TOPIC 5 SOIL BEHAVIOUR

Course : S0705 – Soil Mechanic

Year : 2008

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CONTENT

• SOIL STRENGTH (SESSION 17-18 : F2F)• STRESS – STRAIN RESPONSE (SESSION 19-20 : OFC)

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SESSION 17-18

SOIL STRENGTH

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SOIL STRENGTH

• DEFINITION

The maximum or ultimate stress the material can sustain against the force of landslide, failure, etc.

• APPLICATION

Soil Strength can be used for calculating :– Bearing Capacity of Soil– Slope Stability– Lateral Pressure

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VERTICAL SLOPERETAINING EARTH WALL

EMBANKMENT LANDSLIDE

GLOBAL FAILURE OF SHALLOW FOUNDATION

LOCAL FAILURE OF SHALLOW FOUNDATION

SOIL STRENGTH

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SOIL STRENGTH

• FIELD INFLUENCE FACTOR– Soil Condition : void ratio, particle shape and size– Soil Type : Sand, Sandy, Clay etc– Water Content (especially for clay)– Type of Load and its Rate– Anisotropic Condition

• LABORATORY– Test Method– Sample Disturbing– Water Content– Strain Rate

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SHEAR STRENGTH OF SOIL

• PARAMETER – Cohesion (c)– Internal Friction Angle ()

• CONDITION– Total (c and )– Effective (c’ and ’)

• GENERAL EQUATION (COULOMB)

= c + n.tan

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SOIL TYPES

• COHESIVE SOIL– Has cohesion (c)– Example : Clay, Silt

• COHESIONLESS Soil– Only has internal friction angle () ; c = 0– Example : Sand, Gravel

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SHEAR STRENGTH PARAMETER

• COHESION (C)

Sticking together of like materials.

• INTERNAL FRICTION ANGLE ()

The stress-dependent component which is similar to sliding friction of two or more soil particles

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SHEAR STRENGTH PARAMETER

• UNDRAINED SHEAR STRENGTH

Use for analysis of total stress

Commonly = 0 and c = cu

• DRAINED SHEAR STRENGTH

Use for analysis of effective stress, with parameter c’ and ’

’ = c’ + (n – u) tan ’

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MOHR COULOMB CONCEPT

Mohr envelope line

Mohr-Coulomb envelope line

c

3 3 11

= c + .tan

1 = 3 +

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MOHR COULOMB CONCEPT

2Cos.22

3131n

3

1

1

3

1 > 3

3

1

n

2Sin.2

31

(1)

(2)

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MOHR COULOMB CONCEPT

tan.2.5.0

tan.2

331

CosSin

c

245o

= c + n.tan(1) and (2)

The failure occurs when the value of 1 is minimum or the value of (0.5 . Sin2 - Cos2 . tan) maximum

2/45tan.c.22/45tan. oo231

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MOHR COULOMB CONCEPT

2c

3 1n

Failure Envelope Line

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EXAMPLE

Determine :

- n

-

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EXAMPLE

kPa322

1252

231

Center of Circle =

kPa202

1252

231

Radius of Circle =

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EXAMPLE

kPaCosCos on 84.3870.

2

1252

2

12522.

223131

kPaSinSin o 8.1870.2

12522.

231

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EXAMPLE

Determine :

-

-

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EXAMPLE

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SHEAR STRENGTH OF SOIL

• LABORATORY TESTS– Unconfined Compression Test– Direct Shear Test– Triaxial Test (UU, CU, CD)

• FIELD INVESTIGATION– Vane Shear Test

• PARAMETER CORRELATIONS – Cone Resistance (qc)– N-SPT Value – California Bearing Capacity

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UNCONFINED COMPRESSION TEST

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UNCONFINED COMPRESSION TEST

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2

qsc uuu

UNCONFINED COMPRESSION TEST

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DIRECT SHEAR TEST

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DIRECT SHEAR TEST

Pasir

Clay/Silt

c

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3 Conditions– Unconsolidated Undrained (UU)– Consolidated Undrained (CU)– Consolidated Drained (CD)

TRIAXIAL TEST

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TRIAXIAL TEST

Test Condition Stage 1 Stage 2

Unconsolidated Undrained (UU)

Apply confining pressure 3 while the drainage line from the specimen is kept closed (drainage is not permitted), then the initial pore water pressure (u=uo) is not equal to zero

Apply an added stress at axial direction. The drainage line from the specimen is still kept closed (drainage is not permitted) (u=ud0). At failure state =f ; pore water pressure u=uf=uo+ud(f)

Consolidated Undrained (CU)

Apply confining pressure 3 while the drainage line from the specimen is opened (drainage is permitted), then the initial pore water pressure (u=uo) is equal to zero

Apply an added stress at axial direction. The drainage line from the specimen is kept closed (drainage is not permitted) (u=ud0). At failure state =f ; pore water pressure u=uf=uo+ud(f)=ud(f)

Consolidated Drained (CD)

Apply confining pressure 3 while the drainage line from the specimen is opened (drainage is permitted), then the initial pore water pressure (u=uo) is equal to zero

Apply an added stress at axial direction. The drainage line from the specimen is opened (drainage is permitted) so the pore water pressure (u=ud) is equal to zero. At failure state =f ; pore water pressure u=uf=uo+ud(f)=0

