It is the most important mechanism and helpfull in every field related to rocks and soil. It is also very important in construction.So,these are the ways that convey information about testing of strength of soil and Rocks.

2. Submitted to:
prof. Yasir Sarfraz
Submitted by:
Group 1
Roll no. 1-12

3. Assignment no.
1
Topic:
Mohr-coulomb Failure criterion

4. MOHR COULOMB
FAILURE CRITERION
Definition:
The variation of peak stress σ1 with confining pressure σ3 is known as
criterion of failure. The simplest and best known criterion of failure is
Mohr coulomb criterion.

5.  Many geotechnical analysis method require use of this
strength model
 The Mohr coulomb criterion describe a linear
relationship b/w principle 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 principle stress

6. Equation:
𝑇p = 𝑆𝑖 + 𝜎𝑡𝑎𝑛∅
𝑇p = Confinement
Si = Shear stress intercept
𝜎 = Principle stress
∅ = Angle of internal friction

10.  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

11.  Mohr circle is plotted b/w principle stresses
and residual stress.
 The value of 𝜎1 and 𝜎3 are obtained from
the instrument.
 Shear intercept line indicates the peak
shear strength
 The radius of the circle describe the
strength of rock

12.  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

13. 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

14. 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.

15. Mohr Coulomb Criterian
Limitations
Mohr coulomb criterion is not particularly
satisfactory criterian for rock
 It 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.

16.  It is linear and peak strength envelope determine
experimentally are usually non-linear
 It will be noticed that only 𝜎1 and 𝜎3 are used and 𝜎2
is ignore in case of rock sample
 It implies direction of shear failure which does not
agree with observation particularly in brittle rock