1. 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

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: 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

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 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. 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

9. 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

10. 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

11. 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

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. 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

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

16. 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.

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

18. 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.

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. 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

23. 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.

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

25. 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.

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 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. 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

28. 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

29. 1.9 Definitions
Void Ratio for saturated soils

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

31. 1.9 Definitions

32. 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

33. 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

35. 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