Problem description

For the calculation of active pore pressures in Plaxis we distinguish between two types of analyses, a semi-coupled analysis and a fully coupled analysis:

• In a semi coupled analysis active pore pressures are split in a steady-state part, generated prior to the deformation analysis based on either phreatic levels or a steady-state groundwater flow analysis, and an excess part that results from undrained loading in the deformation analysis:
  p_active = p_excess + p_steady
• A fully-coupled analysis, on the other hand, calculates all pore pressures during the deformation analysis thus coupling deformation, consolidation and (transient) groundwater flow. This results in just active pore pressures without information on steady-state or excess pore pressures. Additionally, a fully coupled analysis also performs an independent steady-state groundwater flow analysis with the sole purpose to be able to compare the pore pressure results of the fully-coupled analysis with a pure steady-state flow only analysis.

A Safety analysis is a semi-coupled analysis based on steady-state and excess pore pressure. At the start of the Safety analysis these steady-state and excess pore pressures are the results of the phase for which a factor of safety is to be calculated. However, if the factor of safety must be determined from a fully coupled flow-deformation analysis, the steady-state pore pressures of this fully coupled phase are in fact the pore pressures from the independent steady-state groundwater flow analysis and the excess pore pressures are zero. The real pore pressures that result from the fully coupled analysis are the active pore pressures. This means that a Safety analysis following on a fully coupled analysis will by default use the results of the steady-state groundwater flow instead of the real active pore pressures which leads to an incorrect factor of safety. Additionally, the steady-state pore pressures always includes suction even when the fully coupled analysis used the “Ignore suction” option, which is likely to cause an overestimation of the factor of safety.

Solution

This issue is solved in PLAXIS 2D AE.02: This problem is overcome by making the Safety analysis read resulting active pore pressures from the previous phase and consider them as steady-state pore pressures in case the previous phase is a fully-coupled analysis. This will ensure that the Safety analysis will calculate the factor of safety based on the correct pore pressures.

For PLAXIS 3D, the issue is solved in PLAXIS 3D 2017.00