Problem description

There are three calculation options for a consolidation analysis in PLAXIS:

1. Staged construction. Here we set the time interval and we can activate/deactivate soil, structures and loads, or change load values.
2. In minimum excess pore pressure, we do not set a time interval, and we cannot activate/deactivate soil, structures and loads: these settings are kept the same as the previous stage while the calculation continues until the maximum (absolute) value of the excess pore pressure is below the specified value for the excess pore pressure, |P-stop| One of the results is then the time it needs to consolidate.
3. In a Degree of consolidation type of analysis the calculation is similar to the Minimum excess pore pressure calculation option, in which the Minimum excess pore pressure parameter (see above) is set to a value as defined by the maximum excess pore pressure in the previous phase (P_{max,previous phase}) and the defined Degree of consolidation ( U ):
       Minimum pore pressure = (100 – U)% * P_{max,previous phase}
   The calculation should stop when the maximum absolute excess pore pressure is below this calculated value of Minimum excess pore pressure.

However, in PLAXIS 2D AE, this stop criteria for the degree of consolidation does not work properly: the calculation does not stop when the degree of consolidation criteria is met. The calculation will continue until the number of steps has been reached or that the maximum absolute excess pore pressure in the model is very small (<10^-40).
This can give a much longer time interval for this phase than would be necessary to reach the specified degree of consolidation.

Solution

This issue is now solved since PLAXIS 2D 2015.00.

In PLAXIS 2D AE, since this automatic stop criteria does not stop the calculation, we cannot use the Degree of consolidation type for the Consolidation analysis to see how long it will take to reach that level of consolidation directly. To still perform a similar calculation, you can do this:

1. calculate the starting-from-phase for this Degree of Consolidation analysis
2. From this starting phase, look up the maximum excess pore pressure. This number can be found in the results in the Output program, but also directly in the phase’s calculation window. Open the phase window, and view the Pmax (Reached max pp) to get the maximum (absolute) excess pore pressure |p_excess|:
   
3. Change the Degree of Consolidation phase to a Minimum excess pore pressure phase.
4. Calculate what the minimum excess pore pressure calculation should be: ( 100 - Degree of consolidation )% * Pmax,previous phase

Example
We want to calculate a degree of consolidation U of 90% for a model in which the start-from-phase has a Pmax of 98 kN/m². Now we need to fill in a minimum excess pore pressure of :
    (100 - 90)% * 98 kN/m² = 9.8 kN/m²