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Weak Layer Overview

The Weak Layer option allows you to define a surface with its own material properties. This can be used to model any type of joint, discontinuity or interface along which failure might occur.

There are two options available to define a Weak Layer surface:

See Weak Surface Tutorial and Multiple Weak Layers Tutorial for examples of how to use the Weak Layer options.

How weak layers work in Slide3

A weak layer works to clip the slip surface. Generally, any slip surface (e.g. an ellipsoid) that touches or contains a weak layer will be clipped by the weak layer. Some exceptions to this behaviour are explained below.

Automatic case generation

If a weak layer is underneath another weak layer, it will not be seen unless “Automatic case generation” of weak layers is enabled in Surface Options.

If “Automatic case generation” is enabled, Slide3 will automatically check which layer(s) will generate the minimum factor of safety for a given slip surface by activating and deactivating the various weak layers intersected by the slip surface. See Multiple Weak Layers Tutorial for a demonstration.

If there are too many weak layers close together this can result in a considerable slowdown in the computations due to the increased number of combinations of the weak layers that need to be tried. Consider combining these layers if you don’t want to consider them individually.

Entire weak layer

Note that each weak layer defined in a model is assumed to exist in its entirety. Slip surfaces affected by the weak layer are either cut by the entire weak layer or, when considering the various cases during automatic case generation, not cut at all.

Suppress weak layer

It is possible to completely exclude a weak layer defined in the model from the analysis, without having to delete it. You can do this by checking off the “Suppress” checkbox in the weak layer properties.

Discontinuous layers

If a weak layer enters a given slip mass but does not fully exit the mass, either via the ground surface or through the slip surface, then it is discontinuous.

Discontinuous weak layers can significantly increase the difficulty of the search routine for finding the critical slip surface. For example, the image shows a blue weak layer that ends inside of a slip mass (delineated by the red line) near the ground surface.

Discontinuous Layers

The discretization process will initially snap the portion of the slip surface found below this weak layer up to the weak layer. In Slide2, the rest of the slip surface will not be changed, resulting in a slip surface with a local discontinuity which may render it invalid. However, for Slide3 an extrapolation scheme described in the following section will be attempted to produce a valid surface.

Horizontal extrapolation

For the above example, Slide2 will discard the slip surface with the local discontinuity shown, because it is relatively easy to generate a better slip surface such as the one shown in the following image. This is because in Slide2, slip surfaces can be drawn using simple polylines or circular arcs.

Horizontal Extrapolation

However, in Slide3, the geometrical parameters defining the slip surfaces are much more complicated. For this reason, it can sometimes be helpful to apply an extrapolation of the discontinuous layer to cut the slip surface beyond the extents of the weak layer, as shown in the figure below.

With and Without Extrapolation examples

The extrapolated portion of the cut does NOT take on the material properties of the weak layer. Rather, it samples the surrounding material.

In Slide3, we will horizontally extrapolate the bottom-most weak layer if there is a discontinuity in the post-cut slip surface caused by one or more weak layers.

To help visualize this, a few examples of how this logic is applied are shown below.

Extrapolation examples

Note that weak layers deactivated for a given case during the automatic case generation will not be considered during the extrapolation. The above examples only show cases where the visible weak layers are all active. For example, if the green layers were deactivated during weak layer handling, the end of the blue layer will be extrapolated horizontally.

Finally, please be advised that the extrapolation of discontinuous weak layers may cause material blocks beneath the extrapolation to be ignored in the analysis. For example, the grey layer in the example below will be missed due to the extrapolation.

Example of materials being ignored

The grey layer will only be analyzed for the case where the blue weak layer is not considered (i.e. during automatic case generation).

Results naming

For a given user-defined slip surface that intersects multiple weak layers, there will be several cases evaluated during Automatic Case Generation. This will produce multiple slip surfaces, which if they converge, will show up in the Show All Surfaces interface with suffixes a, b, c, etc.

Example 1 using Automatic Case Generation

Example 2 using Automatic Case Generation

Example 3 using Automatic Case Generation

Example 4 using Automatic Case Generation

Vertical weak layers

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.

At this time, Slide3 only supports vertical weak layers if they clip the slipping mass completely up to the ground surface. Otherwise, the vertical portion may not be detected during discretization.

Valid Surface

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 (Zone of Compression)

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 (Steep Base Angle)

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 > Advanced).

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