PLAXIS 3D 2016: Fully parametric geometry

30 September 2016

Introduction

Developments for PLAXIS 3D AE and PLAXIS 3D 2016 changed the internal structure of the geometry definition. In versions prior to PLAXIS 3D AE (3D 2013, 3D 2012, and older), the internal geometry data structure was entirely based on triangles. In more complex projects, especially those with curved shapes, this created issues and limitations. PLAXIS 3D AE is a transitional version; it is a hybrid version: parametric geometry for natively created surfaces and volumes, while triangulated geometry from boreholes and imported CAD geometry. In PLAXIS 3D 2016, however, the entire geometry is solely based on parametric geometry.

Issues with tessellated geometry

In PLAXIS 3D 2013 and older, the internal structure of the entire geometry is tessellated: all geometric surfaces and the volume sides (so called BReps or Boundary Representations) are internally made up of triangles.


Figure 1. Rectangle and soil block and their BRep triangles (right)

This works great when using basic, straight sided shapes. But in the case of curved shapes, like arcs, circles and cylinders, this tessellation leads to an approximation of these curved shapes: the straight sides of the triangles have to form the boundary of these shapes.

Of course, when defining a circular shape ideally you want to retain this shape, and not have a shape that approximates this arc. When trying to connect other geometry to these shapes, due to this approximation, this may be very challenging. You can think about connecting tunnels, side tunnels and galleries, but also combi-walls when connecting the tubular piles (cylinders) with sheet pile walls (surfaces).

Not having a fully curved shape can also have consequences on the model results, for instance, due to interface locking when such an approximated circular shape contains significant corners in the interface definition loaded under large shearing conditions.

Linked problem: Possible over-estimation of lateral circular pile bearing capacity [link]

A second issue is caused by the internal triangulation of multiple shapes that have to be intersected. This intersection takes place when we change from the geometry creation mode (Soil / Structures mode) to one of the green modes where we will work with the final geometry for meshing and calculations (Mesh / Flow conditions / Staged construction). This intersection is necessary to be able to uniquely identify parts of the entire model’s geometry, e.g., soil inside or outside an excavation, or surface loads left and right from a wall.
To ensure that the program does not incorrectly change the geometry, the original tessellation is retained. This implies that the original triangles in the tessellation will be kept, and may be subdivided into several parts for discretizing the intersected "cut objects".

See the example below of a tunnel shape (Figure 2) with cutting planes to simulate tunnel construction stages. When zooming in the behaviour of one cutting plane with the tunnel (Figure 3), we can still recognise the original two triangles (B) in the intersection result (C).


Figure 2. Tunnel with cutting plane to simulate staged construction segments


Figure 3. Tunnel side view and cutting plane (magenta) with (internal) tessellation rendering (B,C)

When the intersection of two geometric objects is close to e.g., edges, then this can become complicated. If from the previous example the surface is very long and the cutting plane is close to the left edge of the surface, we will get a small triangle in the intersection results, see Figure 4 below:


Figure 4. Long tunnel side face and cutting plane with (internal) tessellation rendering

When the dimensions of such small triangles are below the geometrical tolerances used in the PLAXIS 3D program, these small triangles will be considered to be obsolete and will be filtered out of the geometry, which will lead to failing intersection actions.

Linked problem: Tunnels in PLAXIS 3D: Extrusions and cutting planes [link]

Solution: Parametric geometry

Approximating real 3D curves shaped using tessellated geometry gives problems in some cases, as explained above. In order to solve these issues related to approximation and discretization, we worked hard on implementing a geometric data structure based on mathematical modelling of shapes and objects, i.e. an arc is an arc, not an approximation. Since the geometry is now based on mathematical equations using parameters to describe geometry, e.g., radius and centre point for an arc, we refer to this as parametric geometry.

With this parametric geometry, we now also have true intersections of the true shapes: we do not have to approximate the geometry using internal subsections. This avoids one of the above-mentioned problems with the internal triangles.
Also importantly, the Finite Element mesh will define the geometry very accurately: the true shapes can now be given to the meshing algorithms, resulting in an accurately geometric description of the Finite element locations, including arcs.

Noticeable changes

The new PLAXIS 3D geometry data model will show us smoother meshes with higher quality elements for curved shapes like piles, shafts and tunnels:


Figure 5. Cylinder shape and  fine mesh. Left in PLAXIS 3D 2013, right PLAXIS 3D 2016

As can be seen in the image above, the cylindrical shape will give us a smoother shape, more evenly distributed elements and a better geometrical description with fewer elements. Overall this will give us higher quality meshes with fewer elements. Since the calculation time is highly influenced by the number of elements, we will gain calculation speed improvements by having a better description of the Finite Element mesh geometry.

Shape designer

Arcs: no segments

Since PLAXIS 3D AE, the shape designer does not use segments anymore to define a curved shape: we now support parametric geometry for these arc-shapes, and so we do not need to approximate the curved shape anymore using small straight sections. So if you want to describe a circle, it can just be done without using segments, and the internal geometry description will make it a true circle (or cylinder).

Standard shapes

Some standard shapes can be generated using a command. This includes a cylinder, cone and cuboid. In older versions (3D 2013, and before) the commands for a cylinder and a cone still supported segments along the curved shape, but since 3D AE these shapes internally use a mathematical description.


