PLAXIS - From Concept to Construction

Importing geometric and stratigraphic data to PLAXIS 3D

Any infrastructure project begins with a topographic survey, context capture using drone technology or satellite imagery, or ground investigations. The ground investigations produce gINT database of borehole information. Now, it is very important that geotechnical engineers doing the geotechnical designs understand all those soil modeling data. They must also be able to import this data to their analysis software with ease. This is where PLAXIS 3D and associate Bentley tools come to the rescue.

Any infrastructural design starts with OpenSite. It, then, exports the plan of the building along with the ground formations. PLAXIS 3D can import the OpenSite export file (*.obj format). This is done with the help of PLAXIS Designer. It can then process the surface, keeping the building structure at the center. Also, it cuts out the necessary parts around it. It can then save that part as a DXF file. You can now use this DXF file in PLAXIS 3D as the top surface of the model.

Another important Bentley software that interacts with PLAXIS 3D is gINT. You can record Geotechnical investigations in gINT. gINT can export the layer information in PLAXIS 3D format. This, however, needs a little adjustment to the Borehole table of gINT. After that, PLAXIS 3D can utilize that exported data and reproduce the exact same stratigraphic layers.

Importing geometric and stratigraphic data

The SoilTest Module

The SoilTest Module of PLAXIS is a unique tool among all the geotechnical analysis software in the industry. SoilTest Module helps a geotechnical engineer to reassess his soil model. You can use this module to easily determine the stiffness and strength parameters of the soil model. You can also check them against the hand-calculated values using this SoilTest Module. It is like a virtual laboratory. It can conduct various laboratory tests under different drainage and/or initial stress conditions.

The SoilTest module can help a geotechnical engineer refine their advanced soil models to their ground condition. You can also test dynamic soil-models using this virtual laboratory. Here, unlike finite element, only one-point solution is sought using incremental technique. In this module, unknown variables are calculated from six nonlinear equations comprising six variables (stresses, strains). It is fast and very intuitive.

The SoilTest module has a parameter optimization tool. Using this tool, a user can optimize his/her soil-model parameters using the available test data. For example, let’s consider a user has four triaxial test data at four confining stresses. The user can then utilize all the four-test data to compute stiffness and strength parameters of a Hardening Soil Model very accurately.

Generating foundation layouts with structural elements

One of the popular features of PLAXIS 3D is its very intuitive user interface. Yes, it is true! There are multiple ways available to accomplish any geometric modeling task. So, it is very important to know the most efficient way of creating your geometric model. Although the choices are very much subjective, there are ways to reduce the errors and do it faster.

There are two tools in PLAXIS 3D to accomplish repetitive tasks efficiently. These are:

‘Extrude tool’
‘Array tool’

With the help of the Extrude tool, you can create:

A line from a point
A surface from a line
A volume from a surface

On the other hand, using the Array tool, you can copy any point, line, or surface any number of times, and in many intended directions. Combination of these two tools gives you a creative edge to do things that you never created before.

There is a unique snapping tool in PLAXIS 3D. You can use this to connect two points in 3D space visually. You can simply hover around the point. Then, the status bar will say, ‘Hit Tab to snap to the point’ at its lower right. Once you do that, the point is snapped. For example, you can snap at the start of creating a line until you do the same to another point in 3D space to end it. So, you create a line using 3D GUI only. You can use the point command in the command line to create points in 3D as well. Then you can connect those using the Hit Tab and Snap technique. Your inclined anchors, and/or slanted piles will be ready in no time.

Generating foundation layouts with structural elements

Soil-Structure interaction analysis using PLAXIS 3D and STAAD.Pro

The design moments, shear forces, axial forces in any structure depends on the foundation settlements. However, the structural engineers consider the foundation to be fixed at the end of the columns and/or shear walls. To reduce the risks of structural failure, it is important to perform a soil-structure interaction analysis. Now, with the help of the Coupling Tool of PLAXIS 3D, it is possible to perform an interaction analysis with a structural software like STAAD.Pro.

At the beginning, the locations of the end of columns in STAAD.Pro software needs to be located. Subsequently, proper foundations need to be designed in PLAXIS 3D at the same elevation and location. Then the Coupling Tool initiates the analysis by linking the column end points of STAAD.Pro to the foundation midpoints of the PLAXIS 3D. After this, it starts to run STAAD.Pro and transfers the reactions to PLAXIS 3D at the foundation points. Then PLAXIS 3D runs with those reactions as loads. Subsequently, the application will calculate the deformations and transfer to STAAD.Pro for further analysis. The soil-structure interaction analysis converges when the norm of deformations for successive iterations is less than the tolerance limit.

After the convergence, STAAD.Pro shows the bending moments, shear force and axial forces corresponding to the foundation deformations. So, the structure designed in this way will sustain its stability even the foundation has heterogenous (differential) settlements in the long term.