transient stability analyses

This week I’ll explore one of the new features introduced in version 7.1:  “transient” analyses in SLOPE/W.

In Theory

The theory behind transient stability analyses is fairly simple.  In the past a SLOPE/W analysis has calculated a factor of safety for a specific snapshot in time.  You determined what that snapshot was by identifying a SEEP/W analysis (for example) and a time step to use for the pore water pressure conditions.

A transient stability analysis simply performs the same calculations over and over at several successive snapshots in time.

In Practice

To perform a single stability analysis (at a single point in time) you have always been able to use KeyIn Analyses to pick another analysis from which to obtain pore water pressures or stress conditions.  In the past you had to identify the other analysis and the time step to use.  The stability analysis would give you a factor of safety for that snapshot in time.

Now you can choose “(all)” as the time step.

When you choose “(all)”, the SLOPE/W solver will repeat its calculations once for each time step it finds in the other analysis.  You end up with one factor of safety for each slice in time.

That’s it!  That’s all it takes to set up a transient stability analysis.

The fun of course starts when you can look at the results. 

Snapshots

The simplest way to look at results is just to pick a specific snapshot in time, a specific time step, and use SLOPE/W CONTOUR as you normally would. 

You pick the time step from the Analysis toolbar.

If the Time dropdown list has the focus (you can press Alt-I to give it the focus) you can use the Up and Down arrow keys to step through each time step.  You can watch how the critical slip surface and factor of safety change over time.

All the other features of SLOPE/W CONTOUR will work too, like drawing graphs, viewing slice forces, etc.  They all show you data for the current time step.

Graphing FOS over Time

To get the bigger picture, you can graph factor of safety over time. 

This graph is NOT under Draw Graph as you might expect.  Because you also need to select which slip surface to use, you find this graph under Draw Slip Surfaces.

There are two graphs to choose from.

The “Factor of Safety vs Time” graph uses the current slip surface, showing its FOS at each time step.  Use this graph if you care about a specific mode of failure.

The “Minimum Factor of Safety vs Time” graph ignores the current slip surface, instead showing the FOS of the most critical slip surface at each time step.  This graph is useful to see if a slope will be stable over time, at any slip surface.

Sometimes the two graphs will be the same (especially if you’re using the Auto-Locate method or the “optimized” slip surface option, because those are by definition the most critical), but if PWP conditions vary enough over time that at a particular step the critical slip surface is at a different location than at another step, the graphs will be different.

If you are doing a Probability analysis, you will also be able to see graphs of “Probability of Failure vs Time” and “Maximum Probability of Failure vs Time”.

Animation

As with any of the FE products, you can use the View Movie command to create a movie of the slope analysis.  Each time step becomes a frame in the movie. 

Next Week

Next week I’ll look at another new feature of 7.1.  Drop me an email or leave a comment if you are especially interested in one of them.