General Mechanical

General Mechanical

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Using INISTATE to apply pore pressure

    • Yuxing Wu


      I am trying to simulate the rock under subsurface conditions. I used INISTATE to apply pore pressure. Since my model is thin, I assumed the pore pressure equals to 22.5MPa and does not change with depth. I used the code INISTATE,set,dtyp,stre
      and INISTATE,defi,all,,,,22.5,22.5,22.5,0,0,0. Also, in my model, I have subsurface horizontal and vertical stresses. However, the results are the same without pore pressure. Can anyone let me know how to correct it? I attched the boundary conditions and code below. 


    • Bill Bulat
      Ansys Employee

      Hi Yuxing,

      I'm not quite sure what the problem with your model is. I created a test case with pretty much the same INISTATE commands you used:


      There are no externally applied loads in my model (just constraints). I get significantly nonzero stresses:

      The initial stresses are listed in the solve.out file by the INISTATE,LIST command:



      If I comment out the commands in the command object and rerun, I see (as expected) zero stress:

      Maybe your constraints only prevent rigid body motion? Maybe you can do some testing without additional applied pressures (as I did) to help you identify the problem?








      • Yuxing Wu

        Hello, Bill 

        Thank you so much for the reply. I re-run the model again and compare the results with and without INISTATE codes. Two models show different results. However, I am not sure if it is the correct result. From my expectation, the stress in the model without pore pressure (total stress) should be equal to the stress with pore pressure (effective stress) minus 22.5 (pore pressure), since the model is in compression and the pore pressure is in an opposite direction. However, the result is not the same as I expected. First figure is the model without pore pressure and the second figure is the model with pore pressure. 

        Also, after I apply the code, there is always a warning message that says: Solver pivot warnings or errors have been encountered ensuring the solution. This is usually a result of an ill conditioned matrix possibly due to unreasonable material properties, an under constrained model, or contact related issues. Check results carefully. 

        Any suggestion on these two issues? 

        Thank you so much in advance for your help. 




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