shear test

Ciao!
i'm trying to simulate a Shear test of a cubic sample:
at first there's an isotropic compression
and then the shear simulation, where the mean stress is manteined costant at 100kPa.

In the first part i used 'O.cell.velGrad(x,0,0, 0,x,0 0,0,x)' and a 'limitMeanStress',
 in the second one i used O.cell.velGrad(0,0,x, 0,0,0, 0,0,0), so shear in xz plane, and a limitation of distorsion O.cell.trsf [0,2].

How can i impose the mean stress '(sigma1+sigma2+sigma3)/3' costant during the second part of simulation?

My target is to observe behavior of loose and dense soil, during the shear test.

thanks

Giulia