thinning of vegetation; boulders or large rock are required

56 In addition to soil bioengineering, there are many other cost-effective and sustainable slope stabilization tech- niques that do not necessarily incorporate vegetation; these are grouped in the reinforced soil slope section. This sec- tion covers the use of retaining walls, geosynthetics, and other artificial and/or nonbiodegradable slope stabilizers. Table 8 summarizes these slope stabilization techniques and products. Earthwork techniques involve the physical movement of soil, rock, and/or vegetation for the purpose of erosion con- trol and slope stabilization. Grading work is done as part of the original road building project but can also be used to prepare a slope for a stabilization treatment. Table 9 sum- marizes different earthwork techniques. The following are a few key findings to consider when planning a road slope stabilization project: • Plan ahead. • Know the site conditions—water, soil, topography. • Consider current and future user needs of a road. • Consider using cost-effective and sustainable treatments. KNOWLEDGE GAPS A significant body of information about erosion control and near- surface slope stabilization is available in the literature and in the experiences of practitioners. Knowledge gaps that still remain were compiled from review of the literature and the results of the interviews. Further research areas include the following points: Developing and Conducting Laboratory and Field Testing • Independent testing of the effectiveness of erosion- control products TABLE 8 SUMMARY OF MECHANICAL STABILIZATION TECHNIQUES Treatment Component Stabilization Method or Depth Pros Cons Walls Masonry (rock, concrete) Shallow to deep; pro- tect against toe scour and undermining of cut slopes Reduce steepness above wall, prevent shal- low slope failures, work where space is lim- ited; provide extra space for a road shoul- der; built external to slope; easily conform to slope shape Do not tolerate settlement or movement; require a drainage system behind the wall Gabion Shallow to deep; at base of slope Reduce steepness, prevent shallow slope failures, work where space is limited; pro- vide extra space for a road shoulder; can accommodate slope movement; allow for water drainage May require the use of an experienced contractor; bas- kets are rigid and can be restrictive in building Mechanically Sta- bilized Earth and Geosynthetic Reinforced Soil MSE walls (reinforce- ment: metal strips, welded wire, or geosynthetic fac- ing: concrete panels, con- crete blocks, metal sheets, gabion baskets, etc.) Shallow to deep Reduce steepness, prevent shallow slope failures, work where space is limited; pro- vide extra space for a road shoulder; easily conform to slope shape; can accommodate complex geometries; simple and fast con- struction; somewhat tolerant of settlement Good-quality backfill should be used Geotextile Walls Shallow to deep; pro- tect against toe scour and undermining of cut slopes Reduce steepness, prevent shallow slope failures, work where space is limited; pro- vide extra space for a road shoulder; built within the slope; tolerant of settlement; can incorporate vegetation May require the use of an experienced contractor; geo- textile surface must be pro- tected from ultraviolet light Reinforced Soil Slopes Shallow to deep Can provide extra space for a road shoulder; tolerant of differential settling; less restric- tive soil type criteria; can incorporate vegetation Require extensive excavation for deeper instabilities Deep Patch Embankment Repair Shallow Less excavation than if repairing full depth of slope Only applicable to failures in fill slope Tire Walls Shallow No skilled labor or special equipment required Settlement occurs; visually unappealing In Situ Reinforcement Launched Soil Nails Shallow Little to no excavation required and little disturbance to existing vegetation Need to catch problem before slope has failed Pin Piles (Micropiles) Shallow or deep Work with shallow and deep instabilities No accepted standard design; more difficult installation than launched soil nails Plate Piles Shallow Promising new technique New technique; more case studies need to be documented

57 • Independent testing of the effectiveness and appropri- ate applications for geogrids and geotextiles • Development of a suite of standard test methods for erosion-control products • Methods to determine when a slope is on the verge of failure, and preventive actions (which are more cost- effective than reactive measures) • Cost-benefit analysis, predictive models, and perfor- mance evaluation criteria for each technique • Definition of the life expectancy of each technique and actual capability of products • Analysis techniques and understanding of pin piles • Effects of overdigging • Behavior of cut slopes in frozen soils and freeze/thaw issues on new slopes • Evaluation of erosion prediction models Vegetation and Ecology • A complete record of root establishment timing, spa- tial distribution, and contribution to slope stability for different climates and soil compositions. Some of this information is available, but the record is incomplete. • Identification of the current knowledge base of vegeta- tion root behavior, which may be expanded with labo- ratory and field studies. • Understanding of how soils develop on overdisturbed sites. • The carbon sequestration potential of vegetative solu- tions and the contribution of these techniques to soil and water conservation. • Highway slopes viewed as part of an ecosystem that may require restoration based on the need for increased safety, stability, and/or maintaining roadside ecology. • Compatibility between mechanical and vegetative components of slope stabilization techniques. • Implementation of site-specific warning systems for domestic and international travelers. Training and Resources • Greater dissemination of information and training. • Development of a single source of good information— a one-stop-shop toolbox and/or a glove box field guide. • Definitive standards or specifications for civil engi- neers who have little knowledge and training in soil science and plant science. • Solution tailoring for specific sites. Too often, the cho- sen technique is based on a narrow field of candidates and without considering all possible alternatives. • More widespread practice of proper soil analyses in the planning stages of projects. • Mandates for the inclusion of erosion control and slope stabilization on all projects. • Implementation of site-specific warning systems for domestic and international travelers. TABLE 9 SUMMARY OF EARTHWORK TECHNIQUES Treatment Component Stabilization Method or Depth Pros Cons Benched Slopes Benches, terraces, steps, serrations Surface to shallow; reduce surface water velocity by breaking up the slope Shallow failures are limited to one bench at a time; reduce suspended solids in runoff Do not work well on decom- posed granite or slopes with high water tables; maintenance may include removal of accu- mulated sediment Soil Roughening Microtopography, rip- ping, extreme roughening Surface to shallow; reduces surface water velocity by breaking up the slope Increases infiltration rates; reduces sediment loss Temporary; requires the use of heavy equipment; does not work well on rocky slopes; regrading may be necessary if heavy precipitation occurs Flattening Oversteepened Slopes Shallow to deep Increases slope stability Additional right-of-way may be required; need to find a location for soil disposal; need to determine a practical place to start excavating Landforming or Geomorphic Modification Shallow to deep Slopes less likely to erode; overall slopes are more stable; can be used over large or small areas Requires the use of heavy equipment