Tutorial 28 Seismic Analysis

Tutorial 28 Seismic Analysis • Pseudo-static seismic loading • Critical seismic coefficient (ky) analysis • Newmark displacement analysis • Multi Scenario modeling

Slide v.7.0 Tutorial Manual Tutorial 28: Seismic Analysis

Introduction This tutorial will demonstrate how to model a multi-material slope with seismic loading in a multiple scenario model. We will demonstrate three different seismic analysis options including displacement analysis using the Newmark method.

The finished product of this tutorial can be found in the Tutorial 28 Seismic Analysis.slmd data file. All tutorial files installed with Slide 7.0 can be accessed by selecting File > Recent Folders > Tutorials Folder from the Slide main menu.

Model 1 – No seismic loading If you have not already done so, run the Slide Model program by double-clicking on the Slide icon in your installation folder. Or from the Start menu, select Programs → Rocscience → Slide 7.0 → Slide.

If the Slide application window is not already maximized, maximize it now, so that the full screen is available for viewing the model.

From the Slide main menu, select File → Recent Folders → Tutorials Folder and read in the Tutorial 28 Seismic (initial).slim file. This model is based on the non-homogeneous, three layer slope found in Slide Verification Problem #4.

Project Settings

The file we have read in is a regular “Single Scenario” model file. For this tutorial we will be using the Multi Scenario option, so we need to select this in Project Settings.

Select: Analysis → Project Settings

Select the Scenarios page and select the Multiple Scenarios option. Select OK.

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The Document Viewer pane, which is used to manage and summarize Scenarios and Groups, is now visible in the sidebar. See Tutorial 24 for more information on multi scenario modeling, groups and scenarios.

In the Document Viewer:

• right-click on “Group 1” select Rename, and change the name to “Seismic Analysis”. • right-click on “Scenario 1”, select Rename and change the name to “No Seismic”.

Now return to the Project Settings dialog.


Select the Methods page from the list at the left of the dialog.

Make sure that only the Spencer checkbox is selected in the Methods list. This is the method used for this slope stability analysis.

Do not change any settings in the dialog. Select OK.

Material Properties

Let’s examine the material properties of the model. Select Define Material from the toolbar or the Properties menu.

Select: Properties → Define Materials

With the first (default) material selected in the Define Materials dialog, notice the following properties:

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• Name = soil 1 • Unit Weight = 19.5 • Strength Type = Mohr-Coulomb • Cohesion = 0 • Phi = 38

Select the second material, and notice the following properties:

• Name = soil 2 • Unit Weight = 19.5 • Strength Type = Mohr-Coulomb • Cohesion = 5.3 • Phi = 23

Select the third material, and notice the following properties:

• Name = soil 3 • Unit Weight = 19.5 • Strength Type = Mohr-Coulomb • Cohesion = 7.2 • Phi = 20

Select Cancel to close the Define Material Properties dialog when finished.

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Surface Options

For this tutorial, we will be performing a circular surface Grid Search, to attempt to locate the critical slip surface (i.e. the slip surface with the lowest safety factor).

Select the Surfaces workflow tab.

Let’s take a look at the Surface Options dialog.

Select: Surfaces → Surface Options

You will see the Surface Type is set to Circular and the Search Method is set to Grid Search, with the Radius Increment = 10.

Select Cancel to close the Surface Options dialog when finished.

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Compute

Before you analyze your model, save it as a file called Seismic Tutorial.slmd.

Select: File → Save

Use the Save As dialog to save the file. You are now ready to run the analysis.

Select: Analysis → Compute

Select the current scenario, No Seismic, to Compute. Select OK. The Slide Compute engine will proceed in running the analysis. When completed, you are ready to view the results in Interpret.

Interpret

To view the results of the analysis:

Select: Analysis → Interpret

This will start the Slide Interpret program. You should see the following critical slip surface with FS = 1.373.

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Model 2 – Pseudostatic seismic loading We will now duplicate the scenario, add a pseudostatic seismic load to the new model and re-run the analysis to determine its effect on the Safety Factor.

Return to the Slide Model program.

In the Document Viewer, right-click on No Seismic and select Duplicate Scenario. Right-click on this new scenario, Scenario 2, and select Rename. Enter Seismic = 0.15 as the scenario name.

