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Civil Engineering - Texas Tech University
CE 3121: Geotechnical Engineering Laboratory
Class 6
Shear Strength
(Direct Shea...
Civil Engineering - Texas Tech University
 Shear strength in soils
 Introduction
 Definitions
 Direct shear test
 Int...
Civil Engineering - Texas Tech University
Shear Strength
 The strength of a material is the greatest
stress it can sustai...
Civil Engineering - Texas Tech University
Slope Failure in Soils
Failure due to inadequate
strength at shear interface
Civil Engineering - Texas Tech University
Shear Failure in Soils
Civil Engineering - Texas Tech University
Bearing Capacity Failure
Civil Engineering - Texas Tech University
Transcosna Grain Elevator Canada
(Oct. 18, 1913)
West side of foundation sank 24...
Civil Engineering - Texas Tech University
Significance of Shear Strength
 Engineers must understand the nature of
shearin...
Civil Engineering - Texas Tech University
Shear Strength in Soils
 The shear strength of a soil is its resistance to
shea...
Civil Engineering - Texas Tech University
Shear Strength in Soils (cont.)
 Soil derives its shear strength from two
sourc...
Civil Engineering - Texas Tech University
Shear Strength of Soils; Cohesion
 Cohesion (C), is a measure of the forces tha...
Civil Engineering - Texas Tech University
Shear Strength of Soils; Internal Friction
 Internal Friction angle (f), is the...
Civil Engineering - Texas Tech University
Mohr-Coulomb Failure Criteria
 This theory states that a material fails
because...
Civil Engineering - Texas Tech University
Shear
Strength,S
Normal Stress, n =  = g h
C
f = f
Mohr-Coulomb Failure Cri...
Civil Engineering - Texas Tech University
General State of Stress
σ1
σ1 major principle stress
σ3 σ3
Minor principle stres...
Civil Engineering - Texas Tech University
State of Stresses in Soils
σ1
Shear
stress σ3
σ3
Normal stress σn
Consider the f...
Civil Engineering - Texas Tech University
Determination of Shear Strength
Parameters
 The shear strength parameters of a ...
Civil Engineering - Texas Tech University
Direct Shear Test
 Direct shear test is Quick and Inexpensive
 Shortcoming is ...
Civil Engineering - Texas Tech University
Direct Shear Test (cont.)
 The test equipment consists of
a metal box in which ...
Civil Engineering - Texas Tech University
Direct Shear Test
Civil Engineering - Texas Tech University
Direct Shear Test
Civil Engineering - Texas Tech University
Civil Engineering - Texas Tech University
Direct Shear Test
Civil Engineering - Texas Tech University
Direct Shear Test Data
Shearstress
Residual Strength
Peak Strength
Civil Engineering - Texas Tech University
Direct Shear Test Data
Volume change
DH
Civil Engineering - Texas Tech University
Direct Shear Test (Procedure)
1.Measure inner side or diameter of shear box and ...
Civil Engineering - Texas Tech University
Direct Shear Test (Procedure)
8. Start the motor with selected speed (0.1 in/min...
Civil Engineering - Texas Tech University
Calculations
1. Determine the dry unit
weight, gd
2. Calculate the void
ratio, e...
Civil Engineering - Texas Tech University
Figures
Shearstress,s
Peak Stress
N1 = 10 kg
N2 = 20 kg
N3 = 30 kg
Horizontal di...
Civil Engineering - Texas Tech University
Figures (cont)
ShearStress,s(psf)
C
f
(1,s1)
(3,s3)
(2,s2)
Normal Stress , ...
Civil Engineering - Texas Tech University
Figures (cont)
Verticaldisplacement
Horizontal displacement
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Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

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Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

