ENV-2E1Y: Fluvial Geomorphology: 2004 - 5

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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

• 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
• 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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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

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berms
Steep scar to rotational failure
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Mans Influence (Agriculture /Development)
Cut / Fill Slopes
Construction
Drainage
Pumping
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
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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

10
Mans Influence (Agriculture /Development)
Cut / Fill Slopes
Construction
Drainage
Pumping
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
Mans Influence (Agriculture /Development)
Cut / Fill Slopes
Construction
Drainage
Pumping
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 gt 60mm
• 60mm gt gravel
  gt 2mm
• 2mm gt sand
  gt 60 ?m
• 60 ?m gt silt
  gt 2 ?m
• 2 ?m gt clay
• Each class may is sub-divided into coarse, medium
  and fine.
• for sand
• 2mm gt coarse sand gt
  600 ?m
• 600 ?m gt medium sand gt 200
  ?m
• 200 ?m gt fine sand gt
  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

• 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

• Fig. 6 Cation forming a bridge between two clay
  particles.

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1.6 Classification of Soils
Atterberg Limits
Semi-plastic material
Liquid sediment transport
volume
Plastic material
Solid brittle
weight
Shrinkage Limit
Liquid Limit
Plastic 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)
Liquid Limit
100 80 60 40 20 0
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
0
Increase in toughness and dry strength decrease
in permeability
A-line
0.2 0.4 0.6 0.8 1.0 Liquid
Limit/100

• Fig. 9 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
1.7 170 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 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).
1.7 170 log stress (kPa)

• Fig. 11 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
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
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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 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
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1.9 Definitions
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
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1.9 Definitions
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1.9 Definitions
Definition 8
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) (?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

?1 ?2 ?3
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1.10 Estimation of effective vertical stress at
depth
Method 2

• 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

?1 ?2 ?3
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Mans Influence (Agriculture /Development)
Cut / Fill Slopes
Construction
Drainage
Pumping
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