Dear all

I am recently working on the mechanical characteristics of large diameter monopiles under horizontal loads.The numerical simulation part of this project was carried out in ABAQUS/STANDARD and for the modelling we invoked the UMAT of the SANISAND intrinsic model.
From the results file I found that the soils generated very large negative pore pressures, which is clearly not possible in reality. To solve this problem we need to set the cavitation limit for the fluid in the soil in ABAQUS/STANDARD.
I would like to ask for advice on how to set the cavitation limit for pore water in the ABAQUS/STANARD simulation.
Is it can be done through some codes in subroutine, could anyone please tell me about that.
I would be very grateful!

  1. Arie Koot
    Arie Koot 1 year ago

    Place after
    next line
    “pore=max(pore,-100) ”
    Note: pore is negative thus tension cannot be more than 100kN/m2 if units are KN,m

    Note: Umat can only handle complete drained or undrained behavior.
    You can improve the model to apply water permeability (Darcy)

    • Zhentao Liu Author
      Zhentao Liu 1 year ago

      Thank you for your advice!

      I have tried setting the undrained condition in abaqus and calling SANISAND (using the “Soil” steps). However, under these conditions (totally undrained) the volume of the elements does not change (depsv=0) and the value of STV29 (PorePressure) is always close to 0, which is very different from the pore pressure calculated by ABAQUS.
      Is there a problem with my settings?

  2. Arie Koot
    Arie Koot 1 year ago

    correct: if volume doesnot change, pore pressure will not change.
    Due to shear strain and dilation/compaction props the effective isoptropic stress changes.
    These stress courses volume change if boundary condition alwows this. And if volume changes then pore pressure chnges.

    What do you mean with “pore pressure calculated by Abaqus”?

    • Zhentao Liu Author
      Zhentao Liu 1 year ago

      My reference to “pore pressure calculated with ABAQUS” refers to one of the default output variables of ABAQUS, “POR”.
      I compared the pore pressure from UMAT (STV29) with the pore pressure from ABAQUS (POR) and found the difference to be quite significant.

      Regarding the effect of dilation and shear contraction, do they lead to changes in volume strain under undrained conditions?

      If not, the calculation is based on the UMAT method
      Here “depsv-np1” will remain at 0 and UMAT should calculate pore=0.

      I’m not sure if I have it right, thanks again for your patience

  3. Arie Koot
    Arie Koot 1 year ago

    “POR” is not a fieldoutput of Sanisand-UMAT.
    Material model input=strain
    Material model output=stress
    stresses => forces => acceleration (a=F/M) => displacement => strain ect

    So: if you have model a cube and you have add vertical strain and a horizontal stress, you will get shear plasticity, change of isotropic effective pressure and change of water pressure.

    • Zhentao Liu Author
      Zhentao Liu 12 months ago

      Dear Arie
      sorry I havent been on for about 1 month. Thank you for your reply, I I have changed the model settings.

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