How to run a Limit Equilibrium Method analysis from PLAXIS 2D

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 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 engineering point of view and the global stability would be of larger 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 of 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 the PLAXIS 2D to PLAXIS 2D LE phase converter you will be 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 PLAXIS LE to run your Limit Equilibrium Method analysis.

Usage instructions

• Download the package from Downloads below
• Extract the zipped package and copy the content (phase_plaxis_le.pyw file and the entire plaxis_fele folder to:
  [PLAXIS 2D install folder]\pytools\input
  When using the default PLAXIS 2D installation folder this will be:
  C:\Program Files\Bentley\Geotechnical\PLAXIS 2D CONNECT Edition V21\pytools\input
• Start PLAXIS 2D Input
• Load your PLAXIS 2D model and select the required phase
• Via the menu item, please select Expert > Run Python tool > phase_plaxis_le
• The PLAXIS 2D to PLAXIS 2D LE phase converter will be launched
• Verify you selected the correct phase
• Select the appropriate PLAXIS LE options
• Click on the Convert button
• After converting, the tool will display required actions and other relevant information
• Now we can launch PLAXIS LE to either directly run the Limit Equilibrium analysis or adjust the model to your needs.

Figure 1. PLAXIS 2D to PLAXIS LE phase converter tool user interface directly after launching

If you do not have write access to add files to the PLAXIS 2D installation folder, you can use this alternative approach:

• Save and extract the zipped package in a local folder
• Then, in PLAXIS 2D Input, go to Expert > Run Python script > Open … and browse to phase_plaxis_le.pyw and select that one to launch the PLAXIS 2D to PLAXIS 2D LE phase converter

Example

In the video below, a slope has been modeled in PLAXIS 2D, and the safety analysis predicts one global factor of safety. With the PLAXIS 2D to PLAXIS LE phase converter, we can investigate multiple parts of the analysis to run an analysis for.

Limitations

The phase configuration of a PLAXIS 2D model can be converted into a PLAXIS LE model. By default, the geometry, material assignments and available parameters, distributed loads and the water table will be converted. The PLAXIS LE model starts, by default, with a Circular Slope Search using the GLE (Fredlund) calculation method for a first, fast, robust approximation of the slope stability.

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 PLAXIS LE. 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 behavior (suctions) is not converted.
• Structural elements can be converted to PLAXIS LE supports. 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 PLAXIS LE analysis for a Kulhawy type of analysis.
• Soil polygons inside a bigger soil polygon that are not connected to the edge of the bigger cluster are not supported in PLAXIS LE. 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 PLAXIS LE. The conversion tool will discretize any arc into multiple segments with straight lines.
• Design approaches and design standards are not converted.

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

Requirements

The PLAXIS 2D to PLAXIS 2D LE phase converter requires:

• PLAXIS 2D CONNECT Edition V21 Update 1 or newer
• PLAXIS LE 2D CONNECT Edition V21 or newer
• PLAXIS 2D started with activated Geotechnical SELECT Entitlements [GSE]. See here how to do this.

This tool requires access to the PLAXIS Python Remote scripting library using Python 3.7.x.

Updates and release version info

This tool is released as a Technology Preview and available as a separate download version only. Any updates will be added to this page, so check this page regularly.

• [2021-05-10] Version 0.9.3: Initial release