Triaxial Compression Test (part 2) - Determination of soil strength parameters

Morh's Circle of Stress

To obtain the Mohr - Coulomb failure line using the triaxial test result, it is normal practice to make use of a graphical solution known as the Mohr's circle of stress.

Generally the principal stress with the largest magnitude is denoted by sigma 1 and called the Major principal stress, and the smallest principal stress is denoted by sigma 3 and called the minor principal stress. (the stress in the 3rd dimension is the intermediate principal stress sigma 2 and it is neglected)

The state of the stress ( in the figure ) is similar to that of a soil specimen in a triaxial test. we need to know the stress state on any plane making on angle alpha to the horizontal since failure cannot occur on either the vertical or horizontal plane.

The stress acting on the plane (EE section) making an angle alpha to the horizontal consist of two components: a stress perpendicular to the plane, known as the normal stress ( sigma n) and a stress acting parallel to the plane, known as the shear stress (tau).

we can develop below equations for above stresses,


using above 2 equations we can plot a graph, actually circle. This is called as the Mohr's circle.

Mohr circle of stresses as explained previously can be used for plotting the triaxial test results from which the shear strength parameters can be obtained.

 We need to do 3 tests, using that test result we can plot those 3 circles and mark 3 points tangentially (shear stress which soil will fail). Then connect those 3 points using a straight line. that is the critical value line which we called as Mohr - Coulomb failure line.

To prepare best possible line we need to do the test 3 times.

There are situations where it is useful to express the Morh - Coulomb failure criteria in terms of principal stresses.


The angle of failure plane to the horizontal,

Skempton's pore pressure parameters A and B

  

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Please check next blog for part 3