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Weak Layer Overview

A Weak Layer boundary allows you to model a potential sliding interface by defining a polyline with assigned strength properties. This is useful for modelling very thin weak layers or interfaces with low strength properties, such as a geomembrane interface.


To add a weak layer to a model use the Add Weak Layer option.

A Weak Layer boundary is an independent modelling entity and is not considered part of the model boundary geometry. It does NOT get intersected with other boundary types, and cannot be used to define material regions. It is only used for the specific purpose of defining a boundary which has strength properties.

How the weak layer works

A weak layer works to crop or clip the failure surface. When failure surface starts at the ground surface, it continues down into the soil, and if it hits a weak layer it will traverse along the weak layer until either:

  1. It hits the ground surface.
  2. It finds the original slip surface intersection on its way back to the ground surface.
  3. It subducts another weak layer with higher elevation


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, Slide2 will employ a different searching algorithm which automatically checks 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 Tutorial 35 for more information.

There are currently two algorithms which can be chosen:

  • Automatic case generation: Evaluates the factor of safety for all configurations of including or excluding each of the weak layers intersected by a slip surface. Use this if you know there are weak layers or weak soils below the uppermost weak layer in your model which you wish to consider in the analysis, such as illustrated in the images below.
FIgure- weak layer
Automatic case generation is recommended in these examples if (left) the purple layer is of concern, or (right) the grey material is of concern

Due to its rigorous nature, this algorithm runs for a longer duration. It will usually (but not always – see * Note below) find surfaces with equivalent or lower factor of safety than the other algorithm.

  • Always snap to highest layer: Simply snaps every point on the slip surface vertically upwards to the highest weak layer found at that location and does not consider multiple cases. This will save computational time if you know that only the top weak layer needs to be analyzed.
Always snap to highest layer is recommended if only the highest weak layer is of concern
The rigorous nature of the Automatic case generation algorithm makes it more reliable, but it is not guaranteed to always outperform the Always snap to highest layer algorithm. During iteration-based searching (e.g. Cuckoo Search, PSO, auto-refine, etc.) with Automatic case generation, the combination that yields the lowest factor of safety for a given slip surface will be recorded, whereas the Always snap to highest layer will record only the factor of safety with clipping to the highest weak layers. Because slip surfaces generated during future iterations of the search will depend on the result of this surface, the two algorithms may sometimes converge to differing regions of the model during searching. Either algorithm can therefore converge to differing local minima in the factor of safety and thus outperform the other.


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.


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.

Note that active weak layers found outside of the external geometry can cause slip surface errors during discretization if they are located above the slip surface. Ensure that these are suppressed.


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 are 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.

Limitations 1

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.

Limitations 2

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.

Limitations 3

If the base angle of a column, θ, exceeds the maximum allowable angle, θmax, then the slip surface will be discarded with Error -126. The default value of θmax is 80° can be changed via Project Settings > Advanced. Weak layers with inclinations greater than this value are not recommended they can otherwise create convergence problems and incorrect results.

Weak layers should NOT be used as tension crack surfaces since there are many tension crack cases with water pressures and support that would not be handled properly.


Note: The Convex Surfaces Only checkbox will not apply where weak layers are present and affect the slip surface. While this means that some concave surfaces can be generated, rejecting concave surfaces with weak layers would invalidate too many surfaces which the user is likely trying to analyze.

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