Course
Year
: S0705 – Soil Mechanic
: 2008
TOPIC 5
SOIL BEHAVIOUR
CONTENT
•
•
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SOIL STRENGTH (SESSION 17-18 : F2F)
STRESS – STRAIN RESPONSE (SESSION 19-20 : OFC)
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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SOIL STRENGTH
EMBANKMENT LANDSLIDE
GLOBAL FAILURE OF
SHALLOW FOUNDATION
LOCAL FAILURE OF
SHALLOW FOUNDATION
VERTICAL SLOPE
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RETAINING EARTH
WALL
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
–
–
–
–
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Test Method
Sample Disturbing
Water Content
Strain Rate
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
 = c + .tan

Mohr-Coulomb envelope line

Mohr envelope line
c
3
3
1
1 = 3 + 
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1

MOHR COULOMB CONCEPT
1
n
3

3
3

1

1 > 3
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1
(1)
n 
(2)

1   3
  3
 1
.Cos 2
2
2
1   3
.Sin 2
2
MOHR COULOMB CONCEPT
(1) and (2)
 = c + n.tan
1   3 

3
. tan   c

0.5 . Sin 2  Cos  .
2
tan 

The failure occurs when the value of 1 is minimum or
the value of (0.5 . Sin2 - Cos2 . tan) maximum
  45o 
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
2



 1   3 . tan 2 45o   / 2  2.c. tan 45o   / 2

MOHR COULOMB CONCEPT

Failure Envelope Line

c
3
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
2
n
1

EXAMPLE
Determine :
- n
-
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EXAMPLE
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Center of Circle =
1   3 52  12

 32kPa
2
2
Radius of Circle =
1   3 52  12

 20kPa
2
2
EXAMPLE
n 

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1   3
2
1   3
2

1   3
2
.Sin 2 
.Cos 2 
52  12 52  12

.Cos70o  38.84 kPa
2
2
52  12
.Sin 70o  18.8 kPa
2
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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UNCONFINED COMPRESSION TEST
qu
cu  s u 
2
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DIRECT SHEAR TEST
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DIRECT SHEAR TEST
Pasir
Clay/Silt

c
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TRIAXIAL TEST
3 Conditions
– Unconsolidated Undrained (UU)
– Consolidated Undrained (CU)
– Consolidated Drained (CD)
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TRIAXIAL TEST
Test Condition
Stage 1
Stage 2
3
3

3
3
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=ud0). 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=ud0). 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
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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 TEST
Soil type
Cohesive
Type of construction
Type of tests and shear strength
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 dam
No 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
Stress, 
Stress, 
Linear and Elastic
Strain, 
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Non-Linear and
Elastic
Strain, 
STRESS-STRAIN MODELS
Stress, 
Stress, 
Elasto-Plastic
Strain, 
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Elastic Perfectly
Plastic
Strain, 
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
dz
H gxz
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gxz = dx/H
z = - dz/H = v

SAND BEHAVIOUR
Depends on:
•
Mean Effective stress (Normal effective stress in simple shear)
•
Relative density, Id
emax - e
Id =
emax - emin
gd
1
Id
=
g dm in
1
g dm in
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Gs g w
=
1+ e
-
-
1
gd
1
g dm ax
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 BEHAVIOUR
Essentially the same as sands. However, data presented as a function of OCR rather
than relative density. OCR is defined as
 pc
OCR 

e
swelling line
NCL - normal consolidation line
CSL
log s’
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It is found that NCL and CSL have the same slope in e-log s’
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
su 
cos  ult
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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)
e
Clay
Loose
Sand
Dense
NCL
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0.1
1
10
NCL
100
log ’ (MPa)
APPLICATION
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