1. Course : S0705 – Soil Mechanic
Year : 2008
TOPIC 5 SOIL BEHAVIOUR

2. CONTENT SOIL STRENGTH (SESSION 17-18 : F2F)
STRESS – STRAIN RESPONSE (SESSION : OFC)
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3. SESSION 17-18 SOIL STRENGTH
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4. 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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5. SOIL STRENGTH EMBANKMENT LANDSLIDE
GLOBAL FAILURE OF SHALLOW FOUNDATION
LOCAL FAILURE OF SHALLOW FOUNDATION
RETAINING EARTH WALL
VERTICAL SLOPE
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6. SOIL STRENGTH FIELD INFLUENCE FACTOR LABORATORY
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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7. GENERAL EQUATION (COULOMB)  = c + n.tan
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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8. SOIL TYPES COHESIVE SOIL COHESIONLESS Soil Has cohesion (c)
Example : Clay, Silt
COHESIONLESS Soil
Only has internal friction angle () ; c = 0
Example : Sand, Gravel
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9. 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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10. 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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11.  = c + .tan MOHR COULOMB CONCEPT c  3 1   1 = 3 + 
Mohr-Coulomb envelope line c  3 1  
Mohr envelope line
1 = 3 + 
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12. MOHR COULOMB CONCEPT n  1 > 3  1 3 3  1 (1) (2)
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13.  = c + n.tan MOHR COULOMB CONCEPT (1) and (2)
The failure occurs when the value of 1 is minimum or the value of (0.5 . Sin2 - Cos2 . tan) maximum
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14. MOHR COULOMB CONCEPT 2 c   3 1 n   Failure Envelope Line
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15. EXAMPLE
Determine :
- n
- 
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16. EXAMPLE
Center of Circle =
Radius of Circle =
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17. EXAMPLE
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18. EXAMPLE
Determine :
- 
- 
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19. EXAMPLE
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20. SHEAR STRENGTH OF SOIL LABORATORY TESTS FIELD INVESTIGATION
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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21. UNCONFINED COMPRESSION TEST
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22. UNCONFINED COMPRESSION TEST
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23. UNCONFINED COMPRESSION TEST
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24. DIRECT SHEAR TEST
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25. DIRECT SHEAR TEST
Clay/Silt
Pasir  c
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26. TRIAXIAL TEST 3 Conditions Unconsolidated Undrained (UU)
Consolidated Undrained (CU)
Consolidated Drained (CD)
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27. 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=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 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
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28. TRIAXIAL TEST
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29. TRIAXIAL TEST
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30. TRIAXIAL TEST
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31. TRIAXIAL TEST
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32. Type of tests and shear strength
SHEAR STRENGTH OF SOIL
SELECTION OF TRIAXIAL TEST
Soil 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
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33. EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE
Embankment constructed rapidly over a soft clay deposit
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34. 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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35. EXAMPLE USE OF UU STRENGTH IN ENGINEERING PRACTICE
Footing placed rapidly on clay deposit
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36. EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE
Embankment raised (2) subsequent to consolidation under its original height (1)
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37. 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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38. EXAMPLE USE OF CU STRENGTH IN ENGINEERING PRACTICE
Rapid construction of an embankment on a natural slope
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39. EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE
Embankment constructed very slowly, in layers, over a soft clay deposit
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40. EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE
Earth dam with steady-state seepage
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41. EXAMPLE USE OF CD STRENGTH IN ENGINEERING PRACTICE
Excavation or natural slope in clay
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42. SELECTION OF SHEAR STRENGTH PARAMETER
CU with pore water pressure measurement
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43. SESSION 19-20 STRESS-STRAIN RESPONSE
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44. STRESS-STRAIN MODELS Non-Linear and Elastic Linear and Elastic
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45. STRESS-STRAIN MODELS Elastic Perfectly Plastic Elasto-Plastic
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46. 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. s dx t dz H
gxz
gxz = dx/H ez = - dz/H = ev
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47. SAND BEHAVIOUR Depends on:
Mean Effective stress (Normal effective stress in simple shear)
Relative density, Id
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48. SAND BEHAVIOUR
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49. 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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50. SAND BEHAVIOUR
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51. 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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52. CLAY BEHAVIOUR
Essentially the same as sands. However, data presented as a function of OCR rather than relative density. OCR is defined as e
log s’
CSL
swelling line
NCL - normal consolidation line
It is found that NCL and CSL have the same slope in e-log s’
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53. CLAY BEHAVIOUR – DRAINED CONDITION
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54. 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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55. CLAY BEHAVIOUR – UNDRAINED CONDITION
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56. 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, s’ plot.
Once the critical state has been determined in the e, s’ 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.
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57. 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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58. 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
NCL
log s’ (MPa)
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0.1 1 10
100

59. APPLICATION
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