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
ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
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Introduction ~ 4 lectures
Seepage and Water Flow through Soils
Consolidation of Soils ~ 4 lectures
Shear Strength ~ 1 lecture
Slope Stability ~ 4 lectures
River Bank Stability ~ 2 lectures
Special Topics
–
–
–
–
~ 2 lectures
Decompaction of consolidated Quaternary deposits
Landslide Warning Systems
Slope Classification
Microfabric of Sediments
1. Introduction
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General Background
Classification of Soils
Basic Definitions
Basic Concepts of Stress
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
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):
Fs =
Forces resisting landslide movement arising
from the inherent strength of the soil.
Forces trying to cause failure
(i.e. the mobilizing forces).
berms
Heave at toe
Landslide in man made Cut Slope at km 365 west of
Sao Paolo - August 2002
berms
Steep scar to
rotational failure
Man’s Influence (Agriculture /Development)
Pumping
Drainage
Hydrology (rainfall)
Construction
Earthquakes
Ground
Water
Ground Loading
Surface
Water
Material Properties
Geology
(Shear Strength)
Erosion/Deposition
Glaciation
Weathering
Geochemistry
Stability Assessment
Slope Profile
(Consolidation)
Landslide
Preventive
Measures
Design
Landslide Warning
Cost
Build
Cut / Fill Slopes
No Danger
Safe at the moment
Landslide
Consequence
Remedial
Measures
Remove
Consequence
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
Stability Assessment
Slope Profile
Landslide
Preventive
Measures
Design
Landslide Warning
Cost
Build
No Danger
Safe at the moment
Landslide
Consequence
Remedial
Measures
Remove
Consequence
Man’s Influence (Agriculture /Development)
Pumping
Drainage
Hydrology (rainfall)
Construction
Earthquakes
Ground
Water
Ground Loading
Surface
Water
Material Properties
Geology
(Shear Strength)
Erosion/Deposition
Glaciation
Weathering
Geochemistry
Stability Assessment
Slope Profile
(Consolidation)
Landslide
Preventive
Measures
Design
Landslide Warning
Cost
Build
Cut / Fill Slopes
No Danger
Safe at the moment
Landslide
Consequence
Remedial
Measures
Remove
Consequence
Man’s Influence (Agriculture /Development)
Pumping
Drainage
Hydrology (rainfall)
Earthquakes
Ground
Water
Ground Loading
Surface
Water
Material Properties
Landslide
Preventive
Measures
Design
Construction
GIS
Geology
(Shear Strength)
Erosion/Deposition
Glaciation
Weathering
Geochemistry
Stability Assessment
Slope Profile
(Consolidation)
Slope Management
Landslide Warning
Landslide
Cost
Build
Cut / Fill Slopes
No Danger
Safe at the moment
Temporarily Safe
Consequence
Remedial
Measures
Remove
Consequence
1.6 Classification of Soils
• Particle Size Distribution
boulders
60mm >
gravel
2mm >
sand
60 m >
silt
2 m >
clay
> 60mm
> 2mm
> 60 m
> 2 m
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'.
1.6 Classification of Soils
Particle Size Distribution (continued)
• Data often presented as Particle Size Distribution Curves with
logarithmic scale on X-axis
clay
silt
sand
• S - shaped - but some conventions of curves going left to right,
others, the opposite way around
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
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
1.6 Classification of Soils
Soil Fabric
Open honey comb fabric
as deposited
Collapsed fabric after consolidation
- note particles are not fully aligned
Fig. 5 Typical clay fabrics.
1.6 Classification of Soils
Soil Fabric
Cation
+
O
H+
H
+
Fig. 6 Cation forming a bridge between two clay particles.
1.6 Classification of Soils
Atterberg Limits
Semi-plastic
material
volume
Liquid
sediment
transport
Solid
brittle
Shrinkage
Limit
Plastic
material
weight
Plastic
Limit
Liquid
Limit
Fig. 7 Volume of saturated soil against weight.
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.
1.6 Classification of Soils
Atterberg Limits - Derived Indices
1) Liquidity Index
m/c - PL
(LI) = ----------LL - PL
---------------- (1)
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
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
------ =
% clay
LL - PL
------- .
% clay
% clay is determined from the size distribution
- i.e. proportion less than 2 m in equivalent spherical diameter
1.6 Classification of Soils
(%)
40
London (1)
Liquid Limit
Shear strength at
Liquid Limit
~ 1.70 kPa
Critical State
Soil Mechanics:
London (2)
Moisture
60
Content
Selby
80
Culham
100
Middlesborough
Atterberg Limits - Derived Indices
Plastic Limit
20
shear strength of
Plastic Limit is
~ 170 kPa
(i.e. 100 times that of LL)
0
Decreasing particle size
Fig. 8 Relationship between mean particle size and moisture content
for some soils
1.6 Classification of Soils
Atterberg Limits - Derived Indices
Plasticity
Index
(PI)
High plasticity
Increase in
toughness and
dry strength
Inorganic clays
0.8
decrease in
permeability
0.6
0.4
0.2
Cohesionless
sands
Inorganic silts
/ organic clays
0
0.2
0.4
0.6
0.8 1.0
Liquid Limit/100
Fig. 9 Plasticity Chart.
1.6 Classification of Soils
Atterberg Limits - Derived Indices
LL
PL
Each line represents a
particular soil.
Void
Ratio
point
Lines from different soils
appear to converge on a
single point
(known as the  - point)
1.7
170
log stress (kPa)
Fig. 10 Typical Plots of Voids Ratio Content against shear strength.
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)
0
(Note:
log(170) - log(1.7) = log(170/1.7)
= log 100 = 2)
This is an estimate of
1.7
Fig. 11
170
log stress (kPa)
the compression index (Cc).
Liquidity Index against shear strength.
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 = ------- or
1+e
e =
n
-------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
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
0.8

