Slope Stability Reference Guide

AbstractSlope stability studies in the USDA Forest Service have been conducted successfullyin accordance with a three-level concept.Level ILevel I, the most general, is conducted for watershed analyses, ecosystemmanagement support, and timber sale area planning. Level I slope stability studiesinclude reviews of air photo coverage and geologic and geotechnical reports; a briefreconnaissance to verify slope processes and to map soil and rock units andlandforms; and development of geomorphic zones based on slope form, soil and rockcharacteristics, and geologic processes. Each geomorphic zone is assigned soil androck units for analysis and evaluation for potential failure due to natural processesand forest management activities. The LISA (Level I Stability Analysis) and DLISA(Deterministic Level I Stability Analysis) programs are often used in the process.Level IILevel 11, an intermediate level used for evaluation of slope stability along roadcorridors and other route studies, consists of the office elements of a level I study, areconnaissance (including sketches of slope characteristics of each alternativetransportation corridor), and defining geomorphic and road design segments based onslope, rock, soil, and drainage characteristics and geologic processes. These analysesare often completed using slope stability charts and the DSARA (DeterministicStability Analysis for Road Access) slope stability program. The probabalistic SARA(Stability Analysis for Road Access) program is still under development.Level IllLevel 111, the most detailed (site-specific) level, is intended for design of stabilizationmeasures. In addition to the elements identified under level I1 for the transportationroute evaluation, the level I11 study includes measurement of field-developed crosssections and installation of site-specific monitoring instrumentation, as appropriate.Level 111 analyses are often completed using the XSTABL (method of slices)interactive program for soil and the Federal Highway Administration's rock slopestability analysis method for rock slopes.In addition to the three-level method, this guide discusses the forest planning process,geomorphology, channel processes and sediment budgets, climatology, soil and rockmechanics, hydrogeology, exploration and testing methods, and risk analysis as theyapply to slope stability. The guide also contains sample problems for analysis,remediation design, and specifications for construction.Abstract

Table of ContentsPageVOLUME I.Dedication to Rod Prellwitz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii.Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .viiAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix.Section I . Introduction and Suggestions for Use1A GuidetotheGuide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1B Introduction to the Three-Level Stability Analysis Concept . . . . . . . . . . . . . . . . . . . . . . . .9.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Section 2. Initial Slope Stability Assessment in Resource Planning2A2B2C2D2E2FIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33.Climatic. Geologic. and Hydrologic Influences on Slope Stability . . . . . . . . . . . . . . . . . . . 39Basin-Scale Assessment of Geologic Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47Considerations for Rock Slope Stability in Reconnaissance-Level Projects . . . . . . . . . . . . . 53Cumulative Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Watershed Analysis Case Histories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77.Section 3. Site Investigation3A3B3C3D3EProblem Definition and Application of the Scientific Method . . . . . . . . . . . . . . . . . . . . . 131Soil and Rock Classification for Engineering Analysis and Design . . . . . . . . . . . . . . . . . 135Surface Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139.Subsurface Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .177.Geotechnical Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .205.Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-1Section 4. Parameters for Stability Analysis4A484C4D4E4FFundamental Stress-Strain Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .331Soil WeightNolume Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .345.Strength and Behavior of Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .377.Strength and Behavior of Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427Ground Water Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.Root Strength and Tree Surcharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .543.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .551Table of Contents

Table of Contents (continued)PageSection 5. Slope Stabil@ Analysis.5A Fundamentals and Derivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .563I Analysis-Natural Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .583.5C Soil Slopes-Level 11 Analysis onstructedSlopes . . . . . . . . . . . . . . . . . . . . . . . . . . 5955D Soil Slopes-Transition From Deterministic to Probabilistic Analyses . . . . . . . . . . . . . . . .633.5E Soil Slopes-Level I and I1 Probabilistic Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.SF Soil Slopes-Level I11 Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6455G Soil SlopesSample Problem Including All Three Levels of Analysis . . . . . . . . . . . . . . .663.5H Rock Slopes-Fundamentals and Sample Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-15B Soil Slopes-LevelVOLUME III.Section 6 n Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .733.Modification of Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737.Surface and Subsurface Drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .767.Horizontal Drains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783.Buttresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803.Soil Slope Stabilization-Reinforced Fills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 817Shear Trenches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 841.Rock Slope Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .851Comparison of Alternatives and Decision Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .861.Construction Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 897Post-Construction Monitoring of the Technical Structures and Projects . . . . . . . . . . . . . . .913.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 927Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1.Table of Contents