  1. 1. Civil Engineering - Texas Tech University CE 3121: Geotechnical Engineering Laboratory Class 6 Shear Strength (Direct Shear Test) Sources: Soil Mechanics – Laboratory Manual, B.M. DAS (Chapter 15) Soil Properties, Testing, Measurement, and Evaluation, C. Liu, J. Evett
  2. 2. Civil Engineering - Texas Tech University  Shear strength in soils  Introduction  Definitions  Direct shear test  Introduction  Procedure  Calculation  Results and Figures Class Outlines
  3. 3. Civil Engineering - Texas Tech University Shear Strength  The strength of a material is the greatest stress it can sustain  The safety of any geotechnical structure is dependent on the strength of the soil  If the soil fails, the structure founded on it can collapse
  4. 4. Civil Engineering - Texas Tech University Slope Failure in Soils Failure due to inadequate strength at shear interface
  5. 5. Civil Engineering - Texas Tech University Shear Failure in Soils
  6. 6. Civil Engineering - Texas Tech University Bearing Capacity Failure
  7. 7. Civil Engineering - Texas Tech University Transcosna Grain Elevator Canada (Oct. 18, 1913) West side of foundation sank 24-ft
  8. 8. Civil Engineering - Texas Tech University Significance of Shear Strength  Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as;  Bearing capacity  Slope stability  Lateral earth pressure on earth-retaining structures  Pavement
  9. 9. Civil Engineering - Texas Tech University Shear Strength in Soils  The shear strength of a soil is its resistance to shearing stresses.  It is a measure of the soil resistance to deformation by continuous displacement of its individual soil particles  Shear strength in soils depends primarily on interactions between particles  Shear failure occurs when the stresses between the particles are such that they slide or roll past each other
  10. 10. Civil Engineering - Texas Tech University Shear Strength in Soils (cont.)  Soil derives its shear strength from two sources:  Cohesion between particles (stress independent component)  Cementation between sand grains  Electrostatic attraction between clay particles  Frictional resistance between particles (stress dependent component)
  11. 11. Civil Engineering - Texas Tech University Shear Strength of Soils; Cohesion  Cohesion (C), is a measure of the forces that cement particles of soils  Dry sand with no cementation  Dry sand with some cementation  Soft clay  Stiff clay
  12. 12. Civil Engineering - Texas Tech University Shear Strength of Soils; Internal Friction  Internal Friction angle (f), is the measure of the shear strength of soils due to friction
  13. 13. Civil Engineering - Texas Tech University Mohr-Coulomb Failure Criteria  This theory states that a material fails because of a critical combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone.  The relationship between normal stress and shear is given as f  tancs frictioninternalofangle cohesionc strengthshears    f
  14. 14. Civil Engineering - Texas Tech University Shear Strength,S Normal Stress, n =  = g h C f = f Mohr-Coulomb Failure Criterion
  15. 15. Civil Engineering - Texas Tech University General State of Stress σ1 σ1 major principle stress σ3 σ3 Minor principle stress Confining stress
  16. 16. Civil Engineering - Texas Tech University State of Stresses in Soils σ1 Shear stress σ3 σ3 Normal stress σn Consider the following situation: - A normal stress is applied vertically and held constant - A shear stress is then applied until failure
  17. 17. Civil Engineering - Texas Tech University Determination of Shear Strength Parameters  The shear strength parameters of a soil are determined in the lab primarily with two types of tests;  Direct Shear Test  Triaxial Shear Test Soil Normal stress σn Shear stress σ3 3 1
  18. 18. Civil Engineering - Texas Tech University Direct Shear Test  Direct shear test is Quick and Inexpensive  Shortcoming is that it fails the soil on a designated plane which may not be the weakest one  Used to determine the shear strength of both cohesive as well as non-cohesive soils  ASTM D 3080
  19. 19. Civil Engineering - Texas Tech University Direct Shear Test (cont.)  The test equipment consists of a metal box in which the soil specimen is placed  The box is split horizontally into two halves  Vertical force (normal stress) is applied through a metal platen  Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen Soil Normal stress σn Shear stress σ3
  20. 20. Civil Engineering - Texas Tech University Direct Shear Test
  21. 21. Civil Engineering - Texas Tech University Direct Shear Test
  22. 22. Civil Engineering - Texas Tech University
  23. 23. Civil Engineering - Texas Tech University Direct Shear Test
  24. 24. Civil Engineering - Texas Tech University Direct Shear Test Data Shearstress Residual Strength Peak Strength
  25. 25. Civil Engineering - Texas Tech University Direct Shear Test Data Volume change DH
  26. 26. Civil Engineering - Texas Tech University Direct Shear Test (Procedure) 1.Measure inner side or diameter of shear box and find the area 2.Make sure top and bottom halves of shear box are in contact and fixed together. 3.Weigh out 150 g of sand. 4.Place the soil in three layers in the mold using the funnel. Compact the soil with 20 blows per layer. 5.Place cover on top of sand 6.Place shear box in machine. 7.Apply normal force. The weights to use for the three runs are 2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10 kg, 20 kg, and 30 kg, if the load is applied directly. Note: Lever arm loading ratio 1:10 (2kg weight = 20 kg)
  27. 27. Civil Engineering - Texas Tech University Direct Shear Test (Procedure) 8. Start the motor with selected speed (0.1 in/min) so that the rate of shearing is at a selected constant rate 9. Take the horizontal displacement gauge, vertical displacement gage and shear load gage readings. Record the readings on the data sheet. 10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the diameter.
  28. 28. Civil Engineering - Texas Tech University Calculations 1. Determine the dry unit weight, gd 2. Calculate the void ratio, e 3. Calculate the normal stress & shear stress 1 d wGs e g g A V A N   ;
  29. 29. Civil Engineering - Texas Tech University Figures Shearstress,s Peak Stress N1 = 10 kg N2 = 20 kg N3 = 30 kg Horizontal displacement, DH s3 s2 s1
  30. 30. Civil Engineering - Texas Tech University Figures (cont) ShearStress,s(psf) C f (1,s1) (3,s3) (2,s2) Normal Stress , psf
  31. 31. Civil Engineering - Texas Tech University Figures (cont) Verticaldisplacement Horizontal displacement
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Class 6 Shear Strength - Direct Shear Test ( Geotechnical Engineering )

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