Problem description

When using phases with Updated Mesh (UM), the Safety analysis should use the deformed mesh (with the updated node locations) as starting point, while the node locations should not be not updated any more during the Safety analysis.
Deformations that you get from a Safety analysis are fictitious (see the related article on Safety analysis and displacements), and are only used to drive the process and to indicate the failure mechanism (displacement increments rather than total displacements). If updated mesh would be used during a Safety analysis, the geometric changes would become unrealistic. For example, a safety analysis with Updated Mesh performed on a slope would lead to the slope becoming flat in the end, with an infinite safety factor. This is not the desired outcome of such a calculation.

During the safety calculation you will get additional displacements of course (in the parameter u), but the node locations should not change. This can be of importance when an embankment in undeformed state is just too steep, while in the deformed mesh it is stable. Another example for which this can be important is an embankment that will sink partly below the water table due to its self weight when using the Updated Water Pressures option (available in PLAXIS 2D). This will reduce the effective weight of the embankment structure on the subsoil.

Unfortunately in the PLAXIS 3D 2013 calculation the nodal coordinates are still updated after each step, which is incorrect as explained above. This may give incorrect values for the safety factor ΣMsf, for example as described above for embankments.

Solution

This is now solved for PLAXIS 3D AE.00!

Unfortunately, no work around is available for PLAXIS 3D 2013 and earlier.
Please note that when resetting displacements to zero at the beginning of the Safety analysis and disabling the Updated Mesh option, it would reset the geometry back to the original geometry, and so:

• it will introduce an out-of-balance force;
• and you do not 'benefit' from the positive effects from the geometric changes that have been introduced in the construction phases prior to the safety phase.