SECTION 1INTRODUCTION AND SUGGESTIONS FOR USEPrincipal contributors:Joe Bailey, Engineering Geologist (Section Leader)USDA Forest ServiceRegional Office EngineeringP.O. Box 3623Portland, OR 97208Tom Koler, Research Engineering GeologistUSDA Forest ServiceIntermountain Research Station1221 S. MainMoscow, ID 83843Ken Neal, Engineering GeologistKenneth Neal & Associates2014 Baker TerraceOlympia, WA 98501Rod Prellwitz, Geotechnical EngineerUSDA Forest ServiceIntermountain Research Station1221 S. MainMoscow, ID 83843

Page1A GuidetotheGuide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.lA.l Purpose of the Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1A.2 Contents and Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1A.3 Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.1A.3.1 Land Managers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.1A.3.2 GeologistslEngineering Geologists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71A.3.3 Geotechnical Engineers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.1A.3.4 Non-Geotechnical Specialists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1B Introduction to the Three-Level Stability Analysis Concept . . . . . . . . . . . . . . . . . . . . . . . .9lB.l Role of Analysis in the Decision-Making Process . . . . . . . . . . . . . . . . . . . . . . . . . 9.1B.2 Land Management Decisions Requiring Stability Input . . . . . . . . . . . . . . . . . . . . . .91B.2.1 Level I: Resource Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1B.2.2 Level 11: Project Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111B.2.3 Level 111: Site Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111B.3 Stability Analysis Applicable at the Three Levels . . . . . . . . . . . . . . . . . . . . . . . . . 111B.3.1 Level I: Infinite Slope Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .121B.3.2 Level 11: Stability Number Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . .121B.3.3 Level 111: Method of Slices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13Appendix1.1 Introduction to the Three-Level Stability Analysis Concept-Chestershireand Backdrop Timber Sales: Case Histories of the Practice of EngineeringGeology in the Olympic National Forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

1A. Guide to the GuideDaniel Joe Bailey, Regional Engineering Geologist, Pacijc Northwest RegionThomas E. Koler, Research Engineering Geologist, Intermountain Research Station1A.1 Purposeof the GuideAll geotechnical specialists experience, at some time in their careers, the problem ofhow to provide an appropriate, accurate, understandable answer to a question posedby a land manager or project engineer. The specialist realizes that the managerprobably has little knowledge of the scientific complexity involved in answering aseemingly simple geotechnical question. Of several legitimate answers, which is"correct"? The anxiety level rises if a geotechnical mentor is not readily available toguide the specialist through the process of providing the correct answer. Of severalpossible ways to begin gathering and analyzing data, which is the most valid? Themost cost effective? The most truly useful? We have all gone through thisexperience, and continue to do so. Therefore, we have compiled this slope stabilityreference guide to help us and those who work with us to better understand howgeotechnical specialists get work done. We have included real-world examples ofcommon geotechnical problems, along with the reasons behind, and the methods of,the investigative techniques employed. The purpose of this guide is to provide apractical document that geologists and engineers can use in their work. We hope thatyour copy will become tattered and dog-eared as you use it to provide these answers.1A.2 ContentsandOrganizationThis guide can be viewed either as a textbook describing concepts and theories or asa laboratory workbook with practical problems. Each section contains both theoryand practical examples; we strongly recommend that both be understood and used.A knowledge of the theory behind the methods employed in a particular examplehelps you to recognize the differences between that example and your specific realworld problem. However, knowing the theories will not help much unless yourecognize which are relevant to your particular problem.Section 1 contains the introduction to the guide and an introduction to the three-levelstability analysis concept.Section 2 discusses the role of stability analysis in cumulative effects analysis andempirical analysis of an area.Sections 3 and 4 describe the concepts central to the analysis of slope stabilityaffecting both natural and constructed slopes.Sections 5 and 6 give detailed presentations on application of the three-level systemto typical field problems. Section 5 emphasizes analysis of slope stability, whilesection 6 emphasizes methods of stabilization.

