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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 modeling very thin weak layers or interfaces with low strength properties, such as a geomembrane interface.

ADD WEAK LAYER

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

Note: A Weak Layer boundary is an independent modeling 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 it’s way back to the ground surface.
  3. It subducts another weak layer with higher elevation

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, Slide2 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 Tutorial 35 for more information.

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.

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.

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°, which can be changed via Project Settings > Advanced.

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