I was trying to simulate the unloading process of clay consolidation in Abaqus using clay hypoplasticity umat (https://soilmodels.com/download/plaxis-umat-hypoplas-zip/). But I had a convergence issue showing in Fig.1.

The model was meshed with one element (C3D8). The parameters for clay are showing in the Fig.2, initial void ratio was 1.6, calculated using equation in Fig.2 when OCR=1 and p = 1. As Fig.3 shows, the circumferential surfaces are restricted to move in either x or z direction, and the bottom surface is restricted in y direction. In Step 1, 25KPa pressure was applying on the top of the specimen and removed in step 2, linearly with time in both steps. These boundary conditions are expected to make a consolidation process.

However, when the pressure on the top started to decrease in the Step 2, there was a convergence issue. I checked the output files that were used to record message from umat, error was 0 and umat seemed working good. Intergranular strain concept and small stiffness concept were not included.

Did anyone encounter a similar problem? I think this clay umat can be used to simulate unloading process but I can’t make it run. I appreciate any help from you.

Best regards,


Fig.1-0.PNG Fig.2-1.PNG Fig.3-2.PNG

  1. David Mašín
    David Mašín 1 year ago

    Hi Sheng,
    First of all, can you try to run the simulation with non-zero value of pt (for example, 5 kPa)?
    Also, please note you have to sub-divide the unloading processinto many steps, as the model is highly non-linear Abaqus will probably not be able to find solution in iterative process.
    Regards David

  2. Shen Wang Author
    Shen Wang 1 year ago

    Hi Prof. Masin,

    Thank you for replying. I was using 1KPa as pt and increments in one loading step were pretty small. I checked the code recently and I found in the umat downloaded from https://soilmodels.com/download/plaxis-umat-hypoplas-zip, the part which responsible for dealing with positive mean stress (tensile) case was comment, but I am not sure if this is the reason.

    Thank you again!

    Best regards,

  3. Shen Wang Author
    Shen Wang 1 year ago

    * commented

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