Figure lA.l is an idealized three-level slope. stability analysis flowchart. This chartwill appear in the introduction to each section with the area of analysis discussed inthat section highlighted.LEVEL IllANALYSISILEVEL IllDATA BASESITE-Figure 1 A . I . S l o p e stabiliry analysisflowchart.We hope you find the guide to be useful. If you find portions of it unclear,debatable, or in error, we would like to hear from you. Names and addresses of theauthors for each section are listed at the beginning of the section.1A.3Suggested Use1A.3.1 LandManagersThe first two sections of the guide will serve land managers' needs by introducingthem to the three-level system and its use in resource planning. The remainder ofsection 1 describes the three-level system, its general methods of analysis, and itslimitations. In section 2 we discuss the use of slope stability assessment in resourceplanning. After reviewing these two sections, the manager should be able tocommunicate better with-and understand the degree of confidence communicatedby-the investigator.The details of investigative and analysis techniques (sections 3-5) may not be ofinterest to the land manager, but the methods of stabilization in section 6 may help inunderstanding alternative solutions.

1A.3.2 Geologists/EngineeringGeologistsGeologists and engineering geologists will have use for the entire design guide, butwill probably find the techniques for level I and I1 analyses the most useful. Thesetwo levels, for reconnaissance and project planning, respectively, describe a methodof predicting slope instability over large areas.Reconnaissance-level assessment refers to the analysis of large areas for relativelandslide-hazard potential. This level of analysis is used for input to forest planning,timber sale and resource allocations, environmental assessment reports, andtransportation planning. The LISA (Level I Stability Analysis Ver. 2.0 1991)computer model discussed in the guide can be used to delineate areas susceptible tolandslides on a broad scale to alert land managers to those areas where the risk isgreatest (Hammond et al., 1992).Project planning refers to the analysis of designs proposed to implement amanagement objective. For example, a proposed road corridor is analyzed forpreconstruction and postconstruction slope stability. This also includes a sensitivityanalysis for individual proposed road design segments that cross potentially unstableareas identified in LISA. DLISA (Deterministic Level I Stability Analysis Ver. 1.021991) works well for sensitivity analysis. An evaluation of road prism conditions(e.g., cut slope heights and angles, fill slope angles and depths, fill compaction, anddrainage) can be completed with the results from the sensitivity analysis. A level I1computer program, SARA (Stability Analysis for Road Access), is being developedand DSARA (Deterministic Stability Analysis for Road Access) is being perfected atthe Intermountain Research Station.Experienced geologists and engineering geologists will find level I and I1 analysesuseful when a proposed project may change soil physical properties, slopeconfiguration, or surface and subsurface water flow. Experienced engineeringgeologists understand the basic concepts contained in the guide. Those geologistswith little experience in the engineering arena may not be familiar with the analyticaltechniques used. Those freshly out of school with degrees in classical geologyshould seek additional training to gain a working knowledge in soil mechanics,strength of materials, and basic road design. For the less experienced practitioner, amentor assisting and verifying input data will help ensure accurate results.1A.3.3GeotechnicalEngineersGeotechnical engineers will have use for the entire guide, but will probably find thesections explaining design applications for level I1 (project planning analysis) andlevel I11 (site-specific analysis) the most useful. During a project design, efforts aregenerally made to minimize cost, which usually translates into the minimumfunctional design. However, site constraints or aesthetic considerations may require adeviation from the standard textbook procedure. This guide shows how to calculatefactors of safety for these minimal or unique solutions. It is also useful in answeringdesign questions where standard designs are inappropriate.During failure analysis, the guide is useful in leading one through the backcalculation process. By back-calculation, the failure parameters can be deduced.Designing a repair is then a matter of bringing the factor of safety up to a desiredlevel by buttressing, drainage, or other means. Usually, several alternatives forfailure correction are possible. The guide shows ways to analyze these alternativerepairs and calculate their factors of safety. Coupled with cost data, thesealternatives can be presented to management.

Geotechnical engineers understand the concepts and analysis techniques contained inthe guide. Most civil engineers are familiar with these methods. Those who havenot worked a number of these problems in the past may want a peer review of theirmethods and nnel without formal geotechnical training-such as many hydrologists, soilscientists, foresters, and other specialists-may use the guide to gain anunderstanding of the types of slope stability problems that geotechnical personnel areable to analyze and solve. Non-geotechnical specialists having extensive exposure toand experience with the underlying concepts and analytical techniques of the guidewill find it useful in the understanding of slope stability problems.The reader must bear in mind that an understanding of the slope-movement processesbeing modeled, as well as how the model represents the processes, is nee

slope, rock, soil, and drainage characteristics and geologic processes. These analyses are often completed using slope stability charts and the DSARA (Deterministic Stability Analysis for Road Access) slope stability program. The probabalistic SARA (Stability Analysis for Road Access) program is still under development.