env-2e1y: fluvial geomorphology: 2004 - 5 slope stability and geotechnics landslide hazards river...
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ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
Slope Stability and Geotechnics
Landslide Hazards
River Bank Stability
N.K. Tovey
Landslide on Main Highway at km 365 west of Sao Paulo: August 2002
Lecture 2 Lecture 3 Lecture 4Lecture 1 Lecture 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
ENV-2E1Y: Fluvial Geomorphology: 2004 - 5
• General Background
• Classification of Soils
• Basic Definitions
• Basic Concepts of Stress
1. Introduction
• 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
1.1 Aims of the Course
• 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):
1.2 Background
Forces resisting landslide movement arising from the inherent strength of the soil.
Forces trying to cause failure(i.e. the mobilizing forces).
Fs =
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
Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
GeologyErosion/DepositionGlaciationWeatheringGeochemistry
Cut / Fill SlopesConstructionDrainage Pumping
Man’s Influence (Agriculture /Development)
Earthquakes
Material Properties (Shear Strength)
Ground Loading(Consolidation)
Hydrology (rainfall)
Ground Water
Surface Water
Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
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
1. Introduction continued
Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
GeologyErosion/DepositionGlaciationWeatheringGeochemistry
Cut / Fill SlopesConstructionDrainage Pumping
Man’s Influence (Agriculture /Development)
Earthquakes
Material Properties (Shear Strength)
Ground Loading(Consolidation)
Hydrology (rainfall)
Ground Water
Surface Water
Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger Temporarily Safe
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
GeologyErosion/DepositionGlaciationWeatheringGeochemistry
Cut / Fill SlopesConstructionDrainage Pumping
Man’s Influence (Agriculture /Development)
Earthquakes
Material Properties (Shear Strength)
Ground Loading(Consolidation)
Slope Management
Hydrology (rainfall)
Ground Water
Surface Water
GIS
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'.
• Data often presented as Particle Size Distribution Curves with logarithmic scale on X-axis
1.6 Classification of SoilsParticle Size Distribution (continued)
• S - shaped - but some conventions of curves going left to right, others, the opposite way around
sand
siltclay
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
Particle Size Distribution (continued)
• Individual voids are larger in the loose-packed sample.• Void Ratio is higher in loose sample
1.6 Classification of SoilsSoil Fabric
Dense Sand Loose Sand
Fig. 5 Typical clay fabrics.
1.6 Classification of SoilsSoil Fabric
Open honey comb fabric as deposited
Collapsed fabric after consolidation - note particles are not fully aligned
Fig. 6 Cation forming a bridge between two clay particles.
1.6 Classification of SoilsSoil Fabric
H
H
O+
+
H
H
O
+
+
+Cation
Fig. 7 Volume of saturated soil against weight.
1.6 Classification of SoilsAtterberg Limits
volu
me
weight
Liquid
sediment transport
Solid brittle
Plastic material
Shrinkage Limit
Liquid Limit
Plastic Limit
Semi-plastic material
1.6 Classification of SoilsAtterberg 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.
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 SoilsAtterberg Limits - Derived Indices
1) Liquidity Index
m/c - PL (LI) = ----------- ---------------- (1) LL - PL
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
1.6 Classification of SoilsAtterberg Limits - Derived Indices
PI LL - PL ------ = ------- . % clay % clay
% clay is determined from the size distribution - i.e. proportion less than 2 m in equivalent spherical diameter
Fig. 8 Relationship between mean particle size and moisture content
for some soils
1.6 Classification of SoilsAtterberg Limits - Derived Indices
Decreasing particle size
100
80
60
40
20
0
Moisture
Content
(%)
Cu
lham M
idd
lesb
orou
gh
Sel
by L
ond
on (
1)
Lon
don
(2)
Liquid Limit
Plastic Limit
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)
Fig. 9 Plasticity Chart.