Figure 6. From left to right: a cylinder, cone and cube

Command examples

cylinder
Signature: cylinder radius height (xO yO zO) (xV yV zV)
To create circular volume pile with its top centre point at (5 5 0), a length of 8 m vertically downward (vector direction will be (0 0 -1)) and a diameter of 0.80 m (radius = 0.4 m) we can use this command:

 cylinder 0.4 8 (2 2 0) (0 0 -1)

cone
Signature: cone base_radius height (xO yO zO)
To create a cone with a base radius of 1.2 m, a height of 4 m with its base centre point at (6 2 -5) we can use this command, pointing up (0 0 1):

 cone 1.2 4 (6 2 -5) (0 0 1)

cuboid
Signature: cuboid sidelength (xO yO zO)
To create a cube with sides of 2.5 m, with the bottom side's centre located at the (10 0 -6), use this command:

 cuboid 2.5 (10 0 -6)

Pile geometry
The cone command can now also be used to create a tapered pile directly:


Figure 7. Tapered pile

See also the Command and objects reference and the compatibility notes for commands for more details.

Reading older PLAXIS 3D files

When reading older PLAXIS 3D files that contain (triangle based) tessellated geometry,  PLAXIS 3D 2016 will attempt to convert this tessellated geometry to the parametric geometric data structure.

  • Boreholes: Soil layers defined via boreholes used to be stored as tessellated geometry in PLAXIS 3D AE, 3D 2013 and older. When loading a model in PLAXIS 3D 2016 (and newer), these boreholes and their resulting soil layer definition will be regenerated using parametric geometry.
  • Surface and volumes: Surfaces and volumes created in PLAXIS 3D AE are already defined as parametric geometry, and so these can directly be loaded in PLAXIS 3D 2016. However, geometry created in older versions (3D 2013 and older) as well as geometry imported from CAD files are based on triangulated, tessellated definition, and cannot be directly loaded into PLAXIS 3D 2016. PLAXIS will attempt to load these triangulated shapes by converting all internal triangles into parametric definitions. When the geometry has a limited amount of triangles, this can be achieved. However, if the geometry consists of many triangles, this will not be possible, and these surfaces and volumes will not be loaded in PLAXIS 3D. Hence, these geometric objects are removed from the Plaxis model.

Whenever PLAXIS 3D detects it needs to change the geometry to make the transition from triangulated/tessellated geometry to fully parametric geometry, the new geometry will require a new intersection. Following from this, a new mesh will need to be generated and the project will need to be completely recalculated.
Hint: if you have selected points for curves, then make sure to reselect them again before the calculation via the Select points for curves option in Output or directly via the command line in Input.
The program will directly store the file under a new name to prevent accidentally overwriting the original file. The new PLAXIS 3D file will be stored in the same folder and "_converted" will be added to the filename.

In some cases, the required changes are too great, or critical triangulated shapes are removed from the model. In that case, it is recommended to use a hybrid version of PLAXIS 3D that supports both triangulated and parametric geometry. This hybrid version will be available as PLAXIS 3D Classic (similar to 3D AE but with some updates).  PLAXIS 3D Classic will be located in a subfolder of the PLAXIS 3D 2016 installation folder called "classic". To start Input for PLAXIS 3D Classic, start Plaxis3DInput.exe from this subfolder "classic".

If you want to use a later version of PLAXIS 3D, e.g. PLAXIS 3D 2017:

  1. first, install PLAXIS 3D 2016 with the Classic version
  2. and then update to the latest/desired version (e.g. PLAXIS 3D 2017)

Import of CAD geometry files

Since PLAXIS 3D 2016 only allows parametric geometry and does not accept triangulated geometry, the program can no longer import CAD files based on triangulated geometry. This means PLAXIS 3D will no longer support the import of *.3DS, *.DWG, and triangulated *.DXF.

For more details on the support import types, please see the page on How do I import a geometry in PLAXIS 3D?

If you still need to import these CAD files using triangulated geometries, PLAXIS 3D Classic is available for this. PLAXIS 3D Classic will be located in a subfolder of the PLAXIS 3D 2016 installation folder called "classic". To start Input for PLAXIS 3D Classic, start Plaxis3DInput.exe from this subfolder "classic".

If you want to use a later version of PLAXIS 3D, e.g. PLAXIS 3D 2017:

  1. first, install PLAXIS 3D 2016 with the Classic version
  2. and then update to the latest/desired version (e.g. PLAXIS 3D 2017)

Conclusion

With the fully parametric geometry data in PLAXIS 3D 2016, we solved problems with intersection, mesh quality for curved shapes and interface locking. PLAXIS 3D 2016 will provide you with:

  • Fewer problems during intersection;
  • Fewer performance issues;
  • Fewer issues in mesh generation;
  • Improved mesh quality with fewer elements required;
  • Faster calculations due to fewer elements when having curved shapes;
  • More accurate results when using curved shapes, especially when loaded in shear.

This major change to PLAXIS 3D will make the program faster, more reliable, more accurate, and better equipped for geotechnical challenges, especially when dealing with large and complex geometries.

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