Soil Strength and Slope Stability

1 soil Strength andSlope StabilityJ. Michael DuncanStephen G. WrightWILEYJOHN WILEY & SONS, FA 411 PAGE 001 CHAPTERCHAPTERCHAPTERCHAPTERCHAPTERCHAPT ERCONTENTS12456 PrefaceINTRODUCTIONEXAMPLES AND CAUSES OF Slope FAILUREE xamples of Slope FailureCauses of Slope FailureSummarySOIL MECHANICS PRINCIPLESD rained and Undrained ConditionsTotal and Effective StressesDrained and Undrained Shear StrengthsBasic Requirements for Slope Stability AnalysesSTABILITY CONDITIONS FOR ANALYSESEnd-of-Construction StabilityLong-Term StabilityRapid (Sudden) DrawdownEarthquakePartial Consolidation and Staged ConstructionOther Loading ConditionsSHEAR STRENGTHS OF soil ANDMUNICIPAL SOLID WASTEG ranular MaterialsSiltsClaysMunicipal Solid WasteMECHANICS OF LIMIT EQUILIBRIUMPROCEDURESD efinition of the Factor of SafetyEquilibrium ConditionsSingle Free-Body ProceduresProcedures of Slices: Generalix5514171919212226313132323333333 5354044545555565763vTJ FA 411 PAGE 002viCONTENTSCHAPTERCHAPTERCHAPTERCHAPTE RCHAPTER7891011 Procedures of Slices: Circular Slip SurfacesProcedures of Slices.

2 Noncircular Slip SurfacesAssumptions, Equilibrium Equations, and UnknownsRepresentation of Interslice Forces (Side Forces)Computations with Anisotropic Shear StrengthsComputations with Curved Failure Envelopes andAnisotropic Shear StrengthsAlternative Definitions of the Factor of SafetyPore Water Pressure RepresentationMETHODS OF ANALYZING Slope STABILITYS imple Methods of AnalysisSlope Stability ChartsSpreadsheet SoftwareComputer ProgramsVerification of AnalysesExamples for Verification of Stability ComputationsREINFORCED SLOPES AND EMBANKMENTSL imit Equilibrium Analyses with Reinforcing ForcesFactors of Safety for Reinforcing Forces and soil StrengthsTypes of ReinforcementReinforcement ForcesAllowable Reinforcement Forces and Factors of SafetyOrientation of Reinforcement ForcesReinforced Slopes on Firm FoundationsEmbankments on Weak FoundationsANALYSES FOR RAPID DRAWDOWND rawdown during and at the End of ConstructionDrawdown for Long-Term ConditionsPartial DrainageSEISMIC Slope

3 STABILITYA nalysis ProceduresPseudostatic Screening AnalysesDetermining Peak AccelerationsShear Strength for Pseudostatic AnalysesPostearthquake Stability AnalysesANALYSES OF EMBANKMENTS WITH PARTIALCONSOLIDATION OF WEAK FOUNDATIONSC onsolidation during ConstructionAnalyses of Stability with Partial Consolidation63718383909O919510310310510 7107111112137137137139139141142142145151 151151160161161164165166169175175176TJ FA 411 PAGE 003 CHAPTERCHAPTERCHAPTERCHAPTERCHAPTER12131 41516 CONTENTSviiObserved Behavior of an Embankment Constructed in Stages178 Discussion179 ANALYSES TO BACK-CALCULATE STRENGTHSBack-Calculating Average Shear StrengthBack-Calculating Shear Strength Parameters Based on SlipSurface GeometryExamples of Back-Analyses of Failed SlopesPractical Problems and Limitation of Back-AnalysesOther UncertaintiesFACTORS OF SAFETY AND RELIABILITYD efinitions of Factor of SafetyFactor of Safety CriteriaReliability and Probability of FailureStandard Deviations and Coefficients of VariationCoefficient of Variation of Factor of SafetyReliability IndexProbability of FailureIMPORTANT DETAILS OF Stability ANALYSESL ocation of Critical Slip SurfacesExamination of Noncritical Shear SurfacesTension in the Active ZoneInappropriate Forces in the Passive ZoneOther DetailsVerification of CalculationsThree-Dimensional EffectsPRESENTING RESULTS OF STABILITYEVALUATIONSSite Characterization and RepresentationSoil Property EvaluationPore Water PressuresSpecial FeaturesCalculation ProcedureAnalysis Summary FigureParametric StudiesDetailed Input DataTable of ContentsSLOPE STABILIZATION AND REPAIRUse of Back-AnalysisFactors Governing Selection of Method of

4 Stabilization183183185187195197199199200 2002022052062062132132192212242282322332 37237238238238239239241243243247247247TJ FA 411 PAGE 004viiiCONTENTSAPPENDIXD rainageExcavations and Buttress FillsRetaining StructuresReinforcing Piles and Drilled ShaftsInjection MethodsVegetationThermal TreatmentBridgingRemoval and Replacement of the Sliding MassSLOPE Stability CHARTSUse and Applicability of Charts for Analysis ofSlope StabilityAveraging Slope Inclinations, Unit Weights, andShear StrengthsSoils with = 0 Soils with > 0 Infinite Slope ChartsSoils with = 0 and Strength Increasing with DepthExamplesReferencesIndex248253254256 2602612612622632652652652662702722742742 81295TJ FA 411 PAGE 005 BASIC REQUIREMENTS FOR Slope Stability ANALYSES27surface and (2) the shear stress required for factor of safety for the shear surface is the ratioof the shear Strength of the soil divided by the shearstress required for equilibrium.

