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

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

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  1 Geotechnical Engineering–II [CE-321] BScCivil Engineering – 5th Semester by Dr. Muhammad Irfan Assistant Professor Civil Engg. Dept. – UET Lahore Email: [email protected] Lecture Handouts: https://groups.google.com/d/forum/geotech-ii_2015session Lecture # 1 6-Sep-2017
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  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
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  3 SOIL STRENGTH SOIL • Mostlyloaded in compression • But fails mostly in shear Embankment Strip footing Failure surface Mobilized shear resistance
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  4 • Greatest shearstress 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)
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  5 • Resistance toshearing 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)
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  7 SHEAR FAILURE OFSOIL 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
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  8 SHEAR FAILURE OFSOIL Retaining wall Soils generally fail in shear
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  9 SHEAR FAILURE OFSOIL 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
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  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?
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  11 The relationship betweennormal 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
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  12 MOHR-COULOMB FAILURE CRITERIA   f Non-cohesive/GranularSoils (c = 0; f > 0)   Cohesive Soils (c > 0; f = 0) c   c-f Soils (c > 0; f > 0) c f
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  13 N T Area: A For acontinuous 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
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  14 MOHR-COULOMB FAILURE CRITERIA  fis 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
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  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 ’.
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  16 CONCLUDED REFERENCE MATERIAL Principles ofGeotechnical Engineering – (7th Edition) Braja M. Das Chapter #12 Geotechnical Engineering – Principles and Practices – (2nd Edition) Coduto, Yueng, and Kitch Chapter #12