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This user guide provides simple, cost-effective methods for stabilizing locally maintained slopes along roadways in. Minnesota. Eight slope stabilization ...
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Minnesota Department of Transportation Research Services & Library 395 John Ireland Boulevard , MS 330 St. Paul, Minnesota 55155-
Report Number: MN/RC 2017-17G Date Published: June 2017
Technical Report Documentation Page
OVERVIEW OF SLOPE FAILURE Slope stability is quantified by a factor of safety—the ratio of the soil’s in situ shear strength to the shear strength required for equilibrium along a given potential failure surface. To stabilize a slope, the factor of safety must be increased, either by introducing stabilizing forces (increasing capacity) or limiting driving forces (decreasing demand). Slopes can be stabilized by adding a surface cover to the slope, excavating and changing (or regrading) the slope geometry, adding support structures to reinforce the slope or using drainage to control the groundwater in slope material. Three site conditions should be considered when choosing an appropriate method for stabilizing a slope: Type of slope failure Type of soil Presence of groundwater (poor drainage) Slope failure is generally classified as either a rotational slide or a surficial soil creep failure. Rotational slide failures generally occur in a circular pattern and typically leave behind exposed soil. In some soil types, cracking at the surface can indicate the slope is nearing a rotational slide failure. Creep failures are slow‐moving, soil surface failures where slope material gradually moves downhill. Common causes of creep failure are seasonal freeze‐thaw cycles and inadequate shear strength properties in soil. Bent trees or signs can indicate creep failure. Examples of common slope failure types (Source: Varnes, D.J. (1978). Slope movement types and processes. Special Report 176: Landslides: Analysis and Control. Washington, D.C.: National Research Council)
Two soil types were considered in this study: cohesive (such as silt and clay) and granular (sand) soils. These soil types can usually be distinguished by a visual inspection, but sometimes laboratory testing is required. In general, slopes made of granular or sandy soil are less likely to experience deep rotational slides. Slopes made of cohesive soils like clay and silt usually have more drainage concerns and are more susceptible to seasonal frost heave. The third major site condition that affects a slope is poor drainage. Drainage is considered poor if groundwater lowers soil shear strength and leads to failure. Water negatively affects soil’s ability to resist shearing, leading to slope instability. An increase in soil’s pore pressure (due to the presence of water) leads to a decrease in effective stress. Because effective stress governs soil strength and deformation characteristics, the presence of water leads to decreased soil shear strength. Groundwater has a significant effect on shear strength. In the research study, removing groundwater provided the greatest difference in the output factor of safety.
Flowchart for slope failure scenarios To use the flowchart to determine the appropriate scenario, users: First, determine the failure type (rotational or creep). Next, choose the soil type of the slope material (cohesive or granular). Finally, determine whether groundwater is present at the site. (Note: “Poor drainage” is interchangeable with “groundwater concerns.”) Descriptions about each of the scenarios, including site conditions and recommended repair techniques to stabilize the slope, are provided in this guide.
SLOPE STABILIZATION RECOMMENDATIONS The following eight scenarios represent site characteristics commonly found in slope failure situations. Each scenario includes the main identifying features of the slope, a recommended stabilization approach and next steps for making the repair. To determine the scenario that best describes a failed slope, use the flowchart above to identify the set of conditions that most closely match the observed slope stabilization site. Then locate the appropriate scenario in the following descriptions to find the recommended stabilization techniques.
Scenario 2: Olmsted County, Minnesota, site Site Conditions Rotational failure Cohesive soil No groundwater concerns Recommended Stabilization Approach: Remove and replace, or regrade and recompact. Add vegetative cover. Rotational failure is visible at these sites. Many factors other than the effects of groundwater can cause soil to lose strength, such as poor compaction. Regrading and recompacting the slope properly will increase soil strength and slope stability. Evaluate the in situ soil properties and either reuse the material or use common^ borrow^ if^ native^ material^ has^ poor^ properties.
