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Design Guidelines for Horizontal Drains Used for Slope Stabilization

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Design Guidelines for Horizontal Drains Used for Slope Stabilization Design Guidelines for Horizontal Drains used for Slope Stabilization WA-RD 787.1 Greg M. Pohll March 2013 Rosemary W.H. Carroll Donald M. Reeves Rishi Parashar Balasingam Muhunthan Sutharsan Thiyagarjah Tom Badger Steve Lowell Kim A. Willoughby WSDOT Research Report Office of Research & Library Services Design Guidelines for Horizontal Drains used for Slope Stabilization Greg M. Pohll, DRI Rosemary W.H. Carroll, DRI Donald M. Reeves, DRI Rishi Parashar, DRI Balasingam Muhunthan, WSU Sutharsan Thiyagarjah, WSU Tom Badger, WSDOT Steve Lowell, WSDOT Kim A. Willoughby, WSDOT 1. REPORT NO. 2. GOVERNMENT ACCESSION 3. RECIPIENTS CATALOG NO NO. WA-RD 787.1 4. TITLE AND SUBTITLE 5. REPORT DATE Design Guidelines for Horizontal Drains used for Slope March 2013 Stabilization 6. PERFORMING ORGANIZATION CODE 7. AUTHOR(S) 8. PERFORMING ORGANIZATION REPORT NO. Greg M. Pohll, Rosemary W.H. Carroll, Donald M. Reeves, Rishi Parashar,Balasingam Muhunthan,Sutharsan Thiyagarjah, Tom Badger, Steve Lowell, Kim A. Willoughby 9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. WORK UNIT NO. Desert Research Institute 2215 Raggio Parkway 11. CONTRACT OR GRANT NO. Reno, NV 89512 GCA6381 12. CO-SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED Washington State Department of Transportation Final Report 310 Maple Park Ave SE 14. SPONSORING AGENCY CODE Olympia, WA 98504-7372 Research Manager: Kim Willoughby 360.705.7978 15. SUPPLEMENTARY NOTES This study was conducted in cooperation with the U.S. Department of Transportation, Federal Highway Administration through the pooled fund, TPF-5(151) in cooperation with the state DOT’s of California, Maryland, Mississippi, Montana, New Hampshire, Ohio, Pennsylvania, Texas, Washington and Wyoming. 16. ABSTRACT The presence of water is one of the most critical factors contributing to the instability of hillslopes. A common solution to stabilize hillslopes is installation of horizontal drains to decrease the elevation of the water table surface. Lowering the water table dries a large portion of the hillslope which increases the shear strength of the soil, thereby decreasing the probability of slope failure. The purpose of this manual is to provide a single comprehensive reference for geotechnical engineers and hydrogeologists on designing horizontal drainage systems to improve slope stability. Guidelines are provided for translational and rotational failure and consider fractured systems. Basics of hydrogeologic and geotechnical terminology, site characterization and conceptualization, groundwater modeling techniques and template projects help to guide the user with respect to identifying important parameters to drainage design. An iterative approach is presented for determining the minimum drain construction to lower water levels enough to keep the factor of safety (FOS) greater than 1.2. 17. KEY WORDS 18. DISTRIBUTION STATEMENT Subsurface drainage, slope stabilization, horizontal drains 19. SECURITY CLASSIF. (of this report) 20. SECURITY CLASSIF. (of this page) 21. NO. OF PAGES 22. PRICE None None 377 ii ACKNOWLEDGMENTS Funding for this project was obtained from the Washington State Department of Transportation through the Transportation Pooled Fund, TPF-5(151). The partner states of TPF-5(151) included: California, Maryland, Mississippi, Montana, New Hampshire, Ohio, Pennsylvania, Texas, Washington and Wyoming. The DRI contract was 650-646-0250. Project oversight and data transfer was facilitated by Tom Badger, Steve Lowell and Kim Willoughby with the Washington State Department of Transportation (WSDOT). Geotechnical modeling was accomplished by Dr. Balasingam Muhunthan and doctoral student Sutharsan Thiyagarjah from Washington State University (WSU). The Desert Research Institute (DRI) performed all hydrologic analysis. Dr. Greg Pohll acted as project manager, Dr. Rosemary Carroll performed all modeling and Dr. Donald Reeves and Dr. Rishi Parashar conducted all fracture analysis. DISCLAIMER The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Washington State Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Authorship of the document: Chapter 1 – Greg Pohll Chapter 2 – Balasingam Muhunthan Sutharsan Thiyagarjah Chapter 3 – Rosemary Carroll Chapter 4 – Rosemary Carroll Donald M. Reeves Rishi Parashar Balasingam Muhunthan Sutharsan Thiyagarjah Chapter 5 – Rosemary Carroll Chapter 6 – Rosemary Carroll Chapter 7 – Rosemary Carroll Balasingam Muhunthan Sutharsan Thiyagarjah Chapter 8 – Donald M. Reeves Rishi Parashar Chapter 9 – Rosemary Carroll Appendices A-D – Rosemary Carroll iii EXECUTIVE SUMMARY The presence