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Geotechnical Mohr Circle

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Hi I am trying to develop Mohr circle. I Have deviator stress which is (sigma 1 - sigma 2)/2. I also have three confining pressures for one sample. Can someone tell me whether confining pressure is sigma 1 or sigma 3?
 
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sigma 3 is ususally termed as the confining pressure in the test cell. The deviator stress being the stress applied over and above sigma 3. Andy Machon


 
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In a triaxial test the hydraulic pressure in the cell is the confining pressure sigma 3.
Sigma 2 is the same as sigma 3 in atriaxial cell.
Sigma 1 is increased above sigma 3 by applying a 'deviator' stress in the vertical (axial) direction by loading the top of the sample.
Sigma 1 - Sigma 3 is the deviator stress.

(Sigma 1 - Sigma 3)/2, half the deviator stress, is the radius of Mohr's Circle and if Sigma 1 is the deviator stress at failure it is the shear strength.

In an unconfined test, Sigma 3 is zero so the shear strength is half the uniaxial compressive strength.

 
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