Problem description

When performing a Safety analysis with a Target Factor of Safety (ΣMsf), the calculation will first pass the target value to ensure that this ΣMsf value is a “stable” value before returning to the specified target value, see also the related article on Safety calculation with a Target Value for more background information.
In PLAXIS 2D 2015.00, when the calculation jumps back to the calculation step prior to reaching the target value, the internal stresses to determine the location in the M-kappa diagram are not (re)set to this calculation step (point C in the image below), but are still taken from the step with a higher ΣMsf (point B) before performing the final calculation step (point D). As a result, the plate’s forces are not correct for this final step of the Safety analysis with a Target ΣMsf.

 

Solution

This is now fixed in PLAXIS 2D 2015.01

For PLAXIS 2D 2015.00, some work arounds are mentioned below:

Obtain correct force values

To find the correct forces in the embedded pile row, the step in which the value for ΣMsf is equal to the value for the Target ΣMsf should be manually determined and then inspected:

• when necessary recalculate the phase and make sure to save all steps by setting the Max number of steps stored equal to the number of Additional steps.
• In the phase’s results, find the step in which the desired Target ΣMsf value is reached first. This can be seen in for example the menu option Project > Calculation per step, and then view the tab called Multipliers. ΣMsf can then be found in the last column.
• Open this step:
  • In Output phase’s drop down, click on the button to select steps
  • Now, from the list, select the step with the ΣMsf value equal to the target value.

Continue calculation

To continue phases from this situation with the correct value of ΣMsf:

• check how many steps are needed to get to this target ΣMsf value
• change the Calculation type to Safety analysis with Incremental multipliers (so not Target ΣMsf)
• recalculate the phase with this number of additional steps
• check the ΣMsf value for the last calculation step
• if this is correct, you can now continue from this phase

Note, this is now fixed in PLAXIS 2D 2015.01