FOS for Geosynthetic Reinforced Retaining Wall

1.0 Introduction

For a reinforced retaining wall, it is crucial to evaluate the minimum safety factor of slip surfaces, where they can be impacted by boundary conditions.

In this tutorial, a geosynthetic reinforced retaining wall will be modelled in Slide2 with the Cuckoo Search method for surface search. This tutorial will help you become familiar with accessing and using the Manufacturer Library available in the program (including Geotextiles, Geogrids, and Strips)

Topics Covered in this Tutorial:

• Adding Geosynthetic Supports to a retaining wall
• Slip Surface Boundaries
• Cuckoo Search (non-circular surface search)
• Support Forces
• Minimum Factor of Safety

Finished Product:

The finished product of this tutorial can be found in the Tutorial 37 Geosynthetic Retaining Wall.slmd data file. All tutorial files installed with Slide2 can be accessed by selecting File > Recent Folders > Tutorials Folder from the Slide2 main menu.

2.0 Model

An initial Slide2 model has been created for this tutorial with proper project settings and material properties. To learn more about basic modelling functions, please refer to Tutorial 01 – Quick Start.

To open the tutorial file:

1. Select File > Recent > Tutorials and open the file Tutorial 37 Geosynthetic Retaining Wall_initial (no rein).slmd

The model is shown below:

2.1 PROJECT SETTINGS

As noted above, the Project Settings are already selected in the starting project file. To view the Project Settings for this tutorial:

1. Select Analysis > Project Settings in the menu or click on the Project Settings icon in the toolbar.
2. Select Methods in the side menu.

For this tutorial, the Bishop simplified checkbox has been cleared and the Spencer checkbox has been selected. All other program defaults remain in place.

2.2 MATERIAL PROPERTIES

As noted above, the Material Properties are already defined in the starting project file. To view the Material for this tutorial:

1. Select Properties > Define Material Properties in the menu or click on the Define Material Properties icon in the tool bar.
   
   Five materials have been defined in the model: Dense silty sandy, Wall, Backfill, Glacial till, and Facing-about. Each material has a different unit weight and strength parameter that can be viewed in the Define Material Properties dialog.
2. Click OK or Cancel to close the dialog.

3.0 Define Support Properties

We will be defining 3 types of Common Geosynthetic Supports in this model. Each support has a different tensile strength.

Tensile strength and pullout strength, especially tensile strength, are the two main properties in determining a Geosynthetic support’s strength.

To begin defining the supports:

1. Select Properties > Define Support from the menu or click on Define Support icon in the toolbar.
2. Select Support 1 on the left.
3. Change the name to Wall C-type 1.
4. Under the Support Type drop-down, select Geosynthetic.
5. Click the Manufacturer Library button. A Manufacturer Library dialog will appear.
6. Select the following geosynthetic support:
• Manufacturer = Tensar International
• Production Set = Tensar RE
• Type = Tensar RE580.

On the right-hand side of the dialog, the default values for the selected support type are displayed. We will change the “Deterioration in Service Factor (RFd)” parameter to 1.02.

This parameter needs to be changed for all three added supports.



7. Click OK to close the dialog.



8. Now in the Support Properties dialog, in the General tab, select Force Application = Passive (Method B).
9. Under the Force Orientation section select Parallel to Reinforcement.
10. Ensure Strip coverage = 100% and Long Term Design Strength = 111.246 kN/m (according to the selected support type).
11. Under the Pullout and Stripping tab, enter Adhesion = 0 kPa and Friction Angle = 41 degrees. Set the Material Dependent to Yes and then click the Define button below it. The following dialog should appear:



12. Select Wall as the Material, ensure the Adhesion = 0 kPa and Friction Angle = 41 degrees.
13. Click OK to close the Material Dependencies dialog.
14. Back in the Define Support Properties dialog, select:
• Shear Strength Model = Linear
• Anchorage = Slope Face
• Connection Strength = 111.246 kN/m (assume same as tensile)

Note: In the Design Factors tab, you can click on Define Manufacturer Library Values to see the Reduction Factor Calculation.

15. Make sure “Use external loads in strength computation” is set to Yes.
16. Similar to Support 1, enter the following properties for Support 2 and Support 3. Leave the other options with default settings.

		Support 1	Support 2	Support 3
Name		Wall C-type 1	Wall C-type 2	Wall C-type 3
Support Type		Geosynthetic
Manufacturer Library	Manufacturer	Tensar International
	Production Set	Tensar RE
	Type	Tensar RE580	Tensar RE560	Tensar RE520
Force Application		Passive (Method B)
Force Orientation		Parallel to Reinforcement
Pullout Strength	Adhesion (kPa)	0
	Friction Angle (degrees)	41
Material Dependent		Wall
Anchorage		Slope Face
Connection strength (kN/m)		111.246	71.868	42.78
Shear Strength Model		Linear
Strip Properties	Strip Coverage (%)	100
	Long Term Design Strength (kN/m)	111.246	71.868	42.78

17. Click OK to close the Support Properties dialog.

4.0 Add Support Patterns

We will now add the support patterns.

1. Select the Support workflow tab.
2. Select Support > Add Support Pattern in the menu or click on the Add Support Pattern icon in the toolbar. A Support Pattern dialog will open.
3. Enter the following:
• Support Property = Wall C-type 1
• Orientation = Horizontal
• Support Length = 9 m
• Distance between support = 0.6 m



4. Select OK to close the dialog.

You now should see a cursor on the screen. You can either select the position with your mouse or enter coordinates in the bottom right of the screen to place the support.

5. Enter coordinates = {-6.896, 101.2}. Hit Enter to place the support.
The x coordinate = -6.896 ensures all supports are anchored along the left edge of the Wall (orange).

A geosynthetic support will be positioned at the entered coordinates. It is anchored at the left edge of the Wall (orange). As you move the mouse above the first support, second support will appear on top of it as shown in the following figure:



6. Click the left mouse button to complete the selection.

The spacing between the two supports is 0.6 m. Supports further above the two supports cannot be extended because a boundary vertex shows within 0.6 m above the second support as circled in red below:



To build the rest of the supports, we need to add a second support pattern.

7. Select Support > Add Support Pattern in the menu or click on the Add Support Pattern icon in the toolbar. The Support Pattern dialog will open.
8. All factors should stay the same as the first time. Select OK to close the dialog.

You now should see a cursor on the screen.

9. Enter coordinates = {-6.896, 102.4} this time and hit Enter.
10. Move the mouse above the first support. A series of supports will show along the boundary.
11. Click the left mouse button when a total of seven supports are positioned as the following:



Now we will add other types of support.

12. Select Support > Add Support Pattern.



13. Enter the following:
• Support Property = Wall C-type 2
• Orientation = Horizontal
• Length = 9 m
• Distance between support = 0.6 m
14. Click OK to close the dialog.
15. Enter coordinates = {-6.896, 106.6}.
16. Move the mouse above the support. When a total of four supports are positioned (as shown in the figure below), click the left mouse button to place them.



We will now add the third support property.

17. Select Support > Add Support Pattern.



18. Enter the following values:
• Support Property = Wall C-type 3
• Orientation = Horizontal
• Length = 9 m
• Distance between support = 0.6 m
19. Click OK to close the dialog.
20. Enter coordinates = {-6.896, 109.0}. Move the mouse above the support. When two supports are positioned above the support (as shown below), click the left mouse button to place.

5.0 Slip Surface Boundaries

By default, the Slide2 model has generated a slope limit based on input geometry. This defines the slip surface boundaries.

In order to avoid a shallow slip surface failure, a secondary set of limits is needed. It further defines the boundaries of a slip surface.

To define the second set of limits:

1. Right-click on the primary slope limit select Define Limits (or select Surfaces > Slope Limits > Define Limits).
   
2. In theDefine Slope Limits dialog, select the checkbox for Second set of limits. Leave coordinates as the defaults for now.
   
3. Click OK to close the dialog.

Now a secondary set of limits should display.

1. Right-click on one secondary slope limit. Select Move To.
2. Use the mouse to drag the slope limit to the edge of the wall. Hit Enter to complete.
3. Repeat the same for the other secondary slope limit. When completed, your model should look like the following:

6.0 Surface Options

In this tutorial, we will be performing the Cuckoo Search Method (Non-Circular Surface) to generate slip surfaces.

1. Select the Surfaces workflow tab.
2. Select Surfaces > Surface Options in the menu or click on the Surface Options icon in the toolbar.
   
3. UnderSurface Type Section, select Non-Circular.
   
4. Keep Search Method = Cuckoo Search.
5. Leave other options as default settings.
6. Click OK to close the dialog.

7.0 Compute

1. Select File > Save As, save the file as a separate file.
2. Select Analysis > Compute in the menu or click on the Compute icon in the toolbar.

8.0 Interpret

1. Select Analysis > Interpret in the menu or click on the Interpret icon in the toolbar.

The result will be displayed in the Slide2 Interpret program.

In the toolbar, the analysis method can be selected. In this case, it can be either Janbu simplified or Spencer.



2. In the top toolbar, click on the Show Support Force Diagram icon to display the force for each support.

With Janbu Method, the following results are shown:

The Global Minimum slip surface is displayed. The minimum factor of safety is 1.350.

With Spencer Method, the following results are shown:

The Global Minimum slip surface is displayed. The minimum factor of safety is 1.528.

This concludes the tutorial on using Common Geosynthetic Supports in your model. You may now close the Slide2 program.

References

Yu, Y., Bathurst, R.J., & Allen, T.M. (2016) Numerical Modeling of the SR-18 Geogrid Reinforced Modular Block Retaining Walls. Journal of Geotechnical and Geoenvironmental Engineering. 142:5