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Slip Surface: Weak Layer

In Slide3, a slip surface is discretized into a set of columns. Then each column is checked whether overlaps with a weak layer or not. If a column is located within a region with weak layer, the failure is governed by the weak layer (i.e, its base is clipped to that weak layer). If the base of the column is above a weak layer, the failure is pertained to distretized columns. However, if it is below a weak layer, the columns are truncated above the weak layer (i.e its elevation of column is changed to be right on top of that weak layer).

There are two scenarios where weak layers can potentially create numerical issues:

1 - Weak Layers that end up inside slope geometry:

Such a weak layer can produce non-convex slip surfaces. For instance, consider the case illustrated below where the red dashed line represents a weak layer. This weak layer, as shown in the image below, does not extend all the way to the end of slip surface. Therefore, only a part of the slip surface is clipped to the weak layer, producing a non-convex slip surface.

Sub Vertical Weak Layers

2 - Sub-vertical weak layers

Weak layers that are defined based on sub-vertical planes, not only induce problems in discretization of slip surface into columns, but also can create columns with very steep base angles. Such steep base angles are known to induce numerical instability in limit equilibrium method and should be avoided if possible.

Automatic Weak Layer Handling

Slide3 will automatically analyze the possible combinations of toggling (on or off) each weak layer intersection of given surfaces. This option is on by default. The option to snap only to the highest layer is still available.

Slip Surface Options Dialog

Surface ID is provided in all surfaces in the results with more description as shown below. For example,

Surface ID Diagram

Discontinuous weak layers

For discontinuous weak layers, Slide3 automatically extrapolates discontinuous weak layers to produce valid surfaces as shown below:

Discontinuous Weak Layers Diagram

Weak layer suppression

Weak layers can now be suppressed to exclude them from analysis without deleting them as shown below:

Add Weak Layer By Surface Dialog

Properties DialogWeak Layer Manager Dialog

Use weak layers as slip surface

This option must now be selected to use the weak layers in the model as user-defined slip surfaces.

Use Weak Layer Surfaces as User Defined Surfaces Model View

Limitations of weak layers:

- Partial deactivation of surfaces are not supported at this time.

- Reversing weak layers are not supported.

Ellipsoidal Surface Diagram

Please refer to the tutorial Weak Layers for examples of Slide3 models with slip surfaces on weak layers.

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