NOTE: since we created the second scenario by duplicating the first scenario, all settings in the second scenario are initially the same as the first scenario by default. However, any subsequent changes made to a scenario, will only apply to that scenario, unless Lock Scenarios is activated. Only changes made to the geometry (External Boundary and Material Boundary) of one scenario are automatically applied to all scenarios within that group. For a more in-depth explanation of the features of multiple model scenarios, refer to Tutorial 24.

Pseudo-Static Seismic Load

In Slide, pseudo-static seismic loads can be applied in the horizontal and vertical directions by specifying the corresponding Seismic load coefficient. The Seismic load coefficient is used to determine the seismic force applied to the slope.

Select: Loading → Seismic Load

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In the dialog, enter a Horizontal Seismic load coefficient = 0.15. Notice that this value is positive in the direction of failure. Select OK when finished.

We are now finished creating this scenario, and can proceed to run the analysis and interpret the results.

Compute

Before you analyze your model, hover the cursor over Seismic = 0.15 in the Document Viewer. In the Document Viewer Legend, notice that Seismic = 0.15 is “Unsaved: Scenario needs to be saved.”


You are now ready to run the analysis.


Select the current scenario, Seismic = 0.15, to Compute. Select OK. The Slide Compute engine will proceed in running the analysis. When completed, you are ready to view the results in Interpret.

NOTE: since we already computed the results of the No Seismic scenario, it is deselected in the “Select scenarios to compute” dialog by default.

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Interpret



This will start the Slide Interpret program. Select the Seismic = 0.15 scenario in the Document Viewer. For the Seismic = 0.15 scenario, you should see the following critical slip surface with FS = 0.988.

With the addition of horizontal seismic loading, the Global Minimum safety factor is now 0.988 compared to 1.373 before adding the seismic load. The seismic load has destabilized the slope. You may find it useful to tile the views, to view the results of both scenarios together.

Select: Window → Tile Vertically

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Under the Document Viewer pane, select Synchronize Views. Select the “Sync Zoom/Pan for all windows” checkbox. Select Done.

Once activated, this feature allows you apply the zoom and pan settings used in one scenario across all scenarios. Use the Zoom options as necessary to achieve the desired view of the slopes.

Model 3 – Critical seismic coefficient (ky) analysis In this tutorial, we have so far considered the effect of a pseudostatic seismic load on the minimum safety factor, by specifying a horizontal seismic load coefficient. In Slide, we can also perform an advanced seismic analysis to determine the critical seismic coefficient (ky) that results in a destabilized slope with FS = 1.


In the Document Viewer, right-click on No Seismic and select Duplicate Scenario. Right-click on this new scenario, Scenario 3, and select Rename. Enter Critical Acceleration as the scenario name.

Project Settings

For the Critical Acceleration scenario we will change the Project Settings in order to determine the critical seismic coefficient.


Select the Seismic page from the list at the left of the dialog.

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Select the “Advanced Seismic Analysis” checkbox. Notice that the “Compute Ky for all failure surfaces” option is selected. This option must be selected in order to compute ky for all failure surfaces. Keep the Target Factor of Safety as 1.0. Select OK.

Compute




Select the new scenario, Critical Acceleration, to Compute. Select OK.

The Slide Compute engine will proceed in running the analysis. When completed, you are ready to view the results in Interpret.

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Interpret



Select the Critical Acceleration scenario in the Document Viewer. You should see the following critical slip surface with the critical seismic coefficient displayed (ky = 0.145).

Now select the All Surfaces option to view all circles generated by the analysis:

Select: Data → All Surfaces

Let’s use the Filter Surfaces option, to display only surfaces with a critical seismic coefficient (ky) below 0.15, the value we specified in the previous scenario Seismic = 0.15.

Select: Data → Filter Surfaces

In the Filter Surfaces dialog, select the “Surfaces with a Ky below” option, enter a value of 0.15, and select Done.

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As you can see, there are a number of unstable surfaces for this model, wherein a seismic coefficient less than 0.15 would result in a destablized slope. This makes sense, since the Global Minimum factor of safety for the Seismic = 0.15 scenario, is 0.988 (i.e. just below one).

Model 4 – Newmark displacement analysis We will now perform a Newmark displacement analysis to determine the critical Newmark displacement that results from seismic loading.


In the Document Viewer, right-click on the Critical Acceleration scenario and select Duplicate Scenario. Right-click on this new scenario, Scenario 4, and select Rename. Enter Newmark Displacement as the scenario name.

Project Settings

We will now change the Project Settings for the new scenario order to determine the Newmark displacements.


Select the Seismic page from the list at the left of the dialog.

