Problem description

When defining a soil dataset using the Soft Soil or Soft Soil Creep model in the material database, only the strength parameters cohesion c_ref, friction angle φ and dilatancy angle ψ are available as input fields. Other soil models, e.g. Mohr Coulomb and Hardening Soil, allow for an additional strength parameter to be defined: a cohesion that increments in depth: c_inc
Although this input field is not present for the Soft Soil and Soft Soil Creep model, in PLAXIS 2D 2017.00 (or earlier) and PLAXIS 3D 2016 (or earlier) this strength parameter is still taken into account in the constitutive soil model’s behaviour.

In most cases, this is not a problem: the value for c_inc is initialized at a zero value, i.e. no cohesion increment with depth. However, when changing an existing material dataset to the Soft Soil or Soft Soil Creep model from a soil model that has this input field available and this c_inc has a value, this c_inc value is internally remembered in PLAXIS 2D and PLAXIS 3D 2016 (or earlier) and used during the calculation when changing to the Soft Soil or Soft Soil Creep model.

Example

First a soil dataset with the Mohr-Coulomb model was defined with a cohesion of c_ref = 14.5 kN/m² and c_inc = 0.5 kN/m²/m. After that the soil model for this dataset was changed from Mohr-Coulomb to Soft Soil Creep. Internally, the program will still remember the cohesion value c_ref = 14.5 kN/m² and c_inc = 0.5 kN/m²/m. The value for the cohesion c_ref is clearly available as an input field, but the user cannot see or change the value for the cohesion increment c_inc value for the Soft Soil Creep model.

Currently this c_inc value cannot be reviewed in the material database in the Input program. Only after the calculation, in the results, one can check the value for the current cohesion. To see this, go to Stresses > State parameters > c
Based on the values in each cluster, one may see if it is a constant value (i.e. c_inc = 0 kN/m²/m), or the cohesion value for this Soft Soil or Soft Soil Creep material changes in depth (i.e. c_inc ≠ 0 kN/m²/m).

Solution

Since PLAXIS 2D 2017.01 and PLAXIS 3D 2017, the c_inc value will always be reset to 0 (zero) when using Soft Soil and Soft Soil Creep.

For older versions of PLAXIS 2D (2017.00 or earlier) and PLAXIS 3D (2016.02 or earlier), please review the cohesion value in the results by inspecting the plot and/or tables for all used Soft Soil and Soft Soil Creep material datasets via the menu item Stresses > State parameters > c and inspect if the expected cohesion value is used.

When unexpectedly a value for c_inc has been used in the calculation, the safest way to solve this is to:

• create a NEW material dataset (so do not copy an existing material dataset)
• define the material dataset with appropriate material parameters
• replace the Soft Soil or Soft Soil Creep model with incorrect cohesion settings in all phases
• recalculate the model
• and again, inspect the used cohesion value via the menu item Stresses > State parameters > c