How to look inside the soil of a 3D model to understand the behaviour

---

When running a PLAXIS 3D model, the most interesting behaviour and results are usually all inside the 3D soil model. For presentation purposes, we can hide a part of the geometry to present the PLAXIS 3D calculation results. We can then have a look to see how soils and structures inside a 3D model behave, such as displacements beneath an excavated surface and behind the walls of an excavation. We can use different techniques to do this:

• Using an iso-plane contour plot to show the iso-areas where we can see the areas that have the same results.
• Or use partial geometry options to hide soil elements so we can focus on what we would like to see.

Iso-areas

An iso-area (or isosurface) is a three-dimensional variation of an isoline. It is a surface that represents points of a constant value (e.g. displacements, strains, stresses, pore pressure, groundwater head) within the volume of the 3D model space. With the iso-areas visualization in PLAXIS 3D Output, the results domain is subdivided in a number of intervals, and for each interval limit, such an iso-area (or isosurface) is shown.
This option can give us quick insight into the model behaviour, however, it may not always give insight when using it as a static image, see Figure 1.

Figure 1. Iso-areas for deformations in Tutorial Lesson 2

Partial geometry options

When using partial geometry options, we can

	hide individual elements by either using the model explorer or using the Click to Hide soil button: Ctrl+click hide individual elements or use Shift+click to hide the entire volume;
	We can also use a tool called Hide soil in the rectangle. With this we can drag a rectangle over the 3D model and all elements inside this rectangle will be hidden;
	And another option is to use the filter options from the Geometry > Filter menu item. Here we can for instance hide elements based on their coordinate, e.g., hide all elements with an X-coordinate > 5.5 m

What all these methods have in common, is that the showing and hiding soil is based on the Finite Elements. This may result in a non-smooth plot due to the unstructured mesh using tetrahedral elements as used in PLAXIS 3D. This can give the following plot when applying it to PLAXIS 3D’s Tutorial Lesson 2, see Figure 2.

Figure 2. Hiding part of the model with partial geometry with a non-smooth cut-surface

Fortunately, there is a way to easily improve the visualization of this. We can do this by introducing helper surfaces at the location where we want to hide the elements.

Example partial geometry with helper surfaces

Example: we want to hide a quarter (or corner) of the 3D model so we can see inside the soil while giving it a clear and smooth visualization. Here we will be using PLAXIS 3D’s Tutorial Lesson 2 again.
We can create this using the following steps:

1. In PLAXIS 3D Input, go to Structures mode
2. Here, create sections (the helper surfaces) along the X- and Y-axis (Z is the vertical axis) that will align with the plot you want to show. This will divide the model into sub-clusters once we enter any of the green modes (mesh, flow conditions and staged construction).
3. Regenerate the mesh and restart the calculation
4. When viewing the results in Output, turn off these soil sub-clusters, and hide any structure in this quarter (or corner) of the model to get the desired Output plot.

Below you can see the final Output plot view in Figure 3 and Figure 4

Figure 3. Partial geometry using helper surfaces – Deformed mesh

Figure 4. Partial geometry using helper surfaces – Total deformations

See also the attached animation to see the steps to create such a nice visualization: