Presentation on theme: "ENV-2E1Y: Fluvial Geomorphology:"— Presentation transcript:
1 ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and GeotechnicsLandslide HazardsRiver Bank StabilityN.K. ToveyLecture 1Lecture 2Lecture 3Lecture 4Lecture 5Landslide on Main Highway at km 365 west of Sao Paulo: August 2002
2 ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Introduction ~ 4 lecturesSeepage and Water Flow through Soils ~ 2 lecturesConsolidation of Soils ~ 4 lecturesShear Strength ~ 1 lectureSlope Stability ~ 4 lecturesRiver Bank Stability ~ 2 lecturesSpecial TopicsDecompaction of consolidated Quaternary depositsLandslide Warning SystemsSlope ClassificationMicrofabric of Sediments
3 1. IntroductionGeneral BackgroundClassification of SoilsBasic DefinitionsBasic Concepts of Stress
4 1.1 Aims of the Course To understand: Subsidiary aims include: the nature of soil from a physical (and chemical) and mechanical standpoint.how water flows in soils and the effects of water pressure on stability.how the behaviour of soils and sediments change with consolidation.- implications for Quaternary Studiesthe nature of shear behaviour of soils and sedimentsthe application of the above to study the stability of soils.Subsidiary aims include:instruction in field sampling and laboratory testing methods for the study of the mechanical properties of soilsManaging Landslide Risk the study of river bank stability.Modification of slope stability ideas to the study of river bank stability
5 Fs = 1.2 Background Geotechnics Soil Mechanics Rock Mechanics "the application of the laws of mechanics and hydraulics to the mechanical problems relating to soils and rocks"Soil MechanicsRock Mechanicsnot covered in this course some references in SeismologyFactor of Safety (Fs):Forces resisting landslide movement arising from the inherent strength of the soil.Fs =Forces trying to cause failure(i.e. the mobilizing forces).
6 bermsHeave at toeLandslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002
8 Geology Landslide Man’s Influence (Agriculture /Development) Drainage PumpingConstructionCut / Fill SlopesHydrology (rainfall)EarthquakesGeologyGround WaterGround Loading(Consolidation)Erosion/DepositionGlaciationWeatheringSurface WaterMaterial Properties (Shear Strength)GeochemistryStability AssessmentSlope ProfileLandslidePreventive MeasuresDesignLandslide WarningLandslideCostBuildNo DangerConsequenceRemedial MeasuresRemove ConsequenceSafe at the moment
9 1. Introduction continued Last Lecture:Water plays an important role in ability of soils to resist deformationSmall amount of water increases strengthLarge amount of water decreases strengthWater pressure affects strengthLandslideConsequenceRemedial MeasuresRemove ConsequenceSafe at the momentCostBuildLandslide WarningNo DangerDesignPreventive MeasuresStability AssessmentSlope Profile
10 Geology Landslide Man’s Influence (Agriculture /Development) Drainage PumpingConstructionCut / Fill SlopesHydrology (rainfall)EarthquakesGeologyGround WaterGround Loading(Consolidation)Erosion/DepositionGlaciationWeatheringSurface WaterMaterial Properties (Shear Strength)GeochemistryStability AssessmentSlope ProfileLandslidePreventive MeasuresDesignLandslide WarningLandslideCostBuildNo DangerConsequenceRemedial MeasuresRemove ConsequenceSafe at the moment
11 GIS Geology Landslide Man’s Influence (Agriculture /Development) DrainagePumpingConstructionCut / Fill SlopesHydrology (rainfall)EarthquakesGeologyGround WaterGround Loading(Consolidation)Erosion/DepositionGlaciationWeatheringSurface WaterMaterial Properties (Shear Strength)GeochemistryStability AssessmentSlope ProfileLandslidePreventive MeasuresSlope ManagementDesignLandslide WarningLandslideCostBuildNo DangerTemporarily SafeConsequenceRemedial MeasuresRemove ConsequenceSafe at the moment
12 1.6 Classification of Soils Particle Size Distributionboulders > 60mm60mm > gravel > 2mm2mm > sand > 60 m60 m > silt > 2 m2 m > clayEach class may is sub-divided into coarse, medium and fine.for sand:2mm > coarse sand > 600 m600 m > medium sand > 200 m200 m > fine sand > 60 mClassification boundaries either begin with a '2' or a '6'.
13 1.6 Classification of Soils Particle Size Distribution (continued)Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axissiltclaysandS - shaped - but some conventions of curves going left to right,others, the opposite way around
14 1.6 Classification of Soils Particle Size Distribution (continued)A Problemclay is used both as a classifier of size as above, and also to define particular types of material.clays exhibit a property known as cohesion(the "stickiness" associated with clays).General PropertiesGravels permeability is of the order of mm s-1.Clays it is 10-7 mm/s or less.Compressibility of the soil increases as the particle size decreases.Permeability of the soil decreases as the particle size decreases
15 1.6 Classification of Soils Soil FabricDense SandLoose SandIndividual voids are larger in the loose-packed sample.Void Ratio is higher in loose sample
16 1.6 Classification of Soils Soil FabricCollapsed fabric after consolidation - note particles are not fully alignedOpen honey comb fabric as depositedFig. 5 Typical clay fabrics.
