Finite Element Analysis - Limit Equilibrium in One Click

Figure 1. Easy conversion from PLAXIS 2D to GeoStudio SLOPE/W

Introduction

For the determination of a Factor of Safety, the strength reduction method is a very powerful tool to obtain the most critical slip surface, a slip surface that may be circular but can also have any other form. However, there may be circumstances in which more information is required than just the most critical slip surface or when the critical slip surface is of minor importance from an engineering point of view. For instance:

• For a large dam, both the global stability as well as the stability of the sloped sections in between berms must be determined.
• For a road project in the mountains, both the stability of the hillside above and below the road must be determined, as failure below the road would lead to a loss of the road whereas a failure above the road would only lead to rockfall and temporary loss of service.
• The critical slip surface is very small, for instance, just the corner of the crown of an embankment. This is not important from an engineering point of view, and the global stability would be of greater interest.

In these cases, determining a Factor of Safety using the Limit Equilibrium Method would be the solution. With Limit Equilibrium, it is possible to specify exactly what part of the model the Factor of Safety must be determined while the method would still have the benefit of determining the most critical, possible non-circular, slip surface in that area. It would also allow within one model to determine different factors of safety for different parts of the model.

With PLAXIS 2D, you are able to quickly open the same model with the same geometry, the same staged construction configuration, the same loading conditions and the same material assignments in GeoStudio SLOPE/W to run your Limit Equilibrium Method analysis.

1 Import from and Export to GeoStudio 

Since the release of PLAXIS 2D 2023.2, you can access the following options under the File menu to:

• Import from GeoStudio 2D to open the GeoStudio model with the same geometry and the same material assignments in PLAXIS 2D to run your Finite Element Method analysis.
• Export to GeoStudio 2D to open the PLAXIS 2D model with the same geometry, the same staged construction configuration, the same loading conditions and the same material assignments in GeoStudio SLOPE/W to run your Limit Equilibrium Method analysis.

2a. Conversion PLAXIS to GeoStudio

Follow these instructions for conversion from PLAXIS 2D to a GeoStudio SLOPE/W (Limit Equilibrium) file: 

1. Ensure the PLAXIS 2D project is saved.
2. Navigate to the menu option, File > Export GeoStudio 2D
3. PLAXIS 2D will prompt for the configuration of the remote scripting server:
   • configure to an available port, and secure password (defaults are OK) and click to Start server.
   • click on Close.
4. The tool will present a dialogue for a selection of what to convert.
5. Select the conversion settings, e.g. which slip direction you want to consider, which phase to consider, etc.
6. Click to Convert.
7. The conversion process will start, resulting in a message with:
   • Conversion warnings and info.
   • Options to view the full log file.
   • And open the newly generated LEM model file.
8. Using the button, you can select to directly open the newly generated GeoStudio model file.
9. After a few seconds, GeoStudio will be loaded with the model.

2b. Conversion GeoStudio SLOPE/W to PLAXIS 2D

Follow these instructions for conversion from a 2D GeoStudio geometry to a PLAXIS 2D model:

1. Ensure the GeoStudio file is saved.
2. Open PLAXIS 2D Input and start a New project.
3. Navigate to the menu option, File > Import GeoStudio 2D
4. Select a source GeoStudio file (*.gsz)
5. Select the appropriate geometry.
6. Click to Convert.
   • The tool will warn when a model is still open in PLAXIS 2D to prevent data loss.
7. The conversion will start a new PLAXIS model and builds up the model according to the data in the GeoStudio model.

Note: Only geometry and soil material assignments are converted from GeoStudio to PLAXIS.

Limitations

Limitations converting PLAXIS 2D to a GeoStudio model

The phase configuration of a PLAXIS 2D model can be converted into a GeoStudio (Limit Equilibrium) model. By default, the geometry, material assignments and available parameters, distributed loads and the water table will be converted.
GeoStudio's SLOPE/W model starts by default with a Morgenstern-Price method and a Circular Entry and Exit method.

In the current version, the following limitations/restrictions apply:

• Only PLAXIS 2D Plane strain models are supported.
• Not all soil models in PLAXIS 2D have an equivalent soil model in GeoStudio. Supported soil models include Mohr-Coulomb, Hardening Soil, HS small, Soft Soil, Soft Soil Creep, and Hoek-Brown.
• Groundwater flow definitions and specific pore pressure definitions are not converted, including settings for the hydraulic conductivity.
• Unsaturated soil behaviour (suction) is not converted.
• Structural elements can be converted to GeoStudio Reinforcements. However, plate elements, vertical embedded beams, fixed-end anchors, and node-to-node anchors (when not part of a tie-back/ground anchor) are not converted.
• PLAXIS 2D stress results cannot yet be included in the Limit Equilibrium analysis for or a stress-based stability analysis (sometimes also referred to as Kulhawy).
• Soil polygons inside a bigger soil polygon that are not connected to the edge of the bigger cluster are not supported. It is suggested to connect the smaller "floating" soil polygon with at least two lines to the edge of the bigger contour.
• Circular/arc shapes are not supported in this converter. The conversion tool will discretize any arc into multiple segments with straight lines.
• Design approaches and design standards are not converted.
• GeoStudio specific:
  • Water levels are partly supported when converting from PLAXIS 2D to GeoStudio
  • Only strength model parameters for SLOPE/W are converted, no SEEP/W and SIGMA/W data conversion.

Note that the PLAXIS 2D to GeoStudio model converter will indicate this in the conversion log. Make sure to read it before continuing in GeoStudio.

Limitations specific to reading GeoStudio files to convert to PLAXIS 2D

• Only geometry and soil material assignments are converted from GeoStudio files to PLAXIS 2D.
• Only strength model parameters for SLOPE/W are converted, no SEEP/W and SIGMA/W data conversion.
• Only reading GeoStudio 2D regions/geometries is supported.

Requirements

The "Import/Export to GeoStudio 2D" tool requires:

• PLAXIS 2D 2023.2 or later
• GeoStudio 2023.1 or later
• PLAXIS 2D with access to Geotechnical SELECT Entitlements [GSE]. See here how to do this.