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Geotechnical Engineering-II [Lec #1: Shear Strength of Soil]

Class notes of Geotechnical Engineering course I used to teach at UET Lahore. Feel free to download the slide show.
Anyone looking to modify these files and use them for their own teaching purposes can contact me directly to get hold of editable version.

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Geotechnical Engineering-II [Lec #1: Shear Strength of Soil]

  1. 1. 1 Geotechnical Engineering–II [CE-321] BSc Civil Engineering – 5th Semester by Dr. Muhammad Irfan Assistant Professor Civil Engg. Dept. – UET Lahore Email: mirfan1@msn.com Lecture Handouts: https://groups.google.com/d/forum/geotech-ii_2015session Lecture # 1 6-Sep-2017
  2. 2. 2 SOIL STRENGTH Construction Materials Concrete, Steel, Bricks, Wood, Aggregate, Soil, etc. Most important property for Civil Engineers? • Concrete, Brick, Wood  • Steel  • Soil ? Material Strength Compressive strength Tensile Strength
  3. 3. 3 SOIL STRENGTH SOIL • Mostly loaded in compression • But fails mostly in shear Embankment Strip footing Failure surface Mobilized shear resistance
  4. 4. 4 • Greatest shear stress a material can sustain before failure • Safety of geotechnical structure dependent on soil shear strength • Failure of soil → Failure of whole structure SHEAR STRENGTH Typhoon triggered landslide Wakayama, Japan (September 2011)
  5. 5. 5 • Resistance to shearing stresses • Shear failure occurs due to sliding or rolling of particles past each other. • Sources of soil shear strength – Cohesion • Cementation between sand grains • Electrostatic attraction between clay particles – Frictional resistance • Interlocking between sand grains SHEAR STRENGTH OF SOILS Sliding of particlesRolling of particles (stress independent component) (stress dependent component)  Cohesion (c)  Angle of internal friction (f)
  6. 6. 7 SHEAR FAILURE OF SOIL Embankment Strip footing Soils generally fail in shear At failure, shear stress/resistance along failure surface (mobilized shear resistance) reaches the shear strength. Failure surface Mobilized shear resistance
  7. 7. 8 SHEAR FAILURE OF SOIL Retaining wall Soils generally fail in shear
  8. 8. 9 SHEAR FAILURE OF SOIL Retaining wall Mobilized shear resistance Failure surface At failure, shear stress/resistance along failure surface (mobilized shear resistance) reaches the shear strength. Soils generally fail in shear
  9. 9. 10 SHEAR FAILURE MECHANISM • Soil grains slide/roll over each other along the failure surface. • No crushing of individual grains. Failure Surface At failure, shear stress/resistance along failure surface () reaches shear strength (f). X Y Difference in shear strength of X & Y?
  10. 10. 11 The relationship between normal and shear stress on the failure plane f tan cf f = shear strength c = cohesion  = normal stress Φ = angle of internal friction )( f σ1 σ3 f Friction angle f    Graphical representation Cohesion c MOHR-COULOMB FAILURE CRITERIA
  11. 11. 12 MOHR-COULOMB FAILURE CRITERIA   f Non-cohesive/Granular Soils (c = 0; f > 0)   Cohesive Soils (c > 0; f = 0) c   c-f Soils (c > 0; f > 0) c f
  12. 12. 13 N T Area: A For a continuous material Normal stress: σ = N / A (compression: +ve) Shear stress: τ = T / A (counter-clock-wise: +ve) Basic Concepts Principle Stress: Max. and min. value of normal stresses Principle Plane: Plane on which principle stresses act • Normal stresses are either max. or min. on principle planes • Shear stresses are zero on principle planes MOHR-COULOMB FAILURE CRITERIA
  13. 13. 14 MOHR-COULOMB FAILURE CRITERIA  f is the maximum shear stress the soil can take without failure, under any particular normal stress of .  f tan cf c f Cohesion Friction angle f  In terms of Total Stress
  14. 14. 15 MOHR-COULOMB FAILURE CRITERIA u   ’ f  tancf f’ Effective friction angle c’ Effective cohesion f ’  = Total stress u = Pore water pressure In terms of Effective Stress f is the maximum shear stress the soil can take without failure, under any particular normal effective stress of ’.
  15. 15. 16 CONCLUDED REFERENCE MATERIAL Principles of Geotechnical Engineering – (7th Edition) Braja M. Das Chapter #12 Geotechnical Engineering – Principles and Practices – (2nd Edition) Coduto, Yueng, and Kitch Chapter #12

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