17 1.6 Classification of Soils Soil FabricHO++CationHO+Fig. 6 Cation forming a bridge between two clay particles.
18 1.6 Classification of Soils Semi-plastic material Atterberg LimitsSemi-plastic materialLiquidsediment transportvolumePlastic materialSolid brittleweightShrinkage LimitPlastic LimitLiquid LimitFig. 7 Volume of saturated soil against weight.
19 1.6 Classification of Soils Atterberg Limitsi) Shrinkage Limit (SL) - The smallest water content at which a soil can be saturated. Alternatively it is the water content below which no further shrinkage takes place on drying.ii) Plastic Limit (PL) - The smallest water content at which the soil behaves plastically. It is the boundary between the plastic solid and semi-plastic solid. It is usually measured by rolling threads of soil 3mm in diameter until they just start to crumble.iii)Liquid Limit (LL) - The water content at which the soil is practically a liquid, but still retains some shear strength.a) Casagrande apparatusb) Fall cone apparatus.
20 1.6 Classification of Soils Atterberg Limits - Derived Indices1) Liquidity Indexm/c - PL(LI) = (1)LL - PLwhere LL - moisture content at the Liquid LimitPL - moisture content at the Plastic Limitand m/c is the actual current moisture content of the soil.LI = 0 at Plastic LimitLI = 1 at Liquid Limit
21 1.6 Classification of Soils Atterberg Limits - Derived Indices2) Plasticity Index (PI)This is defined as PI = LL - PL (2)Soils with high clay content have a high Plasticity Index.3) Activity Index (AI)This is defined asPI LL - PL= % clay % clay% clay is determined from the size distribution- i.e. proportion less than 2 m in equivalent spherical diameter
22 1.6 Classification of Soils Atterberg Limits - Derived IndicesShear strength at Liquid Limit~ 1.70 kPaCritical StateSoil Mechanics:shear strength of Plastic Limit is~ 170 kPa(i.e. 100 times that of LL)London (1)MiddlesboroughLiquid LimitLondon (2)10080604020SelbyCulhamMoistureContent(%)Plastic LimitDecreasing particle sizeFig. 8 Relationship between mean particle size and moisture content for some soils
23 1.6 Classification of Soils Atterberg Limits - Derived IndicesPlasticityIndex(PI)0.80.60.40.2High plasticityIncrease in toughness and dry strengthdecrease in permeabilityInorganic claysA-lineCohesionless sandsInorganic silts / organic claysLiquid Limit/100Fig Plasticity Chart.
24 1.6 Classification of Soils Atterberg Limits - Derived IndicesLL PLEach line represents a particular soil.Lines from different soils appear to converge on a single point(known as the - point)VoidRatio - pointlog stress (kPa)Fig. 10 Typical Plots of Voids Ratio Content against shear strength.
25 1.6 Classification of Soils Atterberg Limits - Derived Indices1.0LiquidityIndex(WLL - WPL)= = 0.5(WLL - WPL)log(170) - log(1.7)………………………..equation (1)(Note:log(170) - log(1.7) = log(170/1.7)= log 100 = 2)This is an estimate ofthe compression index (Cc).log stress (kPa)Fig Liquidity Index against shear strength.
26 1.7 Two Volumetric Definitions VOID RATIO (e)ratio of the volume of the voids to the volume of SOLID.POROSITY (n)ratio of the volume of the voids to the total volume of the SOIL(i.e. solid + voids).e and n are relatede nn = or e =1 + e ne = Gs x (moisture content)Gs is specific gravityratio of mass of unit volume of soil particles) to unit mass of water
27 1.8 Further Applications of the Atterberg Limits Consolidation normally requires the gradient of the consolidation line in terms of voids ratio, and not moisture content as indicated above.Transform equation (1): Cc = (WLL - WPL)Relationship between Plasticity Index and shear strength0.80.60.40.2Correlation is good--- = PI'vApplicable to normally consolidated claysPI
28 1.9 DefinitionsVolume Unit Weight WeightVg~ 0~ 0GasWaterVwwVw.wSolidVoidsVssVs.sVolume of voids (Vv) =Vg VwVolume of voids (Vt) =Vv VsVw = Ww / wand: Vs = Ws / sBut: s = Gs wSo: Vs = Ws / Gs w
32 1.10 Estimation of effective vertical stress at depth Method 1Total Vertical Stress = (i . zi) = ( 3 .3 )where zi is the depth of layer iIf 1 = 16 kN m-3 , 2 = 19 kN m-3 ,and 3 = 17 kN m-3Total stress = (16 x x x 3)= kPa (kN m-3)Deduct the buoyant effect of water= w x = kPa (since w = 10 kN m-3)effective stress = = 97 kPaGround Surface3123Water table113A
33 1.10 Estimation of effective vertical stress at depth Method 2stress at A =16 x x x ( ) x ( )| | |layer layer layer 3[19-10 is submerged unit wt of layer 2 = 2']= kpa as beforeGround Surface3123Water table113A