Embankment Consolidation

Transcription

1 Embankment Consolidation 36-1 Embankment Consolidation In this tutorial, RS2 is used for a coupled analysis of a road embankment subject to loading from typical daily traffic. Model Start the RS2 9.0 Model program. All tutorial files installed with RS2 9.0 can be accessed by selecting File > Recent Folders > Tutorials Folder from the RS2 main menu. The finished product of this tutorial can be found in the Tutorial 36 Embankment Consolidation sub-folder in the Tutorials folder.

2 Embankment Consolidation 36-2 Project Settings Open the Project Settings dialog from the Analysis menu and select the Groundwater tab. Define Method as Transient FEA and initial pore pressures as Steady State FEA. Select the General tab. Select Solid-Fluid Interaction: Coupled (Biot). Make sure the Units option is set to Metric, stress as kpa, the time units to Days and the permeability units to meters/second. Select the Stages tab and change the number of stages to 8. Enter the following staging information: Name Time (days) Drained? Initial Fully Drained Stage 2 1 Drained Stage 3 10 Drained Stage 4 20 Drained Stage 5 60 Drained Stage 6 61 Drained Stage 7 62 Drained Stage 8 65 Drained Close the dialog by clicking OK. Boundaries The problem consists of three soil layers and an embankment fill, therefore an external boundary and material boundaries need to be created.

3 Embankment Consolidation 36-3 Start by creating a rectangular external boundary. Select the Add External option in the Boundaries menu and enter the following coordinates: 0, 0 100, 0 100, -40 0, -40 c (to close the boundary) Press F2 to Zoom All. Now we need to delineate the different material layers within the model. For simplicity, we will import material boundaries from a DXF file; from the File menu, select Import then Import DXF. Navigate to the Tutorial 36 folder in the Tutorials folder, and open the file Tutorial 36 Embankment Consolidation.dxf. Ensure that Boundary Type = Material and the Run Geometry Cleanup after import checkbox is selected. Select OK. The Geometry Cleanup dialog should appear. Ensure the following is selected then select OK. Your model should look like the one below:

4 Embankment Consolidation 36-4 Materials Now select the Materials and Staging workflow tab to assign materials and properties to our model. Since we have four materials to define, let s use the Import Properties option to read in properties from a file. Select: File > Import > Import Properties Navigate to the Tutorial 36 folder. For example: C:\Users\Public\Documents\Rocscience\Phase2 9.0 Examples\tutorials\Tutorial 36 Embankment Consolidation Open the file Tutorial 36 Embankment Consolidation.fez. You will see the Import Properties dialog. Select all of the materials as shown, and select OK. The material properties are now imported.

5 Embankment Consolidation 36-5 Now select Define Materials from the toolbar or the Properties menu. Examine the properties of the four imported materials. Click on each material name and select the Strength and Stiffness tabs, and note the different properties of each material. Notice that the Embankment Fill material has an Initial Element Loading = Body Force Only, since it represents a fill material placed on top of the existing sand layers. When you are finished examining the material properties, select Cancel in the Define Material Properties dialog. Hydraulic When we imported the material properties, the material hydraulic properties were also imported. Let s have a look. From the Properties menu select Define Hydraulic.

6 Embankment Consolidation 36-6 Click on each material name. Notice that the permeability model is Constant for each material, and note the differing permeability values for each material. Select Cancel when you are finished examining the hydraulic properties. Now let s assign the materials to the model. From the toolbar or the Properties menu select Assign Properties. Assign the materials as shown. Excavate Silty Sand I Silty Sand II Dense Silty Sand Notice that the blank region above the "Silty Sand I" is assigned as excavated material for now. This is because we will be constructing the lifts of the embankment in stages. As long as the layers correspond to the correct material properties, the color of the materials is irrelevant. Embankment Fill Still within the Assign Properties dialog, we will fill in our embankment lifts in stages. 1. Select the "Embankment Fill" material at the top of the Assign Properties dialog. 2. Select Stage 2 and apply the material to the bottom lift. The model in Stage 2 should appear as shown.

7 Embankment Consolidation Select Stage 3 and apply the material to the second lift from the bottom. 4. Move on to the next stage and continue until all lifts have been assigned the Embankment Fill material. Stage 4: Stage 5: 5. Select the X button at the upper right to close the Assign dialog. Now select the Stage tabs in order, 1 to 5, to verify the embankment layer assignment.

8 Embankment Consolidation 36-8 Mesh Next we move to the Mesh tab. Here we may specify the mesh type and discretization density for our model. For this tutorial, we will use a 6- noded finite element mesh type. From the Mesh menu select Mesh Setup. We want to use a uniform, 6-noded element mesh. Change the Mesh type to Uniform, make sure the Element type = 6 Noded Triangles and the Approximate Number of Elements = as shown: Click the Discretize button and then the Mesh button. Click OK to close the dialog. The mesh will look like this:

9 Embankment Consolidation 36-9 Groundwater Since there are no liners or additional reinforcement in our model, we will skip the Support tab and move on to the Groundwater tab. In this tutorial, we will model a water table at about the depth of the lowest point on the embankment. From the Groundwater menu select Set Boundary Conditions. The Set Boundary Conditions dialog appears. Follow the steps below to input the transient condition: 1. Ensure you are on Stage 1 tab. From the BC Type drop down menu, select Total Head (H). 2. Enter a Total Head Value (m) of -12 meters. 3. You are now in selection mode. We want to apply the Total Head boundary condition to the side faces of the external boundary. When your selection looks similar to the model above, click Apply from the Add Boundary Conditions dialog. Select Close. Your model should show the following groundwater conditions.

