Published on October 27, 2016
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, tanb = 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.