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ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

ENV-2E1Y: Fluvial Geomorphology: 2004 - 5. Slope Stability and Geotechnics Landslide Hazards River Bank Stability N.K. Tovey. Lecture 4. Lecture 3. Lecture 5. Lecture 1. Lecture 2. Landslide on Main Highway at km 365 west of Sao Paulo: August 2002.

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ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

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  1. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 Slope Stability and Geotechnics Landslide Hazards River Bank Stability N.K. Tovey Lecture 4 Lecture 3 Lecture 5 Lecture 1 Lecture 2 Landslide on Main Highway at km 365 west of Sao Paulo: August 2002

  2. ENV-2E1Y: Fluvial Geomorphology: 2004 - 5 • 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

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

  4. 1.1 Aims of the Course • 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

  5. 1.2 Background • 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 (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. berms Heave at toe Landslide in man made Cut Slope at km 365 west of Sao Paolo - August 2002

  7. berms Steep scar to rotational failure

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

  9. Stability Assessment Slope Profile Landslide Preventive Measures Design Landslide Warning Landslide Cost Build No Danger Consequence Remove Consequence Remedial Measures Safe at the moment 1. Introduction continued • 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

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

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

  12. 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'.

  13. clay silt sand 1.6 Classification of Soils Particle Size Distribution (continued) • Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axis • S - 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 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 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 Fabric • Individual voids are larger in the loose-packed sample. • Void Ratio is higher in loose sample Loose Sand Dense Sand

  16. 1.6 Classification of Soils Soil Fabric Fig. 5 Typical clay fabrics. Collapsed fabric after consolidation - note particles are not fully aligned Open honey comb fabric as deposited

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

  18. 1.6 Classification of Soils Atterberg Limits Semi-plastic material Fig. 7 Volume of saturated soil against weight. Liquid sediment transport volume Plastic material Solid brittle weight Shrinkage Limit Liquid Limit Plastic Limit

  19. 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.

  20. 1.6 Classification of Soils Atterberg Limits - Derived Indices 1) Liquidity Index m/c - PL (LI) = ----------- ---------------- (1) LL - PL 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

  21. 1.6 Classification of Soils Atterberg Limits - Derived Indices 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 PI LL - PL ------ = ------- . % clay % clay % clay is determined from the size distribution - i.e. proportion less than 2 m in equivalent spherical diameter

  22. London (1) Middlesborough Selby Culham London (2) 1.6 Classification of Soils Atterberg Limits - Derived Indices 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) Liquid Limit Fig. 8 Relationship between mean particle size and moisture content for some soils 100 80 60 40 20 0 Moisture Content (%) Plastic Limit Decreasing particle size

  23. High plasticity Inorganic clays Cohesionless sands Inorganic silts / organic clays 1.6 Classification of Soils Atterberg Limits - Derived Indices Plasticity Index (PI) 0.8 0.6 0.4 0.2 0 Increase in toughness and dry strength decrease in permeability Fig. 9 Plasticity Chart. A-line 0.2 0.4 0.6 0.8 1.0 Liquid Limit/100

  24. 1.6 Classification of Soils Atterberg Limits - Derived Indices LL PL Each line represents a particular soil. Lines from different soils appear to converge on a single point (known as the  - point) Fig. 10 Typical Plots of Voids Ratio Content against shear strength. Void Ratio  - point 1.7 170 log stress (kPa)

  25. 1.6 Classification of Soils Atterberg Limits - Derived Indices 1.0 Liquidity Index 0 • (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 of • the compression index (Cc). Fig. 11 Liquidity Index against shear strength. 1.7 170 log stress (kPa)

  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 related e n n = ------- or e = -------- 1 + e 1 - n e = Gs x (moisture content) Gs is specific gravity ratio of mass of unit volume of soil particles) to unit mass of water

  27. 0.8 0.6 0.4 0.2 0 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 = 1.325 (WLL - WPL) Relationship between Plasticity Index and shear strength Correlation is good  --- = 0.22 + 0.74 PI 'v Applicable to normally consolidated clays 0.2 0.4 0.6 0.8 1.0 1.2 1.4 PI

  28. 1.9 Definitions Gas Water Solid Volume Unit Weight Weight ~ 0 Vg ~ 0 Vw w Vw.w Voids Vs s Vs.s Volume of voids (Vv) = Vg + Vw Volume of voids (Vt) = Vv + Vs and: Vs = Ws / s Vw = Ww / w But: s = Gsw So: Vs = Ws / Gsw

  29. 1.9 Definitions Void Ratio for saturated soils

  30. 1.9 Definitions Water Solid Particles Definition 8: Divide top and bottom lines by Vs

  31. 1.9 Definitions

  32. Ground Surface 3 1 Water table 1 3 A 1.10 Estimation of effective vertical stress at depth Method 1 1 2 3 Total Vertical Stress =  (i . zi) = (1 .3 + 2 .2 + 3 .3 ) where zi 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 3 + 19 x 2 + 17 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 = 137 - 40 = 97 kPa

  33. Ground Surface 3 1 Water table 1 3 A 1.10 Estimation of effective vertical stress at depth Method 2 1 2 3 stress at A = 16 x 3 + 1 x 19 + 1 x (19 - 10) + 3 x (17 - 10) | | | layer 1 ---- layer 2 ----------- layer 3 [19-10 is submerged unit wt of layer 2 = 2'] = 97 kpa as before

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

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