STRESS AND STRAIN INVARIANTS
Stress and strain invariants are measures that are independent of the axis system. We will defi ne stress invariants that provide measures of (1) mean stress and (2) deviatoric or distortional or shear stress, and strain invariants that provide measures of (1) volumetric strains and (2) deviatoric or distortional or shear strains.
On a graph with orthogonal principal stress axes s1, s2, s3, the mean stress is the space diagonal (Figure ). Mean stress causes volume changes.
Deviatoric or Shear Stress:
and deviatoric stresses.
A line normal to the mean stress, as illustrated in Figure represents the deviatoric stress. Deviatoric stress causes distortions or shearing of a soil mass. Equation can be written in terms of normal and shear stresses as
Deviatoric or Distortional or Shear Strain:
Axisymmetric Condition, s92 5 s93 or s2 5 s3; ´2 5 ´3:
Therefore, q 5 q9; shear is unaffected by porewater pressures.