Question about the triaxial test

I have a problem about the example in the model library. There is a step in the example, which i don't understand.

Integration 1
1 ) Right-click Definitions and choose Model Couplings>Integration.
2 ) In the Integration settings window, locate the Source Selection section.
3 ) From the Geometric entity level list, choose Boundary.
4 ) Select Boundary 3 only.

Variables 1
1 ) Right-click Definitions and choose Variables.
2 ) In the Variables settings window, locate the Variables section.
Fres=intop1(solid.sGpz*r*2*pi)

why do we need to calculate the axial stress using the formular in the example because normally, the axial stress is gradually increasing in the real triaxial test? I am a little bit confused by the expression.

Any one can tell me how model the viriation of water content of the soil sample and its influence on the stress-strain and strength features?

Thanks a lot!