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- 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 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 SOIL STRENGTH SOIL • Mostly loaded in compression • But fails mostly in shear Embankment Strip footing Failure surface Mobilized shear resistance
- 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 • 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. 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. 8 SHEAR FAILURE OF SOIL Retaining wall Soils generally fail in shear
- 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. 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. 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. 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. 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. 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. 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. 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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