env-2e1y: fluvial geomorphology: 2004 - 5 slope stability and geotechnics landslide hazards river...

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


Cost

Build

Landslide Warning

No Danger

Design



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


Cost

Build

Landslide Warning

No Danger

Design






Earthquakes




Ground Water

Surface Water

Landslide

Consequence

Remedial Measures


Cost

Build

Landslide Warning

No Danger Temporarily Safe

Design






Earthquakes



Slope Management


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.


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.


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


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


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.


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.


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.


(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


Cost

Build

Landslide Warning

No Danger Temporarily Safe

Design






Earthquakes



Slope Management


Ground Water

Surface Water

GIS

env-2e1y: fluvial geomorphology: 2004 - 5 slope stability and geotechnics landslide hazards river...

Documents

stability of soils

lecture slope stability

tovey landslide

landslide risk

toe landslide

landslide movement

introduction slide

course slide