5 The normal stressesalong the slip surface are needed to evaluate the shearstrength: Except for soils with ~b = 0, the shearstrength depends on the normal stress on the potentialplane of effective stress analyses, the pore pressures alongthe shear surface are subtracted from the total stressesto determine effective normal stresses, which are usedto evaluate shear strengths. Therefore, to perform ef-fective stress analyses, it is necessary to know (or toestimate) the pore pressures at every point along theshear surface. These pore pressures can be evaluatedwith relatively good accuracy for drained conditions,where their values are determined by hydrostatic orsteady seepage boundary conditions. Pore pressurescan seldom be evaluated accurately for undrainedcondtions, where their values are determined by theresponse of the soil to external total stress analyses, pore pressures are not sub-tracted from the total stresses, because shear strengthsare related to total stresses.

6 Therefore, it is not neces-sary to evaluate and subtract pore pressures to performtotal stress analyses. Total stress analyses are applica-ble only to undrained conditions. The basic premise oftotal stress analysis is this: The pore pressures due toundrained loading are determined by the behavior ofthe soil . For a given value of total stress on the poten-tial failure plane, there is a unique value of pore pres-sure and therefore a unique value of effective , although it is true that shear Strength is reallycontrolled by effective stress , it is possible for the un-drained condition to relate shear Strength to total nor-mal stress , because effective stress and total stress areuniquely related for the undrained condition. Clearly,this line of reasoning does not apply to drained con-ditions, where pore pressures are controlled by hy-draulic boundary conditions rather than the responseof the soil to external of Drained ConditionsDrained conditions are those where changes in loadare slow enough, or where they have been in place longenough, so that all of the soils reach a state of equilib-rium and no excess pore pressures are caused by theloads.

7 In drained conditions pore pressures are con-trolled by hydraulic boundary conditions. The waterwithin the soil may be static, or it may be seepingsteadily, with no change in the seepage over time andno increase or decrease in the amount of water withinthe soil . If these conditions prevail in all the soils at asite, or if the conditions at a site can reasonably beapproximated by these conditions, a drained analysisis appropriate. A drained analysis is performed using: Total unit weights Effective stress shear Strength parameters Pore pressures determined from hydrostatic waterlevels or steady seepage analysesAnalyses of Undrained ConditionsUndrained conditions are those where changes in loadsoccur more rapidly than water can flow in or out ofthe soil . The pore pressures are controlled by the be-havior of the soil in response to changes in externalloads. If these conditions prevail in the soils at a site,or if the conditions at a site can reasonably be approx-imated by these conditions, an undrained analysis isappropriate.

8 An undrained analysis is performed using: Total unit weights Total stress shear Strength parametersHow Long Does Drainage Take?As discussed earlier, the difference between undrainedand drained conditions is time. The drainage charac-teristics of the soil mass, and its size, determine howlong will be required for transition from an undrainedto a drained condition. As shown by Eq. ( ):02t99 = 4-( )Cvwhere t99 is the time required to reach 99% of drainageequilibrium, D the length of the drainage path, and cothe coefficient of of co for clays vary from about cm2/h(10 ft2/yr) to about 100 times this value. Values of cofor silts are on the order of 100 times the values forclays, and values of co for sands are on the order of100 times the values for silts, and higher. These typicalvalues can be used to develop some rough ideas of thelengths of time required to achieve drained conditionsin soils in the path lengths are related to layer thick-nesses.

9 They are half the layer thickness for layers thatare bounded on both sides by more permeable soils,and they are equal to the layer thickness for layers thatare drained only on one side. Lenses or layers of siltor sand within clay layers provide internal drainage,reducing the drainage path length to half of the thick-ness between internal drainage FA 411 PAGE 006 CHAPTER 5 Shear Strengths of soil and Municipal Solid WasteA key step in analyses of soil Slope Stability is mea-suring or estimating the strengths of the soils. Mean-ingful analyses can be performed only if the shearstrengths used are appropriate for the soils and for theparticular conditions analyzed. Much has been learnedabout the shear Strength of soils within the past 60years, often from surprising and unpleasant experiencewith the Stability of slopes, and many useful researchstudies of soil Strength have been performed.

10 Theamount of information that has been amassed on soilstrengths is very large. The following discussion fo-cuses on the principles that govern soil Strength , theissues that are of the greatest general importance inevaluating Strength , and Strength correlations that havebeen found useful in practice. The purpose is to pro-vide information that will establish a useful frameworkand a point of beginning for detailed studies of theshear strengths of soils at particular MATERIALSThe Strength characteristics of all types of granularmaterials (sands, gravels, and rockfills) are similar inmany respects. Because the permeabilities of these ma-terials are high, they are usually fully drained in thefield, as discussed in Chapter 3. They are cohesionless:The particles do not adhere to one another, and theireffective stress shear Strength envelopes pass throughthe origin of the Mohr stress shear Strength of these materials can be char-acterized by the equations = o- tan ~b ( )where s is the shear Strength , or the effective normalstress on the failure plane, and ~b the effective stressangle of internal friction.