MOHR COULOMB FAILURE CRITERION

MOHR COULOMB FAILURE CRITERION

1. MOHR COULOMB FAILURE CRITERION Definition: The variation of peak stress σ 1 with confining pressure σ 3 is known as criterion of failure. The simplest and best known criterion of failure i s Mohr coulomb criterion .

2. Many geotechnical analysis method require use of this strength model. Many geotechnical analysis method require use of this strength model. The Mohr coulomb criterion describe a linear relationship b/w normal and shear stresses at failure. It represents the linear envelope that is obtained from a plot of the shear strength of the material versus applied normal stress

3. Equation: p =   p = Confinement Si = Shear stress intercept ormal stress = Angle of internal friction  

4. The angle of friction(Ø) depends upon the grain size If Ø = 0, the Mohr coulomb criterion reduces to the tresca criterion If Ø = 90, then Mohr coulomb model is equivalent to the rankine model Higher values of Ø are not allowed

5. Mohr circle is plotted b/w principle stresses and residual stress. The value of 1 and 3 are obtained from the instrument. Shear intercept line indicates the peak shear strength The radius of the circle describe the strength of rock  

6. Small radius of circle indicate the low strength of rock Large radius indicate high strength of rock Each circle is the combination of maximum load and confined pressure. The point of tangency of circle with shear intercept line is the maximum strength of that rock . is the angle of friction of the peak load on the plane of normal stresses  

7. When the load increases to its maximum point the cracks are produced and residual stresses released Rocks having minimum cracks will have maximum residual stresses

8. Rock having fine grain particles will show greater strength. When there will be coarser grains then less stress will be apply to create the failure as compare to fine grains.

9. Mohr Coulomb Criterian Limitations Mohr coulomb criterion is not particularly satisfactory criterian for rock I t implies that a major shear fracture occurs at peak strength. The criterion is likely to give incorrect result if the failure mechanism is not shear. It implies direction of shear failure which does not agree with observation particularly in brittle rock It is linear and peak strength envelope determine experimentally are usually non-linear It will be noticed that only 1 and 3 are used and 2 is ignore in case of rock sample