When doing a semi-coupled transient groundwater flow calculation in the Classical mode that starts from a phase that has excess pore pressures, these excess pore pressures are added to the steady state pore pressures for the starting conditions of the transient groundwater flow. This means that at the start of the phase, the excess pore pressures are in fact doubled. This will result in a different flow field at the beginning of the phase, and in a different effective stress state, which in turn could lead to incorrect displacements.
During the phase, this effect of doubled excess pore pressures will become less due to groundwater flow. Depending on the time interval of the phase and the soil’s permeability, this might not be noticed directly when inspecting only the final phase’s results.
This issue is fixed in PLAXIS 2D 2012.00