1.6 Classification of SoilsAtterberg Limits - Derived Indices
0.2 0.4 0.6 0.8 1.0
Liquid Limit/100
Plasticity Index(PI)
0.8
0.6
0.4
0.2
0
Inorganic silts / organic clays
High plasticity
Inorganic clays
Cohesionless sands
Increase in toughness and dry strength
decrease in permeability
A-line
Fig. 10 Typical Plots of Voids Ratio Content against shear strength.
1.6 Classification of SoilsAtterberg Limits - Derived Indices
Each line represents a particular soil.
Lines from different soils appear to converge on a single point
(known as the - point) -
point
1.7 170
log stress (kPa)
Void
Ratio
LL PL
Fig. 11 Liquidity Index against shear strength.
1.6 Classification of SoilsAtterberg Limits - Derived Indices
(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)
1.0LiquidityIndex
0
1.7 Two Volumetric Definitions
ratio of the volume of the voids to the total volume of the SOIL (i.e. solid + voids).
e and n are related
• VOID RATIO (e)
• POROSITY (n)
ratio of the volume of the voids to the volume of SOLID.
e = Gs x (moisture content)
Gs is specific gravity
ratio of mass of unit volume of soil particles) to unit mass of water
e n n = ------- or e = -------- 1 + e 1 - n
1.8 Further Applications of the Atterberg LimitsConsolidation 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 'vApplicable to normally consolidated clays
0.2 0.4 0.6 0.8 1.0 1.2 1.4 PI
0.8
0.6
0.4
0.2
0
v
Solid
Water
GasVoids
Volume Unit Weight Weight
Vw
Vs
~ 0 ~ 0
w Vw.w
s Vs.s
Volume of voids (Vv) = Vg + Vw
Volume of voids (Vt) = Vv + Vs
Vg
Vw = Ww / w and: Vs = Ws / s
But: s = Gs w So: Vs = Ws / Gs w
1.9 Definitions
Definition Symbol
1Void Ratio(ratio of volume of voids tovolume of solid)
e s
gw
s
v
V
VV
V
Ve
2Porosity(ratio of volume of voids tototal volume)
n t
gw
t
v
V
VV
V
Vn
5 Unit Weight of Water w
6 Unit Weight of SolidParticles
s
7 Specific Gravity Gs
3Degree of SaturationSr
v
wrV
VS
4Water Content (%) w
(or m)
ss
ww
s
w
V
V
W
Ww
Void Ratio for saturated soils
ws
s
w
w
s
v
GW
W
VV
e
sGm
1.9 Definitions
Definition 8:
VolumeTotalWeightTotal
s
vs
wssw
VV
V
GVV
1
e
VVG w
s
ws
1
Divide top and bottom lines by Vs
e
VV
VVG w
s
v
v
ws
1
.
e
eSG wrs
1
sv
ssww
VV
VV
Solid ParticlesWater
1.9 Definitions
8 Bulk Unit Weight )1( e
eSG wrs
9 Saturated Unit Weight sat
)1( e
eG ws
10 Dry Unit Weight d
)1( e
G ws
11 Submerged Unit Weight
’ =
-
w
)1(
1
)1(
e
G
e
eG
ws
wws
1.9 Definitions
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
1.10 Estimation of effective vertical stress at depth
Method 1
Water table
3
3
11
Ground Surface
1
2
3 A
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
1.10 Estimation of effective vertical stress at depth
Method 2
Water table
3
3
11
Ground Surface
1
2
3 A
Landslide
Consequence
Remedial Measures
Remove ConsequenceSafe at the moment
Cost
Build
Landslide Warning
No Danger Temporarily Safe
Design
LandslidePreventive Measures
Stability Assessment Slope Profile
GeologyErosion/DepositionGlaciationWeatheringGeochemistry
Cut / Fill SlopesConstructionDrainage Pumping
Man’s Influence (Agriculture /Development)
Earthquakes
Material Properties (Shear Strength)
Ground Loading(Consolidation)
Slope Management
Hydrology (rainfall)
Ground Water
Surface Water
GIS