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Multiple Weak Layers

1.0 Introduction

As discussed in Tutorial 16a – Weak Surfaces , weak layer surfaces can be used to clip slip surfaces that are generated at their locations. Sometimes, multiple weak layers can exist in the subsurface and the factor of safety associated with each weak layer or combinations thereof needs to be determined. The underlying material behind the weak layer can also be considered in the analysis if it is weaker than the weak layer itself.

In this tutorial, we will demonstrate how to use the “Weak Layer Handling” feature to perform a comprehensive analysis using multiple weak layers within a slope. The feature is demonstrated on both a single user-defined surface as well as a global search.

Want to watch a video version of the tutorial? Check it out here:

1.1 Open the Starting File

Open the tutorial file in File > Recent > Tutorials > Weak Surface and open the file Multiple Weak Surface - starting file.

2.0 Define Materials

The starting file model consists of four different material layers, which have been predefined. There are also two surfaces which will be assigned as weak layers during this tutorial.

Model Starting View with Four Material Layers

To see the defined material properties:

  1. Select Materials > Define Materials or select Define Materials define materials icon from the toolbar.

Material Properties dialog

In addition to the four materials used in the slope geometry, there are also two weak materials (Interface 1 and 2) with low cohesion and friction angles, which will be assigned to the weak surfaces.

3.0 Specify Weak Layers

  1. Select the Geometry workflow tab.
  2. Select Materials > Weak Layers > Add Weak Layer Surface…
  3. Choose Material: Interface 1 and Surface: Surface 1:
  4. Add Weak Layer by Surface dialog

    The “Suppress on creation” checkbox can be toggled to exclude the weak layer from the analysis without needing to delete the entity. Weak layers can also be suppressed via the Properties tree and via Slip Surface Options > Manage… (beside the Weak Layer Handling section)
  5. After selecting OK, repeat the same steps to add the second weak layer with Material: Interface 2 and Surface: Surface 2:
    Add Weak Layer by Surface dialog
  6. Click OK.

4.0 Slip Surfaces

We will first analyze the single, pre-defined Elliptical slip surface in the model.

  1. Select Surfaces > Slip Surface Options Slip surfaces icon
  2. Use the following settings:
  • Surface Generation Method = User-Defined Surfaces, and
  • Weak Layer Handling = Automatic case generation
Since the user-defined surface intersects both weak layers, the “Automatic case generation” algorithm will evaluate the factor of safety for all possible configurations of including or excluding weak layer. The other option, “Always snap to highest layer”, snaps every point on the slip surface to the highest weak layer found at that location, and does not consider multiple cases.

Slip Surface Option dialog

  1. Select OK.

5.0 Results

  1. Save the file and Compute compute icon
  2. To see the results, go to the Results tab results workflow tab
  3. Select Show All Surfaces show surfaces icon from the toolbar.
  4. For easier viewing of the individual surface results, check the Show only selected outline checkbox.

Cycle through the surface results to view the different slip surfaces generated during the analysis. Here we can see that multiple cases were analyzed, with clipping the ellipsoid to each of the weak layers and also without clipping at all.

Notice that despite the deeper shape of the slip surface showing factor of safety 1.507 compared to that of the slip surface with factor of safety 1.472, the results for these two cases are comparable. This is due to the relatively weak properties of Interface 2 compared to Interface 1 as specified during the Materials definition.

User Defined Surface 1 results

User-Defined Surface 1a results

User-Defined Surface 1b results

6.0 Global Searching

Automatic case generation is also performed during global searching. The weak layers intersecting any shape of slip surface (spherical, ellipsoidal, multiplanar, altered, etc.) will be handled in the same way as shown above. We will now showcase the complete analysis of this model using a global search.

Now we switch the Surface Generation Method to Search Method.

  1. Select Surfaces > Slip Surface Options Slip surface options icon
  2. Use the following settings:
    • Surface Type = Ellipsoid,
    • Search method = Partial Swarm Search, and
    • Surface Altering Optimization enabled.

    Slip Surface Options dialog

  3. Select OK and compute the model. It may take a few minutes to complete.

6.1 Results

As expected, the global minimum slip surface looks to be clipped at the bottom by a weak layer.

  1. Click Contours or Show All Surfaces to browse the detailed results.

Search Method Results

7.0 Extra Discussion

Please see Weak Layer Overview for a detailed discussion of the limitations for the weak layers feature in Slide3. These limitations concern models that have discontinuous and/or vertical weak layers.

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