10 Embankment Consolidation Loading We wish to apply a surcharge to the soil surface on top of the embankment to represent traffic loading. Click on the tab for Stage 5. From the Loading menu, select Distributed Loads and then select Add Uniform Load. Set the magnitude to 10 kn/m 2 as shown and click the checkbox for Stage Load. Click on the button for Stage Factors. Set the Stage Factors to 0 except for Stages 5, 6, 7, and 8 as shown. Click OK to close the Stage Factors dialog. Click OK to close the Add Distributed Load dialog. You will now be prompted to select a boundary on which to apply the load. Select the two roadway sections of the top of the embankment and hit enter. Your final model for Stage 5 should now look like this:

11 Embankment Consolidation Field Stress Because the top of the model represents the true ground surface, we want to use a gravity field stress. Go to the Loading menu and select Field Stress. For Field Stress Type select Gravity and click the checkbox for Use actual ground surface and Use effective stress ratio. Also change the Effective Stress Ratio both vertical in and out-of-plane to 0.5. Click OK to close the dialog. 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. Go to the Displacements menu and select Free. Click on the three sections that make up the top external boundary and hit Enter. You will see that the fixed boundary conditions have disappeared from the top external boundaries.

12 Embankment Consolidation The left and right edges of the embankment should be fixed only in the x- direction to allow vertical movement. Select Restrain X from the Displacements menu and select the sections of the left and right boundaries. These boundaries will now be showing rollers instead of pins. Finally, we need to re-establish the fixed boundary condition on the bottom corners. Select Restrain X,Y from the Displacements menu, click on the bottom boundary and hit Enter. Your model should now look like this: You have completed the definition of the model. Save the model using the Save option in the File menu. Compute Run the model using the Compute option in the Analysis menu. The analysis should take a few minutes to run.

13 Embankment Consolidation Interpret Once the model has finished computing (Compute dialog closes), select the Interpret option in the Analysis menu to view the results. Pressure Head The Interpret program starts and reads the results of the analysis. From the drop down menu select Seepage then Pressure Head. Notice the legend at the right side of the view. The legend is automatically adjusted to the values at each stage. From the View menu, select Contour Options. To have a uniform contour range for all stages, select Auto-Range (all stages). Now let s flip through the stages and see what happens to the water table as the embankment is constructed and traffic loads are added. Notice the water table (zero pressure head) is highlighted by a pink line superimposed on the contours. Initial Stage (0 d):

14 Embankment Consolidation Stage 2 (1 d): Stage 5 (60 d): The embankment installation is complete and traffic load is applied. Stage 8 (65 d): Very little change in pressure head occurs between the last few stages.

15 Embankment Consolidation Excess Pore Water Pressure We want to only view the excess pore pressure due to the construction of the embankment. Therefore, we should set the in-situ conditions as the reference stage. Right-click any stage tab at the bottom of your screen and select Stage Settings as shown. The following Stage Settings dialog should appear. Drag the Reference Stage indicator to the right until Not used changes to Initial as shown. The current Visible Stage is not important in this process. We have set the initial stage as our reference stage. The solid results of all other stages are calculated with respect to the solid results of the reference stage. New Results = Current Stage Results Reference Stage Results

16 Embankment Consolidation Now from the drop down menu, select Seepage then Excess Pore Pressure. Notice how the excess pore water pressure builds up under the embankment and then dissipates over time. Stage 2 (1 d): Stage 4 (20 d):

17 Embankment Consolidation Stage 5 (60 d): The Maximum excess pore pressure in the bottom soil layers under the embankment, decreases from near 40 kpa to less than 10 kpa after stage 5. Not much change occurs between the last few stages after the installation of the embankment is complete and the traffic load is added. If you add a vertical material query line from the bottom of the embankment to the bottom of the external boundary, and graph the query, you can see the profile of excess pore pressure underneath the embankment decreasing with time.

18 Embankment Consolidation Settlement To see the settlement of the embankment, with the same stage settings, from the drop down menu, select Solid Displacement; then select Vertical Displacement. In the Contour Options dialog, change the Auto-Format = Cold to Hot. At the point that the first lift is constructed, Stage 2, notice the settlement below the lift. Select through the stages to see the change in settlement as the embankment is constructed and traffic loads are added. Additionally, to see the deformed shape, from the view menu, select Display options. In the Display Options dialog, in the Stress tab under Deformations check Deformed Contours.

19 Embankment Consolidation Click Done to finish. The deformed contour for Stage 5 (60 d) is shown below: Turn off the deformed contour option. Using the same material query, let s plot the settlement below the embankment. Right click on the query line, select Graph Data, and graph the settlement results for all stages.

20 Embankment Consolidation The vertical settlement underneath the embankment increases until stage 5 (addition of traffic loads) and then stays constant for the remaining stages. This concludes the Embankment Consolidation Tutorial.