v
Correlation is good

--- = 0.22 + 0.74 PI
'v
0.6
0.4
Applicable to normally consolidated clays
0.2
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
PI
Voids
1.9 Definitions
Volume
~0
Vw
w
Vw.w
Vs
s
Vs.s
Gas
Solid
Vw = Ww / w
But: s = Gs w
Weight
~0
Vg
Water
Unit Weight
Volume of voids (Vv) =
Vg + Vw
Volume of voids (Vt) =
Vv + Vs
and: Vs = Ws / s
So: Vs = Ws / Gs w
1.9 Definitions
Definition
Symbol
1 Void Ratio
e
(ratio of volume of voids to
volume of solid)
Porosity
2 (ratio of volume of voids to
n
total
volume)
Void
Ratio
for saturated soils
Vv Vw  V g
e

Vs
Vs
Vv Vw  Vg
n

Vt
Vt
Vw
3
w
Vv
V

v
w w
Water
Content
(%)
Ww  w Vw
e



wm
 Gs 
4
(or m)
Ws
 s Vs
Vs W s
Degree of Saturation
W
Sr
G s  ww
5 Unit Weight of Water
6 Unit Weight of Solid
s
Particles
7 Specific Gravity
Gs
Sr 
1.9 Definitions
Definition 8:
Total Weight

Total Volume

Vw  w  Vs  s
Vv  Vs

Vw
 Vs G s   w

Vv 
Vs  1 

Vs 

Divide top and bottom lines by Vs
 G  Vw . Vv
 s
Vv
Vs

Solid
Particles

1  e 
 G  VWater

w
 s
 w
V
s 

1  e 

Gs
 Sr e   w
1  e 

 w

1.9 Definitions
8 Bulk Unit Weight
9 Saturated Unit Weight
10 Dry Unit Weight

sat

G

 S r e  w
s
(1  e )

G

s
 e  w
(1  e )

G 

d
s
w
(1  e )

G

s
 e  w
(1  e )
11 Submerged Unit Weight ’ =  -w

G
s
 1 w
(1  e )
 w
1.10 Estimation of effective vertical stress at depth
Method 1
Total Vertical Stress =
 (i . zi) = (1 .3 + 2 .2 + 3 .3 )
Ground Surface
where zi is the depth of layer i
3
1
1
1
Water
2
table
1 = 16 kN m-3 , 2 = 19 kN m-3 ,
If
3 = 17 kN m-3
and
Total stress = (16 x 3 + 19 x 2 + 17 x 3)
=
137 kPa (kN m-3)
Deduct the buoyant effect of water
3
3
=
w x. 4
= 40 kPa (since w = 10 kN m-3)
effective stress =
A
137 - 40 = 97 kPa
1.10 Estimation of effective vertical stress at depth
Method 2
stress at A =
Ground Surface
16 x 3 + 1 x 19 + 1 x (19 - 10) + 3 x (17 - 10)
3
1
1
1
Water
2
table
|
layer 1
|
---- layer 2 -----------
3
A
layer 3
[19-10 is submerged unit wt of layer 2 = 2']
=
3
|
97 kpa as before
Man’s Influence (Agriculture /Development)
Pumping
Drainage
Hydrology (rainfall)
Earthquakes
Ground
Water
Ground Loading
Surface
Water
Material Properties
Landslide
Preventive
Measures
Design
Construction
GIS
Geology
(Shear Strength)
Erosion/Deposition
Glaciation
Weathering
Geochemistry
Stability Assessment
Slope Profile
(Consolidation)
Slope Management
Landslide Warning
Landslide
Cost
Build
Cut / Fill Slopes
No Danger
Safe at the moment
Temporarily Safe
Consequence
Remedial
Measures
Remove
Consequence