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ENV-2E1Y: Fluvial Geomorphology: Slope Stability and Geotechnics Landslide Hazards River Bank Stability N.K. Tovey Landslide on Main Highway at km 365 west of Sao Paulo: August 2002 Lecture 2 LectureLecture 3 Lecture 4 Lecture 1 Lecture 5

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Introduction ~ 4 lectures Seepage and Water Flow through Soils ~ 2 lectures Consolidation of Soils ~ 4 lectures Shear Strength ~ 1 lecture Slope Stability ~ 4 lectures River Bank Stability ~ 2 lectures Special Topics –Decompaction of consolidated Quaternary deposits –Landslide Warning Systems –Slope Classification –Microfabric of Sediments ENV-2E1Y: Fluvial Geomorphology:

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General Background Classification of Soils Basic Definitions Basic Concepts of Stress 1. Introduction

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To understand: 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 Studies the nature of shear behaviour of soils and sediments the 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 soils Managing Landslide Risk the study of river bank stability. Modification of slope stability ideas to the study of river bank stability 1.1 Aims of the Course

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Geotechnics "the application of the laws of mechanics and hydraulics to the mechanical problems relating to soils and rocks" –Soil Mechanics –Rock Mechanics not covered in this course some references in Seismology Factor of Safety (F s ): 1.2 Background Forces resisting landslide movement arising from the inherent strength of the soil. Forces trying to cause failure (i.e. the mobilizing forces). F s =

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berms Heave at toe Landslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002

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berms Steep scar to rotational failure

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Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Design Landslide Preventive Measures Stability Assessment Slope Profile Geology Erosion/Deposition Glaciation Weathering Geochemistry Cut / Fill Slopes Construction Drainage Pumping Mans Influence (Agriculture /Development) Earthquakes Material Properties (Shear Strength) Ground Loading (Consolidation) Hydrology (rainfall) Ground Water Surface Water

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Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Design Landslide Preventive Measures Stability Assessment Slope Profile Last Lecture: Water plays an important role in ability of soils to resist deformation Small amount of water increases strength Large amount of water decreases strength Water pressure affects strength 1. Introduction continued

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Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Design Landslide Preventive Measures Stability Assessment Slope Profile Geology Erosion/Deposition Glaciation Weathering Geochemistry Cut / Fill Slopes Construction Drainage Pumping Mans Influence (Agriculture /Development) Earthquakes Material Properties (Shear Strength) Ground Loading (Consolidation) Hydrology (rainfall) Ground Water Surface Water

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Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Temporarily Safe Design Landslide Preventive Measures Stability Assessment Slope Profile Geology Erosion/Deposition Glaciation Weathering Geochemistry Cut / Fill Slopes Construction Drainage Pumping Mans Influence (Agriculture /Development) Earthquakes Material Properties (Shear Strength) Ground Loading (Consolidation) Slope Management Hydrology (rainfall) Ground Water Surface Water GIS

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1.6 Classification of Soils Particle Size Distribution boulders > 60mm 60mm > gravel > 2mm 2mm > sand > 60 m 60 m > silt > 2 m 2 m > clay Each class may is sub-divided into coarse, medium and fine. for sand: 2mm > coarse sand > 600 m 600 m > medium sand > 200 m 200 m > fine sand > 60 m Classification boundaries either begin with a '2' or a '6'.

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Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axis 1.6 Classification of Soils Particle Size Distribution (continued) S - shaped - but some conventions of curves going left to right, others, the opposite way around sand silt clay

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A Problem clay 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 Properties Gravels permeability is of the order of mm s -1. Clays it is mm/s or less. Compressibility of the soil increases as the particle size decreases. Permeability of the soil decreases as the particle size decreases 1.6 Classification of Soils Particle Size Distribution (continued)

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Individual voids are larger in the loose-packed sample. Void Ratio is higher in loose sample 1.6 Classification of Soils Soil Fabric Dense Sand Loose Sand

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Fig. 5 Typical clay fabrics. 1.6 Classification of Soils Soil Fabric Open honey comb fabric as deposited Collapsed fabric after consolidation - note particles are not fully aligned

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Fig. 6 Cation forming a bridge between two clay particles. 1.6 Classification of Soils Soil Fabric H H O + + H H O Cation

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Fig. 7 Volume of saturated soil against weight. 1.6 Classification of Soils Atterberg Limits volume weight Liquid sediment transport Solid brittle Plastic material Shrinkage Limit Liquid Limit Plastic Limit Semi-plastic material

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1.6 Classification of Soils Atterberg Limits i)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 apparatus b) Fall cone apparatus.

