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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.

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.

Model Starting View with Four Material Layers

To see the defined material properties,

Select : Materials > Define Materials, or select Define Materials

Define material properties icon

from the toolbar.

Materials Properties Dialog

In addition to the four materials used in the slope geometry, there are also two weak materials 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 > Layer Icon Weak Layers >Add Layer Icon Add Weak Layer Surface…

Choose Material: Interface 1 and Surface: Surface 1:

Add Weak Layer By Surface Dialog

Note: 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)

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

Select OK.

4. Slip Surfaces

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

Slip Surface Icon Select: Surfaces > Slip Surface Options, then use the following settings. Ensure that Automatic case generation is selected in the Weak Layer Handling section.

Note: 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 Options Dialog

Select OK.

5. Results

Results Tab

Calculator Icon Save the file and Compute.

To see the results, go to Results Results Tab tab, and select Show All Surfaces Surfaces Icon 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 1.508 compared to that of the slip surface with factor of safety 1.475, 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.

3D Model of Slip Surfaces with Show All Surfaces Dialog

3D Model of Slip Surfaces with Show All Surfaces Dialog

3D Model of Slip Surfaces with Show All Surfaces Dialog

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.

Slip Surfaces IconNow we switch the Surface Generation Method to Search Method. To do so, select Surfaces > Slip Surfaces Icon Slip Surface Options , then use the following settings, Surface Type = Ellipsoid, Search method = Partial Swarm Search, and Surface Altering Optimization enabled. Ensure that the dialog appears similar to the one shown below.

Slip Surface Options Dialog

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


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.

3D Model with Contours

7. Extra Discussion

Slide attempts to cut slip surfaces using the weak layers that they intersect. Sometimes, discretization issues can occur when these cuts are vertical or near-vertical.

Case a) Vertical cuts in tension zones

In many cases, vertical weak layers can produce valid slip surfaces for analysis. For example, the slip surface below (blue) is cut vertically in a zone of tension by the vertical face of a weak layer (red). The resulting slip surface and slices / columns for analysis is highlighted in yellow. This slip surface is valid because in the zone of tension, no reaction force is assumed to be exerted by the soil to the right side of the cut.

valid Surface with Zone of Tension

Case b) Vertical cuts in compression zones

Suppose the weak layer is modified such that it also cuts the slip surface in a zone of compression near the base of the slope, as shown below. The resulting slip surface is invalid, because the restoring lateral force exerted by the soil left of the cut in the zone of compression is non-zero, and cannot be determined using limit equilibrium methods.

Invalid Surface with Zone of Compression and Tension

If this case is ever encountered, Slide will report Error -149 and prompt a warning when viewing the results. However, if the desired analysis is for the slip surface to pass through (and effectively ignore) the weak layer in the zone of compression, then there are several ways to bypass the error:

  • Replace the vertical faces of the weak layers with Tension Cracks, which vertically cut the slip surfaces only if the soil is in tension at those locations.
  • Alter or delete the left vertical portion of the weak layer so that it does not vertically intersect the slip surface (like case (a)).
  • Split up the weak layer geometry into separately defined weak layers, and use the Weak Layer Handling feature.
  • Use more advanced techniques such as finite element analysis in RS2 & RS3, which consider the behavior of the entire slope.

Case c) Vertical cuts in compression zones

Near-vertical weak layers can also create columns with very steep base angles, which are known to induce numerical instability in limit equilibrium methods and should be avoided if possible.

Invalid Surface where Base Angle exceed Maximum Allowable

If the base angle of a column, θ, exceeds the maximum allowable angle, θmax, then the slip surface will be discarded with Error -413.The default value of θmax is 80°, which can be changed via Project Settings Icon Project Settings > Advanced).

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