Rotational failure in sand, similar to Scenario 3 Soil Conditions Rotational failure Granular soil Groundwater concerns Recommended Stabilization Approach: Remove and replace, or regrade and recompact. Add drainage features and adequate surface cover. As with other rotational failures, excavation and reconstruction is necessary. Surface cover is very important for slopes with granular soil because erosion is a concern. Surface erosion can cause geometric inconsistencies that lead to failure. Erosion can often cause washout failure. Regrade or, if necessary, replace with sand fill. Add drainage features to remove groundwater in the slope.
Scenario 5: Koochiching County, Minnesota, site Site Conditions Creep failure Cohesive soil Groundwater concerns Recommended Stabilization Approach: Regrade and recompact. Add drainage features. If one area of failure, remove and replace. Sites with cohesive soils are more likely to have drainage concerns. Surficial creep failure can be identified by bent signs or trees that lead to pavement damage. With the presence of groundwater and frost‐susceptible cohesive soil, frost heave is a possible cause of soil movement. Add drainage features. If creep is at the top of the slope, also consider replacing that portion of the slope with free‐draining sand. If the failure is near the bottom of the slope, use a buttress to stabilize the slope.
Scenario 6: Murray County, Minnesota, site Soil Conditions Creep failure Cohesive soil No groundwater concerns Recommended Stabilization Approach: Remove, replace and recompact. Surficial creep failure can be identified by bent signs or trees that lead to pavement damage. The image above clearly shows how soil creep at the top of a slope can lead to pavement damage. Replace the failed portion of the slope with sand fill to increase sliding resistance. In the absence of groundwater, poor compaction decreases the soil’s shear strength. If in situ soil has adequate strength properties, consider regrading and recompacting, but keep in mind that creep failure indicates concerns about the strength of native material.
Soil creep in sand, similar to Scenario 8 Soil Conditions Creep failure Granular soil No groundwater concerns Recommended Stabilization Approach: Remove and replace, or regrade and recompact. Add adequate surface cover. Erosion is a concern with granular soils. Surficial damage caused by erosion is not always soil creep, but the movement type and stabilization methods are similar. Surface washout can undermine roadways and cause pavement damage. Ensure adequate groundcover on slopes with granular fill. Repair damage at the top of a slope by regrading.
RECOMMENDED RESOURCES The resources listed below provide more information about the stabilization methods presented in this guide. Users are encouraged to consult these resources before selecting a stabilization method. Drainage Features Cornforth, D. (2005). “Dewatering Systems,” chapter 17 in Landslides in Practice: Investigations, Analysis, and Remedial/Preventative Options in Soils. Hoboken, N.J.: John Wiley & Sons. Dewatering Coduto, D., Yeung, M., Kitch, W. (2011). “Rate of Consolidation,” chapter 11 in Geotechnical Engineering: Principles and Practices (2nd ed.). Upper Saddle River, N.J.: Pearson Education, Inc. Vegetative Cover Abramson, L. W., Lee, T., Sharma, S., Boyce, G. (2002). “Slope Stabilization Methods,” chapter 7 in Slope Stability and Stabilization Methods (2nd ed.). New York: Wiley. Buttressing/Riprap Cover Abramson, L. W., Lee, T., Sharma, S., Boyce, G. (2002). “Slope Stabilization Methods,” chapter 7 in Slope Stability and Stabilization Methods (2nd ed.). New York: Wiley. Geosynthetics Gee, B. (2015). Geosynthetic materials help build optimized infrastructure. Geostrata, 19 (2), 50. Lightweight Fill Abramson, L. W., Lee, T., Sharma, S., Boyce, G. (2002). “Slope Stabilization Methods,” chapter 7 in Slope Stability and Stabilization Methods (2nd ed.). New York: Wiley. Remove and Replace Duncan, J. M., Wright, S. (2005). “Slope Stabilization and Repair,” chapter 16 in Soil Strength and Slope Stability. Hoboken, N.J.: John Wiley & Sons. Regrading and Benching Cornforth, D. (2005). “Earthworks,” chapter 15 in Landslides in Practice: Investigations, Analysis, and Remedial/Preventative Options in Soils. Hoboken, N.J.: John Wiley & Sons. Retaining Walls Cornforth, D. (2005). “Retaining Walls,” chapter 19 in Landslides in Practice: Investigations, Analysis, and Remedial/Preventative Options in Soils. Hoboken, N.J.: John Wiley & Sons.
