Retaining Wall

1.0 Introduction

In this tutorial, RS2 is used to simulate the construction of an earth retaining wall. The wall is subjected to forces from backfill and from ponded water. Joint elements are included between the wall and the soil, so the wall may slip relative to the soil. The model was constructed in four stages:

1. Bring the foundation soil to equilibrium
2. Add a layer of fill and a retaining wall
3. Add water
4. Add another layer of fill on top of the first

2.0 Constructing the Model

1. Select: File > Recent Folders > Tutorial Folder.
2. Select: Retaining Wall (Initial) file

In this file, geometry, materials, and boundary conditions have already been assigned. Click through the stages and to see the addition of the retaining wall and a layer of fill in stage 2 and another layer of fill in stage 4. In this tutorial, a joint will be added between the retaining wall and the soil layer, and ponded water will be added to the left of the wall.

2.1 Add Joint

1. Select the Stage 2 tab.
2. Select: Boundaries > Add Joint
3. In the Add Joint Dialog:
• The joint is man-made and will start and finish at a free surface. Therefore, for Joint End Condition, select Both ends open.
• Note that for natural joints found in geological formations, it is customary to choose both ends closed.
• Set the Install at stage option to 2. The dialog should look like this:



4. Click OK. There will now be a cross-hair cursor to select the points that make up the joint.
5. With the mouse, select the point at the bottom left of the retaining wall (6, 5). The cross-hairs should snap to the existing point. If it does not snap, right-click and turn on all the Snap options.
6. Now select the point at the bottom right of the wall (8.5, 5) and then at the top of the wall (8.5, 11).
7. Right-click and choose Done.

There should be a joint represented by an orange line as shown:

The open circles at the ends of the joint indicate that it is open at both ends. Clicking through the stages shows that the joint is light colour in Stage 1, indicating that it is not installed. It is a dark orange (installed) in all other stages.

To define the properties of the joint:

8. Select: Properties > Define Joints
9. For Joint 1, change the Criterion to Mohr-Coulomb and the Peak Friction Angle to 27 degrees as shown.
   
10. Click OK to close the dialog.

Joint 1 properties do not need to be assigned to the existing joint because it is Joint 1 by default.

2.2 Add Piezometric Line

In Stage 3, there will be ponded water to the left of the wall. To draw the piezo line, first, add a vertex on the wall at the water surface:

1. Select: Boundaries > Edit > Add Vertices
2. The water will be at an elevation of 8 m, so enter the coordinates (7, 8). Hit Enter. Hit Enter again to finish entering points.

There will be a new vertex about halfway up the wall on the left side.

3. Select: Boundaries > Add Piezometric Line
4. Enter (0, 8) for the first point and press Enter.
5. Now click on the new vertex at (7, 8). Click on the bottom right corner of the wall at (8.5, 5) and finally click on the top right corner of the foundation soil at (20, 5). Hit Enter to finish entering points.

6. In the dialog, choose which materials are affected by the piezometric water level:
7. Select the check box next to Foundation and click OK.

The piezometric line should appear as shown (Stage 2):

The water should only be added at Stage 3. So we will modify this using hydraulics properties settings.

8. Select: Properties > Define Hydraulic
9. For the Foundation material, select the Stage Factors tab and turn on the Stage Hydraulic Properties option.
10. Click Add Stage.
11. Next to Stage 1 change the Piezo # to none. Stage 2 should now read none as well. Next to Stage 3 change the Piezo # to 1.

The dialog should look like this:

12. There is no need to set stage 4 since it will automatically be the same as stage 3. Click OK to close the dialog.

Clicking through the stages shows the piezo line plotted at every stage. To change this:

13. Select: Groundwater > View Piezos by Stage.

The piezo line should now only be visible in stages 3 and 4.

2.3 Distributed Load

The water to the left of the wall will exert a hydrostatic force on the wall and foundation soil; this can be simulated by adding a distributed load.

1. Select: Loading > Ponded Water Loads > Add Ponded Water Load
2. Enter Total Head = 8 m.
3. To add water at stage 3 only, click on the Stage Load option.
4. Click on the Stage Total Head button and unclick the Apply boxes for stages 1 and 2 as shown.
   
