Problem description

When during a consolidation phase the only open consolidation boundary is z_max and the top volume is deactivated, the calculation will give an error (Complex error). This error is a result of the fact that for this specific situation the program cannot determine an initial timestep.

Solution

A possible workaround is to do the following:

1. add a prescribed displacement at the current bottom and set all directions (x,y,z) to fixed
2. then add a (relative thin) soil layer with a non-porous material at the bottom
3. then add second (relative thin) soil layer with a porous material (e.g. drained) at the bottom
4. regenerate the mesh
5. in your phases, open the bottom boundary

This will force a drainage border at the bottom, but due to the non-porous cluster there is no drainage possible through this non-porous layer, making your original bottom still closed for consolidation. And by adding the prescribed displacement, you will have the original bottom fixed for displacements. And by adding the porous material, the calculation kernel can determine the drainage boundary correctly for the determination of the minimal time step.

This issue is solved in PLAXIS 3D 2011.