Problem description

When doing a Dynamics calculation, PLAXIS 2D uses the settings and the results of the previous phase as a starting condition. This also includes groundwater flow results: the end result of the previous phase’s groundwater flow calculation (the pore pressures) are used in the dynamics calculation.
In PLAXIS 2D 2011.01 the Manual settings for the Dynamics phase’s Iterative procedure may contain settings related to the iterative settings for groundwater calculations. This might happen when e.g. the previous phase uses a transient or steady state groundwater flow analysis, or when the pore pressure type of this Dynamics phase is set to “From previous phase”. It is not correct that these Dynamics phase iterative settings contain settings for groundwater flow, since the current dynamics calculation cannot handle a groundwater flow analysis due to the different time scales used: dynamics calculations are in seconds, while groundwater flow usually is determined in a time frame of hours or even days.

These parameters only refer to the groundwater flow analysis:

• Tolerated error (flow)
• First time step
• Minimum time step
• Maximum time step

Defined values for these groundwater flow analysis settings are not used in the dynamics calculation, so these settings will not have any influence on the results. However, when trying to close the window for the iterative settings, the values will be validated anyway.
This means that the minimum time step should be smaller than the first time step, and that the first time step should be smaller than the maximum time step:

minimum time step ≤ first time step ≤ maximum time step > 0

Please note that the time step for a dynamics calculation is a fixed time step, determined by:

∂t = ∆t / (m * n )

With ∂t the time step, ∆t the time interval for the phase, m the number of additional steps and n the number of dynamic substeps.

Solution

In order to work around this easily, you could use these fictive settings in a dynamics phase:

• Minimum time step = 1
• First time step = 2
• Maximum time step = 3

This is fixed in PLAXIS 2D 2011.02