Weak Surface

1.0 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

2.0 Define Materials

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

1. Select Materials > Define Materials or select Define Materials from the toolbar.

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.

3.0 Geometry

1. Select the Geometry workflow tab.

First we will define a Plane that will be used to represent the weak layers.

2. Select Geometry > 3D Primitive Geometry > Plane
3. Expand the Plane Definition section and enter Define by = Dip/Dip Direction.
4. Enter the following values into the corresponding fields:
• X = 15, Y =12, Z =11
• Dip = 0, Dip Direction = 0
• Select Role = Construction



5. 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



6. Click OK.

4.0 Specify Weak Planes

To assign weak layers to the two planes defined in the previous step:

1. Select both planes from the visibility pane by holding 'Ctrl' key.
2. Select Materials > Weak Layers > Add Weak Layer Surface
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.
3. Specify the Material to be weak and click OK.

5.0 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.

1. Select Surfaces > Slip Surface Options
   
2. Enable the Use Weak Layer Surfaces as User-Defined Surfaces option and change the Weak Layer Handling to Always snap to highest layer.
   
3. Select OK.
   

6.0 Results

1. Save the file and Compute
2. To see the results, go to Results tab , and select Show contour from the toolbar

The Janbu safety factor is about 2.2.

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.

7.0 Modify Weak Planes and Surface Options

1. Select the non-horizontal weak plane from Visibility pane, then remove it by clicking on
2. Select Surfaces > Slip Surface Options
   
3. Now we switch the Surface Generation Method to Search Method. Use the following settings:
• Surface Type = Sphere,
• Search method = Cuckoo, and
• Surface Optimization disabled.



4. Select OK and compute the model.

8.0 Results

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.

8.1 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.

9.0 Extra Discussion

Please see Weak Layer Overview for a detailed discussion of the limitations for the weak layers feature in Slide3. These limitations concern models that have discontinuous and/or vertical weak layers.