5. Close both dialogs by clicking OK. Now, select the boundary segments on which to apply the load.
6. Click the bottom of the pond and the bottom left boundary of the retaining wall below the piezo line.
7. Right-click and select Done.

Stage 4 should appear as follows:

Note the triangular load applied to the side of the wall. This shows how the hydrostatic force increases with depth.

2.4 Mesh

Before adding the forces caused by the ponded water, the mesh needs to be generated. The mesh options are already set up:

1. Select: Mesh > Discretize and Mesh

2.5 Restraints

By default, all segments of the external boundary are fixed. Since the top of this model represents the actual ground surface, we need to free the top surface.

1. Select: Displacements > Free
2. Select the top boundary sections and the left boundary sections above the foundation soil and press Enter.
   
   The bottom left edge and right edge should be fixed only in the x-direction to allow vertical movement.
   
3. Select: Displacements > Restrain X
4. Select the bottom left and all sections of the right boundary. These boundaries will now be showing rollers instead of pins.
   
   Finally, we need to re-establish the fixed boundary condition on the bottom corners.
   
5. Select: Displacements > Restrain X,Y
6. Click on the bottom boundary and hit Enter.

The model should now look like this:

The model is now complete.

7. Save the model with a different name by selecting File > Save As in the menu.

3.0 Compute

1. Select: Analysis > Compute

4.0 Results and Discussion

1. Select: Analysis > Interpret

The maximum stress in the foundation soil layer for Stage 1 will be displayed.

2. Change the display to show contours of Total Displacement.

There should be almost no visible displacement in the layer since the field stress and body force of the finite elements are in equilibrium in the first stage.

3. Select: Stage 2 tab

Significant deformation is visible in the fill layer as it settles due to gravity. There is little displacement in the retaining wall since it is made of stiff concrete and does not deform much under gravitational loading.

4. Select: Display Deformed Boundaries in the toolbar

The retaining wall is being pushed outwards and rotated as shown.

5. Select: Display Yielded Joints to see the vertical joint sections turn red, indicating that this entire section of the joint has slipped. This shows that sliding along the vertical joint is responsible for the displacement contours behind the retaining wall.
6. Select: Stage 3 tab. The wall is being pushed back to the right slightly due to the force applied by the ponded water.
   
   To see this more clearly, plot the displacement of this stage relative to Stage 2.
   
   
7. Select: Data > Stage Settings. Set the Reference Stage to Stage 2 and click OK.

The Stage 3 plot will appear as follows:

The bottom of the wall is being pushed by the water and this is causing displacement and rotation.

Stage 4 shows significant displacement back in the other direction as the second layer of fill is added.

To look at the joint behaviour in more detail, graph the joint data. First, turn off the reference stage.

8. Select: Data > Stage Settings. Set the Reference Stage to Not Used.
9. Select: Graph > Graph Joint Data
10. In the Graph Joint Data dialog, select Shear Stress for the vertical axis and turn on stages 2, 3 and 4 as shown.
    
11. Click Plot and the plot should appear as follows:
    

The first four points show the stress on the horizontal portion of the joint. These show positive values indicating stress-causing left lateral motion. Stage 2 shows the highest shear stress on this segment and the stress decreases when the water is added in Stage 3. Stage 4 then shows an increase in stress when the extra fill is added. For the vertical section of the joint, the stresses are negative indicating stress tending to cause right-lateral motion. Return to the results.

Let’s plot the joint data directly on the model.

12. Right-click on the joint and select Show Values > Shear Displacement
13. Under the Data heading, turn on the Joints option.
14. For this plot, choose Shear Displacement from the drop-down menu.
15. Click OK.
16. Turn off the deformed boundaries.
17. Turn off the distributed loads by right-clicking, choosing Display Options and deselecting the option for Distributed Loads under the Stress tab.

The Stage 3 plot should now appear as follows:

The maximum and minimum values are denoted with red text and blue text respectively. Note the negative (right lateral) slip on the vertical section of the joint as the soil is moving downwards relative to the wall. The bottom of the joint shows positive (left lateral) displacements since the wall is shifting left relative to the foundation.

This concludes the Retaining Wall Tutorial.