Tutorial_10_Water_Table_Statistics.pdf

Water Table Statistics Tutorial 10-1

Water Table Statistics

In Slide it is very easy to account for a variable water table location, in

either a Sensitivity or Probabilistic Analysis.

1. The Minimum and Maximum locations of the Water Table are

specified graphically, by drawing the location of the limiting

boundaries on the model.

2. A single random variable (a Normalized Elevation ranging between 0

and 1), is then used to generate Water Table elevations between the

Minimum and Maximum boundaries, according to the statistical

parameters entered in the Water Table Statistics dialog.

The finished product of this tutorial (file: Tutorial 10 Water Table

Statistics.sli ) can be found in the Examples > Tutorials folder in your

Slide installation folder.

Sensitivity Analysis

First, we will demonstrate a simple Sensitivity Analysis, using a Water

Table. We will start with the Tutorial 01 file.

Select: File → Open

Open the Tutorial 01 Quick Start.sli file, which you will find in the

Examples > Tutorials folder in your Slide installation folder.

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Project Settings

To enable a Sensitivity Analysis with Slide , you must first select the

Sensitivity Analysis checkbox in Project Settings.

Select: Analysis → Project Settings

In the Project Settings dialog, select the Statistics tab, and select the

Sensitivity Analysis checkbox. Select OK.

Water Table Boundaries

In order to define the upper and lower limits of a Water Table for the

Sensitivity Analysis, we must define the Maximum and Minimum Water

Table boundaries.

Select: Statistics → Water Table → Draw Max Water Table

We will create the Maximum Water Table, by snapping to the vertices

along the slope.

1. First, right-click the mouse and make sure the Snap option is

enabled.

2. Now left click the mouse, and snap the Maximum Water Table to the

slope vertices at (0 , 30) , (50 , 30) , (80 , 50) and (130 , 50).

3. Right-click and select Done from the popup menu, and the boundary

will be added to the model.

Now let’s create the Minimum Water Table boundary.

Select: Statistics → Water Table → Draw Min Water Table

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1. Snap the Minimum Water Table, to the slope vertices at (0 , 30) and

(50 , 30).

2. Now enter the point (130 , 30) in the prompt line. (Or alternatively,

right-click the mouse and make sure the Ortho Snap option is

enabled. Hover the mouse near the point (130 , 30) at the right edge

of the model. When the Ortho Snap icon appears, click the mouse and

you will snap exactly to the point (130, 30) on the boundary.)

3. Right-click and select Done from the popup menu.

4. You will see the Assign Water Table dialog. Select OK to

automatically assign the Water Table to all slope materials (only one

material is actually used in this model).

You have now defined the Maximum and Minimum Water Table

boundaries. When BOTH boundaries have been defined, you will notice

that a THIRD boundary, the MEAN Water Table, is automatically

calculated, and appears on the model.

Your screen should appear as follows.

Figure 10-1 : Maximum, Minimum and Mean Water Table boundaries.

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Mean Water Table

So, how has the Mean Water Table been calculated? First, let’s look at

the Water Table Statistics dialog.

Select: Statistics → Water Table → Statistical Properties

NOTE: a convenient shortcut to access this dialog, is to right-click the

mouse on any of the three Water Table boundaries – Maximum, Mean or

Minimum – and select Statistical Properties from the popup menu.

Normalized Mean

In the Water Table Statistics dialog, you will notice the Normalized Mean

parameter.

The definition of the Normalized Mean water table location, is illustrated

in the following figure. The Normalized Mean is simply the relative

elevation of the Mean Water Table, along any vertical line between the

Minimum and Maximum water table boundaries.

Figure 10-2 : Definition of Normalized Mean water table location.

The default Normalized Mean ( = 0.5 ) produces a Mean Water Table

which is exactly midway between the Minimum and Maximum

boundaries, at all locations.

The Normalized Mean must have a value between 0 and 1.

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Compute

Before we run the analysis, it is important to note the following:

• The MEAN Water Table will be used as the Water Table in the

Deterministic Analysis.

• The Sensitivity Analysis is then performed on the Global Minimum

slip surface located by the Deterministic Analysis.

• The Sensitivity Analysis is carried out by varying the Water Table

location between the Minimum and Maximum Water Table

boundaries, in 50 equal increments, and calculating the safety factor

of the Global Minimum slip surface, for each location of the Water

Table.

First save the file with a new file name: wt_sens.sli .

Select: File → Save As

Use the Save As dialog to save the file. Now select Compute.

Select: Analysis → Compute

When the analysis is complete, view the results in Interpret.

Select: Analysis → Interpret

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