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Weak Surface

1. Introduction

Weak layers can be used to represent interfaces between soil layers, lining material in landfills or any other type of discontinuity. Additionally, weak layers can be used to form a single slip surface.

In this tutorial, first we will demonstrate how to add a weak layer within a slope and use weak layers as user-defined slip surface. Next we see how weak layers can change the shape of slip surfaces to create composite surfaces.

Open the tutorial file in File > Recent > Tutorials > Weak Surface and open the file Weak Surface - starting file

2. Define Materials

The starting file model consists of four different material blocks. To see the defined material properties,

Materials Icon Select: Materials > Define Materials, or select Define Materials Materials Icon from the toolbar.

Material Properties Dialog

In addition to the four materials used in the slope geometry, there is also a weak material defined with lower cohesion and friction angle with respect to the other defined materials.

3. Geometry

Select the Geometry workflow tab.

Add Plane

Select: Geometry > 3D Primitive Geometry > Plane

Expand the Plane Definition dialog. Select Define by = Dip/Dip Direction.

Enter the following values into the corresponding fields:

  • X = 15, Y =12, Z =11
  • Dip = 0, Dip Direction = 0
  • Select Role = Construction

Create Plane Dialog

After selecting OK, repeat the same steps and add another plane with the following values:

  • X = 15, Y = 12, Z = 14
  • Dip = 45, Dip Direction = 90
  • Select Role = Construction

Create Plane Dialog

Select OK.

4. Specify Weak Planes

To assign weak layers to the two planes defined in the previous step, you can select both planes from the visibility pane by holding 'Ctrl' key or repeat the following for each plane individually.

Select: Materials > Weak Layers > Add Weak Layer Surface

Note: Weak Layers can also be defined by specifying a table of coordinates that make up the surface. This feature can be accessed using Add Weak Layer by Location.

Specify the Material to be weak and click OK.

Add Weak Layer By Surface Dialog

5. Slip Surfaces

The layers defined above form a sliding mass which its factor of safety can be computed. But before computing the model, we need to set the Surface Generation Method to User-Defined Surfaces to avoid creation of any other slip surfaces by search algorithms.

Slip Surface Icon Select: Surfaces > Slip Surface Options, then use the following settings.

Slip Surface Options Dialog

Select OK.

6. Results

Results Tab

Calculator Icon Save the file and Compute.

Results Tab To see the results, go to Results tab, and select Show contour Contoured Model Icon from the toolbar.

The Janbu safety factor is about 2.2. Contoured Model View with Weak Layers Intersecting with Slip Surfaces

Now that we have seen how weak layers could be used to define a user-defined slip surface, let’s move on to see how weak layers interact with slip surfaces created by a search method.

A weak layer clips any slip surface that its base intersects with the weak layer plane. To see this effect, let’s slightly modify the model.

6. Modify Weak Planes

Select the non-horizontal weak plane from Visibility pane, then remove it by clicking on Delete Icon.

Modify Surface Options

Now we switch the Surface Generation Method to Search Method. To do so, select Surfaces > Slip Surface Options, then use the following settings, Surface Type = Sphere, Search method = Cuckoo, and Surface Optimization disabled.

Slip Surfaces Options Dialog

Select OK and compute the model.


As expected, the global minimum slip surface looks to be clipped at the bottom by the horizontal weak layer. If you turn on contours and select Base Cohesion, you will see the part that has been clipped by the weak layer, displays a different material property.

Base Cohesion Contours Model View

Safety Map

Additionally, if you click on Safety Map from the toolbar, you notice almost all the contour remains on top of the weak layer.

Safety Map Contours Model View

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.

Zone of Tension and Valid Surface Diagram

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.

Zone of Tension and Compression and Invalid Surface Diagram

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 Diagram where Base Angle is greater than 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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