Overview of results for Displacements

• Total displacements
• Phase Displacements
• Sum Phase Displacement
• Incremental Displacements

 

In any geotechnical finite element analysis, "displacements" is the most common result. However, simply looking at the final deformed mesh does not tell the whole story.

PLAXIS provides several distinct ways to view and manage displacements, each answering a different engineering question. Understanding the difference between these displacements and knowing when to use the "Reset displacements to zero" feature is essential for a correct and insightful analysis.

This article provides a detailed guide to each of these core concepts.

In PLAXIS, consider a situation where a horizontal slab is rigidly connected to a vertical wall while both are modelled as plate elements and activated in different phases. An excavation with a vertical wall was activated in Phase_1, and a horizontal slab was activated in Phase_4.

 

 

The vertical wall has undergone some deformation before the horizontal slab is activated in Phase_4.  

 

PLAXIS provides several ways to visualize displacements. Below we can see the different displacements at shared nodes. 

 

Total Displacements  

These are the accumulated displacements at the end of the current calculation step. These displacements can be selected from the Deformations menu, and they are represented as vectors ∣u∣ or as individual components u_x​, u_y​, and u_z​.  

When looking at the Total displacements of the shared node, different values for the horizontal slab and the vertical plate are observed. 

This key difference occurs because the newly activated horizontal slab is geometrically fitted to the already deformed mesh.

PLAXIS applies an initial, non-mechanical displacement to the slab's nodes to match the position of the deformed vertical wall. The 'Total Displacement' of the slab node, therefore, includes this initial adjustment, whereas the 'Total Displacement' of the wall node reflects its true deformation history from previous phases, too. 

 

Phase Displacements 

These are the accumulated displacement increments at the end of the selected calculation phase. They refer to the displacements that occur during a specific calculation phase and are computed as the sum of incremental displacements over all steps within that phase.

Crucially, these displacements exclude the initial geometric adjustments applied to newly activated elements. Because they only show the movement calculated during the phase, the values at the shared node are identical for both the wall and the slab. This allows you to isolate the deformation caused by the loads and construction steps within that specific phase.

Like total displacements, phase displacements are selected from the Deformations menu and can be displayed as vectors ∣Pu∣, or as individual components P_ux, P_uy, and P_uz.  

The shared node has the same displacement when we see the Phase displacements result. 

 

 

Reset Displacements to Zero 

PLAXIS has an option to reset all displacements to zero at the start of a phase, which is located in the Phase window settings.

This means the newly activated Horizontal slab or vertical wall starts with zero recorded movement. Hence, the Total displacements will now match the Phase displacements, and in both conditions, you will see the shared node has the same displacement value, even though they are activated in different phases. 

 

By using the “Reset displacements to zero” option at the construction phase, where the horizontal slab has been activated (Phase_4), the total displacements and phase displacements will be the same for the shared node.

 

Sum Phase Displacement 

When new elements are activated in a model that has already deformed, PLAXIS pre-deforms these new elements to fit the existing deformed mesh. The Sum Phase Displacements ignore this pre-deformation of newly activated elements. This provides a more realistic settlement profile, especially for layered construction, by showing the settlement of each new layer as if it were placed on the previously deformed geometry without the initial adjustment. 

For example, it shows the settlement of a newly placed embankment layer relative to the already settled ground beneath it, without the initial adjustment. 

Note that, if displacements are set to zero in intermediate phases, the Sum phase displacements may (at some point) be larger than the total displacements. Considering structural elements, the Sum phase displacements retrieve the accumulated deformation of a structural element from the moment the nodes of this element have been activated. 

 

Incremental Displacement 

This option displays the displacement increments that occurred within a single calculation step (or load step). This is particularly useful for visualizing failure mechanisms, as it highlights the areas with the most significant movement in the final (or any intermediate) step of an analysis. 

 

 

Practical example 

If the system had already undergone 12 mm of displacement before the retaining wall was activated, and the wall then displaced 10 mm in Phase 3, followed by 5 mm in Phase _4, the total displacement at the end of Phase _4 becomes: 12 mm + 10 mm + 5 mm = 27 mm.

However, the phase displacement at the end of Phase _4 remains only 5 mm, because it reflects deformation generated solely in that phase.

The sum phase displacement accumulates only the displacements after activation, giving 10 mm (Phase _3) + 5 mm (Phase _4) = 15 mm, without adding the earlier 12 mm.

Meanwhile, the incremental displacement represents the solver step movements inside Phase _4, small increments per step, that always sum exactly to the phase displacement of 5 mm. 

 

Conclusion 

By understanding and selecting the appropriate displacement visualization, i.e., Phase Displacement, or using the Reset Displacement to Zero option, you can interpret the results correctly and ensure they reflect the actual behavior of the model.

A summary of the aforementioned PLAXIS Output results are provided in the table below.

 

Result Type	What It Measures	Frame of Reference	Excavation Use Case
Total displacements	The total deformation accumulated from the very beginning of the analysis.	The initial, undeformed geometry of the model.	Viewing the final, overall deformed shape of the project.
Phase displacements	The displacement occurs only within a selected calculation phase.	The geometry at the end of the previous phase.	Isolating and analyzing the impact of a single construction stage.
Sum phase displacements	The accumulated displacement starts from the phase in which an element was first activated, thus ignoring its initial geometric adjustment.	A modified geometry that excludes artificial pre-deformation of new elements.	Analyzing the realistic settlement profile in staged construction projects.
Incremental displacements	The displacement that occurs in every calculation step of a phase.	Geometry at the beginning of every calculation step.	Identifying failure mechanisms by visualizing where the largest movements occur at the point of failure

 

See also