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Notice that the “Advanced Seismic Analysis” checkbox is selected, as it was in the Critical Acceleration scenario. This option must be selected in order to compute Newmark displacements. The Newmark analysis in Slide is based on the program SLAMMER, developed by the U.S. Geological Survey. The permission to use the SLAMMER code by Dr. Jibson and Dr. Rathje in Slide is gratefully acknowledged1.

Select Newmark Analysis Options and then the Define Seismic Record button.

Notice that in Slide there are a number of ways the seismic record can be entered. Time and acceleration data points can be manually entered into each cell or copied in from a table. Alternatively, the seismic record can be imported from a Slammer or Slide (.ssr) file, or chosen from a list of Example Records containing historical data from a selection of earthquakes.

For this tutorial, we will use data from the Example Record of Mammoth Lakes-1 1980, CVK-090 with a peak ground acceleration (PGA) of 0.416 g. Select the Example Records… button and set Earthquake = Mammoth Lakes-1 1980 and Record Name = CVK-090.

1 Reference: Jobson, R.W., Rathje, E.M., Jibson, M.W., and Lee, Y.W., 2013, SLAMMER – Seismic Landslide Movement Modeled using Eatherquake Records (ver.1.1, November 2014): U.S. Geological Survey Techniques and Methods, book 12, chap. B1, unpaged.

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Notice that a summary of the Earthquake Properties, which includes the PGA and PGV of the selected record, is displayed.

Select OK to close the Example Seismic Records dialog.

Notice that once the time and acceleration data points have been entered, an acceleration vs. time plot is generated in the Define Seismic Record dialog.

Select OK to close the Define Seismic Record dialog when finished reviewing the seismic record data.

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In the Newmark Analysis dialog, notice the Newmark Analysis Type option. In Slide, we are able to define the Newmark Analysis Type as either Rigid, Coupled, or Decoupled. Also, notice that the displacement can be computed by examining the Positive Accelerations, Negative Accelerations, Mean Accelerations, or the Maximum positive/negative accelerations of the seismic record.

For this tutorial, we will set Newmark Analysis Type = Rigid and Displacement computed using = Maximum positive/negative acceleration.

Select OK to close the Newmark Analysis dialog.

Select OK in the Project Settings dialog.

Compute




Select the Newmark Displacement scenario to Compute. Select OK.

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The Slide Compute engine will proceed in running the analysis. When completed, you are ready to view the results in Interpret.

Interpret



Select the Newmark Displacement scenario in the Document Viewer. You should see the following critical slip surface with the critical Newmark displacement displayed = 4.426 cm.

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Additional Exercise From the results in scenarios 1, 3, and 4, the slip center grid was observed to have a blank (white) area which is not contoured, at the left of the grid. As discussed in Tutorial 4 and Tutorial 5, you may want to go back to the modeler and modify the grid size or location.

One advantage of having enabled Multiple Scenarios in the Project Settings, is that you are able to make changes across all scenarios within a group. Here, we want to go back to the modeler and modify the grid size or location across the 4 scenarios within the group.

Return to No Seismic in the modeler.

1. Under the Document Viewer pane, select Lock Scenarios . When activated, Lock Scenarios will duplicate all operations performed in one scenario to all scenarios in that group.

2. Right-click on the EDGE of the grid.

3. A popup menu will appear. Select the Move option. Left-click on the lower RIGHT corner of the grid and enter the following coordinates in the prompt line at the bottom right of the screen.

Enter vertex [esc=cancel]: 46 36 Enter vertex [c=close,u=undo,esc=cancel]:press Enter

4. The grid will be redrawn slightly below and to the right of its original location (near the crest of the slope).

Let’s also increase the Radius Increment, to generate more surfaces at each grid point. Select Surface Options from the Surfaces menu, enter a new Radius Increment = 20, and select OK.

Deselect Lock Scenarios.

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Now let’s see how the new grid affects the analysis in the 4 scenarios.

Compute

Before you analyze your model, save it as a file called Seismic Tutorial 2.slmd.


Use the Save As dialog to save the file. You are now ready to run the analysis.


Select all scenarios to Compute. Select OK. The Slide Compute engine will proceed in running the analysis. When completed, you are ready to view the results in Interpret.

Interpret



This will start the Slide Interpret program. Tile the views. As you can see the grid is now fully contoured with no blank areas for all scenarios.

Compare the results with those computed using the initial grid.

That concludes this tutorial. To exit the program:

Select: File → Exit

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Documents

Tutorial 28 Seismic Analysis