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**ENV-2E1Y: Fluvial Geomorphology: 2004 - 5**

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

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**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

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

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**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 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

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**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 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).

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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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**Geology Landslide Man’s Influence (Agriculture /Development) Drainage**

Pumping Construction Cut / Fill Slopes 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 Remedial Measures Remove Consequence Safe at the moment

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**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 Landslide Consequence Remedial Measures Remove Consequence Safe at the moment Cost Build Landslide Warning No Danger Design Preventive Measures Stability Assessment Slope Profile

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**Geology Landslide Man’s Influence (Agriculture /Development) Drainage**

Pumping Construction Cut / Fill Slopes 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 Remedial Measures Remove Consequence Safe at the moment

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**GIS Geology Landslide Man’s Influence (Agriculture /Development)**

Drainage Pumping Construction Cut / Fill Slopes 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 No Danger Temporarily Safe Consequence Remedial Measures Remove Consequence Safe at the moment

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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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**1.6 Classification of Soils**

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

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**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

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**1.6 Classification of Soils**

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

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**1.6 Classification of Soils**

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

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**1.6 Classification of Soils**

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

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**1.6 Classification of Soils Semi-plastic material**

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

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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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**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

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**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

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**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) London (1) Middlesborough Liquid Limit London (2) 100 80 60 40 20 Selby Culham Moisture Content (%) Plastic Limit Decreasing particle size Fig. 8 Relationship between mean particle size and moisture content for some soils

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**1.6 Classification of Soils**

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

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**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) Void Ratio - point log stress (kPa) Fig. 10 Typical Plots of Voids Ratio Content against shear strength.

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**1.6 Classification of Soils**

Atterberg Limits - Derived Indices 1.0 Liquidity Index (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). log stress (kPa) Fig Liquidity Index against shear strength.

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**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 n e = Gs x (moisture content) Gs is specific gravity ratio of mass of unit volume of soil particles) to unit mass of water

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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): Cc = (WLL - WPL) Relationship between Plasticity Index and shear strength 0.8 0.6 0.4 0.2 Correlation is good --- = PI 'v Applicable to normally consolidated clays PI

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

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1.9 Definitions Void Ratio for saturated soils

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

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1.9 Definitions

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**1.10 Estimation of effective vertical stress at depth**

Method 1 Total Vertical Stress = (i . zi) = ( 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 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 kPa Ground Surface 3 1 2 3 Water table 1 1 3 A

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**1.10 Estimation of effective vertical stress at depth**

Method 2 stress at A = 16 x x x ( ) x ( ) | | | layer layer layer 3 [19-10 is submerged unit wt of layer 2 = 2'] = kpa as before Ground Surface 3 1 2 3 Water table 1 1 3 A

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**GIS Geology Landslide Man’s Influence (Agriculture /Development)**

Drainage Pumping Construction Cut / Fill Slopes 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 No Danger Temporarily Safe Consequence Remedial Measures Remove Consequence Safe at the moment

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