of water is one of the most critical factors contributing to the instability of hillslopes. A common solution to stabilize hillslopes is installation of horizontal drains to decrease the elevation of the water table surface. Lowering the water table dries a large portion of the hillslope which increases the shear strength of the soil, thereby decreasing the probability of slope failure. The purpose of this manual is to provide a single comprehensive reference for geotechnical engineers and hydrogeologists on designing horizontal drainage systems to improve slope stability. Guidelines are provided for translational and rotational failure and consider fractured systems. Basics of hydrogeologic and geotechnical terminology, site characterization and conceptualization, groundwater modeling techniques and template projects help to guide the user with respect to identifying important parameters to drainage design. An iterative approach is presented for determining the minimum drain construction to lower water levels enough to keep the factor of safety (FOS) greater than 1.2. Simple systems may only require an analytic approach to computing maximum water levels. Techniques are supplied for steady state conditions given a flat surface, a sloped surface less than 10° and a discussion is provided on the influence of recharge and hydraulic conductivity on drainage design. Analytic equations for transient solutions for a flat surface are given for different drain depths with respect to the impermeable barrier as well as for sloping surface with a declining water table over time. Past research has found that flow to ditches; water table elevation and the rate of water table decline were independent of slope for slopes less than 15-30%. For these relatively shallower slopes, flat- surfaced assumptions can be maintained with little error. In all cases drainage design based on analytic (and graphical) approaches is focused only on drain spacing or the location of the first interceptor drain in a sloped system. However, analytic results can be used to assess impact of system response to lowering the overall water table prior to a rapid rise caused by a large storm event. Irregular drain networks, heterogeneous or anisotropic aquifer conditions, complex slope geometry, a rapid rise in pore pressures, as well as fractured rock network may mandate a numeric modeling approach. As a general rule, drains installed a significant distance into the hillslide at the lowest possible elevation, will capture the majority of groundwater and have the largest effect on lowering the water table. Drains located in the upper region of a slope are found to have no real significance if additional deeper drains are in the lower part of a slope as the water table will eventually be reduced to the lowest drain level and any drains above the lowest-most drain will no longer be effective. The only exception to this rule might be for site conditions that have the ability to setup significant perched water table conditions. Translational failure of thin geologic sections is found more sensitive to water level increases in the upper slope compared to groundwater seepage in the lower slope. In contrast, rotational failure in the slope toe is susceptible to rising pore pressers in the lower slope region. In both cases, toe drains should be installed, with length and density of drain network increasing with decreasing hydraulic conductivity and storage and with increasing anisotropy. Horizontal drains may be ineffectual at promoting slope stability in low conductive soils with low storage. The ability to stabilize slopes with horizontal drains declines for all soil types with increased anisotropy. iv Table of Contents Chapter 1: Introduction 1.1 Problem Statement ...................................................................................................................... 1 1.2 Background .................................................................................................................................. 2 1.3 Objective of Manual..................................................................................................................... 5 1.4 References Cited .......................................................................................................................... 6 Chapter 2: Slope Stability Analysis 2.1 Introduction ............................................................................................................................... 10 2.2 Factor of Safety .......................................................................................................................... 11 2.3 Methods of Slope Stability Analysis ...........................................................................................
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