Multiple Weak Layers
1. 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. 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.
To see the defined material properties,
Select : Materials > Define Materials, or select Define Materials 
from the toolbar.
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. Specify Weak Layers
Select the Geometry workflow tab.
Select: Materials > 
Weak Layers >
Add Weak Layer Surface…
Choose Material: Interface 1 and Surface: Surface 1:
After selecting OK, repeat the same steps to add the second weak layer with Material: Interface 2 and Surface: Surface 2:
Select OK.
4. Slip Surfaces
We will first analyze the single, pre-defined Elliptical slip surface in the model.
Select: Surfaces > Slip Surface Options
Use the following settings:
- Surface Generation Method = User-Defined Surfaces, and
- Weak Layer Handling = Automatic case generation
Select OK.
5. Results
Save the file and Compute.
To see the results, go to Results tab.
Select Show All Surfaces 
from the toolbar.
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.
6. 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.
Select: Surfaces > Slip Surface Options
Use the following settings:
- Surface Type = Ellipsoid,
- Search method = Partial Swarm Search, and
- Surface Altering Optimization enabled.
Select OK and compute the model. It may take a few minutes to complete.
Results
As expected, the global minimum slip surface looks to be clipped at the bottom by a weak layer. Click Contours or Show All Surfaces to browse the detailed results.
7. 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.