Download Slope Stabilization Guide and more Exams Engineering in PDF only on Docsity! Minnesota Department of Transportation Research Services & Library 395 John Ireland Boulevard , MS 330 St. Paul, Minnesota 55155-1899 Authors: David Saftner, Carlos Carranza-Torres and Mitchell Nelson Report Number: MN/RC 2017-17G Date Published: June 2017 Slope Stabilization Guide for Minnesota Local Government Engineers Technical Report Documentation Page 1. Report No. 2. 3. Recipients Accession No. MN/RC 2017‐17G 4. Title and Subtitle 5. Report Date Slope Stabilization Guide for Minnesota Local Government Engineers June 2017 6. 7. Author(s) 8. Performing Organization Report No. David Saftner, Carlos Carranza‐Torres and Mitchell Nelson 9. Performing Organization Name and Address 10. Project/Task/Work Unit No. Department of Civil Engineering University of Minnesota Duluth 1405 University Drive Duluth, Minnesota 55812 CTS #2016011 11. Contract (C) or Grant (G) No. (c) 99008 (wo) 190 12. Sponsoring Organization Name and Address 13. Type of Report and Period Covered Minnesota Local Road Research Board Minnesota Department of Transportation Research Services & Library 395 John Ireland Boulevard, MS 330 St. Paul, Minnesota 55155‐1899 Manual: September 2015 – June 2017 14. Sponsoring Agency Code 15. Supplementary Notes http://mndot.gov/research/reports/2017/201717G.pdf, http://mndot.gov/research/reports/2017/201717.pdf 16. Abstract (Limit: 250 words) This user guide provides simple, cost‐effective methods for stabilizing locally maintained slopes along roadways in Minnesota. Eight slope stabilization techniques are presented that local government engineers can undertake using locally available materials and equipment. These methods are the result of a research effort that analyzed recent slope failures in Minnesota. The recommendations are based on input from Minnesota county engineers; case studies from site investigations within the state; and slope stability analysis, including limit equilibrium methods. This guide is based on information provided in Slope Stabilization and Repair Solutions for Local Government Engineers, which presents the results of a Minnesota Local Road Research Board research project on slope stabilization methods. Detailed information about the research project along with complete descriptions of the field sites is available in the report. Local government engineers are encouraged to reference the report when using this guide. 17. Document Analysis/Descriptors 18. Availability Statement Slope stability, slope failure, rotational failure, creep, erosion, cohesive soil, granular soil, groundwater, geotechnical engineering No restrictions. Document available from: National Technical Information Services, Alexandria, Virginia 22312 19. Security Class (this report) 20. Security Class (this page) 21. No. of Pages 22. Price Unclassified Unclassified 18 3 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. 4 SLOPE FAILURE SCENARIOS Eight possible scenarios have been developed for slope failure in Minnesota. These scenarios are based on a combination of the three site characteristics: slope failure type, soil type and presence of groundwater. The scenarios and their corresponding site conditions are presented in Table 3.1. Summary of slope failure scenarios Failure Type Soil Type Groundwater Concerns? Scenario 1 Rotational Slide Cohesive Yes Scenario 2 Rotational Slide Cohesive No Scenario 3 Rotational Slide Granular Yes Scenario 4 Rotational Slide Granular No Scenario 5 Surficial Creep Cohesive Yes Scenario 6 Surficial Creep Cohesive No Scenario 7 Surficial Creep Granular Yes Scenario 8 Surficial Creep Granular No The flowchart below illustrates the process for selecting the appropriate scenario. By choosing the site conditions that most closely match the slope failure, users can quickly determine the stabilization method needed to repair the failed slope. 5 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. 8 SCENARIO 2: ROTATIONAL FAILURE, COHESIVE SOIL 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. 9 SCENARIO 3: ROTATIONAL FAILURE, GRANULAR SOIL, POOR DRAINAGE 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. 10 SCENARIO 4: ROTATIONAL FAILURE, GRANULAR SOIL Scenario 4: Lac Qui Parle County, Minnesota, site Site Conditions Rotational failure Granular soil No groundwater concerns Recommended Stabilization Approach: Regrade and recompact. Add vegetative cover or more involved surface cover. Because groundwater is not the primary reason for failure, identify and mitigate the main cause of the soil losing strength. If erosion is evident, consider using a more involved cover, such as riprap or gravel. If slope steepness is a concern, regrade and compact with in situ material. Also, consider using adequate groundcover to protect the slope from erosion damage. 13 SCENARIO 7: CREEP FAILURE, GRANULAR SOIL, POOR DRAINAGE Scenario 7: Carver County, Minnesota, site Soil Conditions Creep failure Granular soil Groundwater concerns Recommended Stabilization Approach: Remove and replace, or regrade and recompact. Add drainage features and adequate surface cover. Bent guardrails are evidence of soil creep, which typically causes pavement damage. Proper drainage can remove groundwater from the area, increasing resistance to soil creep. Install drainage features, and replace failed soil with properly compacted fill or recompact in situ material. Use adequate groundcover to prevent erosion in slopes with sand. 14 SCENARIO 8: CREEP FAILURE, GRANULAR SOIL 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. 15 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.