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where LL - moisture content at the Liquid Limit PL - moisture content at the Plastic Limit and m/c is the actual current moisture content of the soil. LI = 0 at Plastic Limit LI = 1 at Liquid Limit 1.6 Classification of Soils Atterberg Limits - Derived Indices 1) Liquidity Index m/c - PL (LI) = (1) LL - PL

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2) 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 as 1.6 Classification of Soils Atterberg Limits - Derived Indices PI LL - PL = % clay % clay % clay is determined from the size distribution - i.e. proportion less than 2 m in equivalent spherical diameter

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Fig. 8 Relationship between mean particle size and moisture content for some soils 1.6 Classification of Soils Atterberg Limits - Derived Indices Decreasing particle size Moisture Content (%) Culham Middlesborough Selby London (1) London (2) Liquid Limit Plastic Limit Shear strength at Liquid Limit ~ 1.70 kPa Critical State Soil Mechanics: shear strength of Plastic Limit is ~ 170 kPa (i.e. 100 times that of LL)

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Fig. 9 Plasticity Chart. 1.6 Classification of Soils Atterberg Limits - Derived Indices Liquid Limit/100 Plasticity Index (PI) Inorganic silts / organic clays High plasticity Inorganic clays Cohesionless sands Increase in toughness and dry strength decrease in permeability A-line

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Fig. 10 Typical Plots of Voids Ratio Content against shear strength. 1.6 Classification of Soils Atterberg Limits - Derived Indices Each line represents a particular soil. Lines from different soils appear to converge on a single point (known as the - point) - point log stress (kPa) Void Ratio LL PL

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Fig. 11 Liquidity Index against shear strength. 1.6 Classification of Soils Atterberg Limits - Derived Indices (W LL - W PL ) = = 0.5(W LL - W PL ) log(170) - log(1.7) ………………………..equation (1) (Note: log(170) - log(1.7) = log(170/1.7) = log 100 = 2) This is an estimate of the compression index (C c ) log stress (kPa) 1.0 Liquidity Index 0

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1.7 Two Volumetric Definitions ratio of the volume of the voids to the total volume of the SOIL (i.e. solid + voids). e and n are related VOID RATIO (e) POROSITY (n) ratio of the volume of the voids to the volume of SOLID. e = G s x (moisture content) G s is specific gravity ratio of mass of unit volume of soil particles) to unit mass of water e n n = or e = e 1 - n

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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): C c = (W LL - W PL ) Relationship between Plasticity Index and shear strength Correlation is good --- = PI ' v Applicable to normally consolidated clays PI

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Solid Water Gas Voids Volume Unit Weight Weight VwVw VsVs ~ 0 w V w. w s V s. s Volume of voids (V v ) = V g + V w Volume of voids (V t ) = V v + V s VgVg V w = W w / w and: V s = W s / s But: s = G s w So: V s = W s / G s w 1.9 Definitions

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Void Ratio for saturated soils 1.9 Definitions

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Definition 8: Divide top and bottom lines by V s Solid Particles Water 1.9 Definitions

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Total Vertical Stress = ( i. z i ) = ( ) where z i is the depth of layer i If 1 = 16 kN m -3, 2 = 19 kN m -3, and 3 = 17 kN m -3 Total stress = (16 x x x 3) = 137 kPa (kN m -3 ) Deduct the buoyant effect of water = w x. 4 = 40 kPa (since w = 10 kN m -3 ) effective stress = = 97 kPa 1.10 Estimation of effective vertical stress at depth Method 1 Water table Ground Surface A

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stress at A = 16 x x x ( ) + 3 x ( ) | | | layer layer layer 3 [19-10 is submerged unit wt of layer 2 = 2 '] = 97 kpa as before 1.10 Estimation of effective vertical stress at depth Method 2 Water table Ground Surface A

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Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Temporarily Safe Design Landslide Preventive Measures Stability Assessment Slope Profile Geology Erosion/Deposition Glaciation Weathering Geochemistry Cut / Fill Slopes Construction Drainage Pumping Mans Influence (Agriculture /Development) Earthquakes Material Properties (Shear Strength) Ground Loading (Consolidation) Slope Management Hydrology (rainfall) Ground Water Surface Water GIS

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