3

3

3

3

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TRIAXIAL TEST

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TRIAXIAL TEST

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'

'

TRIAXIAL TEST

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TRIAXIAL TEST

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SHEAR STRENGTH OF SOIL

SELECTION OF TRIAXIAL TESTSoil type Type of construction Type of tests and shear strength

Cohesive Short term (end of construction time)

Triaxial UU or CU for Undrained Strength with appropriate level of insitu strength

Staging Construction Triaxial CU for Undrained Strength with appropriate level of insitu strength

Long term Triaxial CU with pore water pressure measurement or Triaxial CD for effective shear strength parameter

Granular All Strength parameter ’ which is got from field investigation or direct shear test

Material c- Long Term Triaxial CU with pore water pressure measurement or Triaxial CD for effective shear strength parameter

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EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE

Embankment constructed rapidly over a soft clay deposit

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EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE

Large earth dam constructed rapidly with no change in water content of clay core

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EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE

Footing placed rapidly on clay deposit

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EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE

Embankment raised (2) subsequent to consolidation under its original height (1)

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EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE

Rapid drawdown behind an earth damNo drainage of the core. Reservoir level falls from 1 2

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EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE

Rapid construction of an embankment on a natural slope

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EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE

Embankment constructed very slowly, in layers, over a soft clay deposit

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EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE

Earth dam with steady-state seepage

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EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE

Excavation or natural slope in clay

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SELECTION OF SHEAR STRENGTH PARAMETER

CU with pore water pressure measurement

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SESSION 19-20

STRESS-STRAIN RESPONSE

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STRESS-STRAIN MODELS

Str

ess,

Linear and Elastic

Str

ess,

Strain,

Non-Linear and Elastic

Strain,

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STRESS-STRAIN MODELS

Strain,

Str

ess,

Elasto-Plastic

Str

ess,

Strain,

Elastic Perfectly Plastic

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STRESS-STRAIN RESPONSE OF SOILS

Triaxial tests are the standard means of investigating the stress-strain-strength response of soils. To simplify, only simple shear tests will be considered.

The simple shear test is an improved shear box test which imposes more uniform stresses and strains.

dx

H

dz

xz

xz = dx/H z = - dz/H = v

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SAND BEHAVIOUR

e - e

e - e = I dminmax

max

e + 1

G = ws

d

dmaxdmin

ddmind 1

- 1

1 -

1

= I

Depends on:

• Mean Effective stress (Normal effective stress in simple shear)

• Relative density, Id

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SAND BEHAVIOUR

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SAND BEHAVIOUR

For tests performed with the same normal stress

• All samples approach the same ultimate shear stress and void ratio, irrespective of the initial relative density

• Initially dense samples attain higher peak angles of friction

• Initially dense soils expand (dilate) when sheared

• Initially loose soils compress when sheared

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SAND BEHAVIOUR

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SAND BEHAVIOUR

• The ultimate values of shear stress and void ratio depend on the applied normal stress

• The ultimate stress ratio and angle of friction are independent of density and stress level

• Initially dense samples attain higher peak angles of friction, but the peak friction angle decreases as the stress increases

• Initially dense soils expand and initially loose soils compress when sheared. Increasing the normal stress causes less dilation (more compression)

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CLAY BEHAVIOUREssentially the same as sands. However, data presented as a function of OCR rather than relative density. OCR is defined as

NCL - normal consolidation line

e

log s’

CSL

swelling line

pcOCR

It is found that NCL and CSL have the same slope in e-log s’

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CLAY BEHAVIOUR – DRAINED CONDITION

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CLAY BEHAVIOUR – DRAINED CONDITION

• In drained loading the change in effective stress is identical to the change in total stress. In a shear box (or simple shear) test the normal stress is usually kept constant, and hence the response is fixed in the t, s’ plot.

• The soil heads towards a critical state when sheared, and this ultimate (or critical) state can be determined from the t, s’ plot.

• The change in void ratio can then be determined.

• Knowing the sign of the volume change enables the likely stress-strain response to be estimated.

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CLAY BEHAVIOUR – UNDRAINED CONDITION

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CLAY BEHAVIOUR – UNDRAINED CONDITION

• In undrained loading the void ratio (moisture content) must stay constant.

• The soil must head towards a critical state when sheared, and knowing e the critical state can be determined from the e, ’ plot.

• Once the critical state has been determined in the e, ’ plot the ultimate shear stress is also fixed. The ultimate shear stress is related to the undrained strength. This relation can be obtained by considering a Mohr’s circle.

ult

ultus

cos

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CLAY BEHAVIOUR – UNDRAINED CONDITION

• In undrained loading the effective stresses are fixed because void ratio (moisture content) must stay constant.

• The total stresses are controlled by the external loads, and the pore pressure is simply the difference between the total stress and effective stress.

• The CSL provides an explanation for the existence of cohesion (undrained strength) in frictional soils

• From the CSL it can also be seen that changes in moisture content (void ratio) will lead to different undrained strengths

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DIFFERENCES BETWEEN SAND AND CLAY

All soils are essentially frictional materials but different parameters are used for sands (Id) and clays (OCR)

0.1 1 10 100 log ’ (MPa)

e

NCL NCL

Loose

Dense

Clay

Sand

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APPLICATION