Although the implementation has been tested to be accurate and robust, the hypoplastic models are highly non-linear which may cause problems during solving complex boundary value problems. When encountering problems, the following steps may help to improve the overall performance:

  1. In the "Iterative procedure" settings under "Parameters" Tab sheet, set "Manual settings" and do not use the arc-length control.
  2. Try to modify the iterative procedure by decreasing the "Desired minimum" and "Desired maximum" number of iterations (for example, 3 and 5 respectively).
  3. The hypoplastic models are undefined in the tensile stress region, which can cause integration problems in the vicinity of the free surface and in the case of staged construction starting from the stress-free state. For this reason, artificial cohesion is introduced in the model implementation through the parameter $ p_t$. The user can specify any $ p_t$ value; in the case $ p_t$=0 kPa, the program replaces it by a default value $ p_t$=1 kPa.
  4. When used with interface elements, proper interface parameters must be input in the "Interface" Tab sheet under "Material models" window. Particularly, as some tensile stress is allowed for in the soil through the parameter $ p_t$, the tensile stress should be allowed for also in the interface by increasing the cohesion value (e.g., 10 kPa). Convergence problems may also be caused by high interface stiffness. In the case PLAXIS keeps on iterating although the global error is below the tolerated value, or does not increase the step size although the number of iterations is below the "desired minimum", problem is the most likely in interfaces.

David Masin 2017-09-12