Tension cut-off for Soft Soil (Creep) and UDSM interfaces

23 July 2013
Date modified 19 August 2019
Known issue ID SW-15553
Applications PLAXIS 2D 2011.00, PLAXIS 2D 2011.01, PLAXIS 2D 2011.02, PLAXIS 2D 2012.00, PLAXIS 2D 2012.01, PLAXIS 2D 2015.00, PLAXIS 2D 2015.01, PLAXIS 2D 2015.02, PLAXIS 2D 2016.00, PLAXIS 2D 2016.01, PLAXIS 2D 2017.00, PLAXIS 2D 2017.01, PLAXIS 2D 2018.00, PLAXIS 2D 2018.01, PLAXIS 2D 2019.00, PLAXIS 2D AE.00, PLAXIS 2D AE.01, PLAXIS 2D AE.02, PLAXIS 2D Classic, PLAXIS 2D CONNECT Edition V20, PLAXIS 3D 2012.00, PLAXIS 3D 2012.01

Problem description

For most soil models with a Mohr-Coulomb failure criterion, a tension cut-off option with a tensile strength value is available in order to limit the (effective) tensile stress.
However, for the constitutive models mentioned below, these edit fields for the tension cut-off in the material database are not available:

  • Soft Soil:  no tension cut-off was considered in the constitutive model up to PLAXIS 2D2012.00 and PLAXIS 3D 2012.02. This has been added since the release of 2D2012.01
  • Soft Soil Creep. Tension cut-off is considered on the Mohr-Coulomb failure criteria, but the input field was not available prior to PLAXIS 2D2012.01. However, the tension cut-off was still remembered from parameter definition prior to changing the material model to Soft Soil Creep. In most cases, this would mean that this has the default value: the tension cut-off is activated and set to 0 kN/m2 tensile strength. But other values could be defined when in the parameter definition prior to changing it to Soft Soil Creep the tension cut-off value was set to a different value.
    Example: first the Mohr-Coulomb model was defined for the current material dataset with a tensile strength of 4.5 kN/m2. After that, the soil model for this dataset was changed from Mohr-Coulomb to Soft Soil Creep. Internally, the program will still remember the tension cut-off activated at a tensile strength of 4.5 kN/m2, although the user cannot see or change this for the soft soil creep model.
  • User Defined Soil models for the interface parameters. Interface behaviour for User Defined Soil Models (UDSM) is completely controlled via the Interface tabsheet. So even if the User Defined Soil Model uses a tension cut-off option, the interface still relies on the parameters set on the Interface tab sheet in the material database. The tension cut-off value is not available here as an input field, but internally the program will use the tension cut-off for the UDSM interface as defined earlier for this material dataset in the same way as described above for the Soft Soil Creep model.

Solution

The tension cut-off option is available in the material dataset and is correctly used in the calculation for:

  • Soft Soil since PLAXIS 2D 2012.01. Not yet in PLAXIS 3D (Version 2012)
  • Soft Soil Creep since PLAXIS 2D 2012.01. Not yet in PLAXIS 3D (Version 2012)

The issue with the UDSM interface tension cut-off is not resolved yet.

If the user wants to control and set the value for the tension cut-off to a different one for Soft Soil Creep (in Versions 2D2012.0 and earlier and 3D2012.02 and earlier), or for the User Defined Soil Model interface parameters, this trick can be used:

  1. When having an already defined soil model, remember the parameters for the soil model.
  2. Change the soil model to e.g. Mohr-Coulomb
  3. Go to the parameters tab, and set the values for the tension cut-off
  4. Now switch back to the original soil model (Soft Soil Creep or UDSM) and set all parameters correct again.

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