# SHEAR STRENGTH THEORY

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Published on October 27, 2016

Author: VibhanshuSingh5

Source: slideshare.net

1. SHEAR STRENGTH THEORY GEOTECHNOLOGY SUBMITTED TO: DR. N. SHANKAR

2. INTRODUCTION  In general, the shear strength of any material is the load per unit area or pressure that it can withstand before undergoing shearing failure.  Shear strength: o Soil’s ability to resist sliding o Primarily depends on interaction between soil particles Important for: o Foundation design o Lateral earth pressure calculations o Slope stability

3. SHEAR STRENGTH OF SOIL  Soil is weak in tension  Soil can resist compression  For excessive compression, failure occur in the form of shearing along the internal surface within the soil  The failure in soil occurs by relative movement of the particles and not by breaking of particles

4. SHEAR STRENGTH IN SOILS  Soil derives its shear strength from two sources: • Cohesion between particles(stress independent component) • Frictional resistance between particles(stress dependent component)

5. General curve for shear strength of soil S =ANGLE OF INTERNAL FRICTION C    tan tan   cs s

6. COULOMB EQUATION S: shear strength C: cohesion : Angle of internal friction Effective intergranular normal pressure  tan cs  :σ

7. MOHR-COULOMB THEORY  According to Mohr, the failure is caused by a critical combination of normal and shear stress  The soil fails when the shear strength ‘s’ on the failure plane at failure is a unique function of normal stress ‘ ’ acting on that plane s=f( )  Failure of material occurs when the Mohr circle of stresses touches the Mohr envelope 'tan''   cf ’ X Y ~ stable X Y ~ failure

8. MOHR-COULOMB FAILURE CRITERIA  This theory states that a material fails because of critical combination of normal stress and shear stress and not from their either maximum normal or shear stress alone  Mohr-coulomb failure criteria (In terms of total stress)    tan cf c  f 

9. MOHR-COULOMB FAILURE CRITERIA (in terms of effective stress) u=pore water pressure  f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.  ’ 'tan''   cf c’ ’ f ’ u '

10. METHODS OF INVESTIGATING SHEAR STRENGTH  Unconfined compression test (for cohesive soil)  Direct shear test  Triaxial compression test  Vane test (for soft clay)  Standard penetration test (for cohesionless soil)  Penetrometer test In which UCC,Direct shear and Triaxial compression are mainly performed laboratory test.

11. UNCONFINED COMPRESSIVE STRENGTH TEST  It is performed mainly on cylindrical,moist clay specimens sampled from bore holes  Measures vertical stress applied to soil sample with no confining pressure  Shear stress on failure plane is determined similarly to undrained triaxial compression test ; =unconfined compressive strength Soil Specimen 2 uq c  uq uq uq

12. DIRECT SHEAR TEST  It can be performed on all type of soil, moist or dry  Measure shear stress at failure on failure plane for various normal stresses  Failure plane is controlled parallel to direction of applied load Shearing Force Shearing Force Shearing Force Shearing Force Normal Load Normal Load

13. TRIAXIAL COMPRESSION TEST  It can be performed on all type of soil ,moist or dry and can consolidate sample to in situ conditions by tracking pore water pressure  Measure vertical stress applied to soil sample and confining pressure  Shear stress on failure plane must be calculated from principal stresses. Porous stone impervious membrane Piston (to apply deviatoric stress) O-ring pedestal Perspex cell Cell pressure Back pressure Pore pressure or volume change Water Soil sample

14. VANE SHEAR TEST  The vane shear test can be used to determine the undrained shear strength of soft clay in laboratory.  It can also be conducted in the field on the soil at the bottom of a bore hole.  The test is simple and quick.  It is ideally suited for the determination of the insitu undrained shear strength of non-fissured,fully saturated clay.  The test can be easily used to determine the sensitivity of soil.

15. SOME OTHER SHEAR STRENGTH THEORIES  Hvorslev’s strength theory According to Hvorslev’s hypothesis, the shear strength of remoulded saturated clay is given by s=ce+ tan e ce=true cohesion; e=true angle of internal friction =effective stress on the failure plane at failure The constants ce and e are also known as Hvorslev shear strength parameters. 

16. Shear strength of partially saturated soils  Bishop (1959) proposed shear strength equation for unsaturated soils as follows Where, n – ua = Net normal stress ua – uw = Matric suction (ua=pore air pressure;uw=pore water pressure) c = a parameter depending on the degree of saturation (c = 1 for fully saturated soils and 0 for dry soils)  Fredlund et al (1978) modified the above relationship as follows Where, tanb = Rate of increase of shear strength with matric suction   'tan)()(' c waanf uuuc  b waanf uuuc  tan)('tan)(' 

17. CONCLUSION  Hence the shear characteristics of soil can be summarized as-: • The shear strength of cohesionless soil such as sand &non-plastic silt, is mainly due to friction between particles. • In dense sand, interlocking between particles also contributes significantly to the strength. • The shear characteristics of a cohesive soil depend upon whether a soil is normally consolidated or over-consolidated. • The stress-strain curve of an over-consolidated clay is similar to that of a dense sand and that of a normally consolidated clay is identical to that of a loose sand.