transient stability analyses

by Nate Hekman on April 23, 2008

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.

Picking the 'all' time step makes this a transient stability analysis

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. 


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.

Pick a time step to see the stability analysis for that moment in time

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.

Graphing factor of safety vs time

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.

Graphing factor of safety over time

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”.


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 7.1Drop me an email or leave a comment if you are especially interested in one of them.

{ 10 comments… read them below or add one }


Ong Yin Hoe 07.11.08 at 3:10 am

Dear Hekman,

i refer to an article related to transient stability analyse posted on 23 Apr 2008.

Engineers understand the modelling steps of transient analysis of slope stability affected by inflow of outflow by using seep/w and slope/w.

We (engineer) face a problem when we want to define the soil–water characteristic curve. We have no idea what is the maximum suction can develop during the time of analysis, so we normally give up.

Also, it will be interesting if you write an artical on boundary conditions used in transient analysis of slope induced by rainfall for different cases.




Nate 07.11.08 at 8:39 am

Thanks for your comments, Ong. As I’m not an engineer myself I’m afraid I can’t answer your questions personally, but I’ll see if I can get some answers from my colleagues.

Do any other readers have ideas to help Ong out? Go ahead and post them in your own comment.


Nate 07.25.08 at 9:29 am

Lori has followed up on this question in this guest post: soil-water characteristic curve for transient stability analyses.


Kevin Ye 01.27.10 at 3:46 pm

Hi, Gokking,

I use Geo SEEP/W for a while, but just found one issue there. When saturated-only materials used, the pore water pressure above phreatic surface is also negative, instead of zero as what I expected. Please check it through the example, dam.gsz at the folder of C:\Program Files\GEO-SLOPE\GeoStudio2007 Resources\Examples.

I’ll appreciate if you have any good idea regarding this.

Thank you!



Nate 01.27.10 at 4:22 pm

From the online help “KeyIn Materials” topic:

Saturated Only: This model defines water properties in the saturated zone only.

Since this model is only defining properties within the saturated zone, it should only be used in situations where it will be fully saturated at all times. In the Dam.gsz example, part of the region is unsaturated, therefore we use the more rigorous Saturated / Unsaturated model.


Harry 08.31.14 at 10:37 am

Dear Hekman,

I am interested to your article related to transient stability analysis. I am a new user with this software. Now I would like to ask you how can I analyse slope instability induced by rainfall (20mm/hr) in this transient stability analysis? And I do not know why I cannot draw a factor of safety graph from Slope/W (limit equilibrium)?

Note: I am using a student license. Thank you
Best regards,


Nate Hekman 09.04.14 at 2:38 pm

Hello Harry. I’m not sure I understand your first question correctly–it seems to me the answer is in the article. Start by creating a transient SEEP/W analysis to model the rainfall, then a SLOPE/W analysis getting its PWP conditions from “(all)” of the SEEP/W analysis’ time steps. That will give you a FOS that changes over time.

It may be simplest if you send a copy of your .gsz file, along with your questions and perhaps some screen shots explaining what you’re trying to do, to support [at] geo-slope [dot] com so an engineer can help.


Harry 12.08.14 at 12:18 am

Dear Mr. Hekman,

Thanks so much for the idea. I will send and discuss more about my problem with you via Linked. Thanks for spending you value time with my problem.

Best regard,


Samuel 05.03.17 at 9:58 am

Currently I’m using Geo-Studio for stability analysis of rainfall induced landslide for my second degree thesis work. But I’ve found one problem I prepare a daily rainfall data and insert it as a boundary as step function and assigned it on the ground surface of my slope i checked for error verification and i found zero errors zero warnings but the solver couldn’t solve it. it displays the error function is not supported dialog box when I hit the solve button.
I really appreciate if you help me out.
I am using a full version
best regards


Nate Hekman 05.03.17 at 1:12 pm

Samuel, thanks for commenting. This feels like the type of problem that will be easiest to help with if we have a copy of your file to look at ourselves. I think you should email support [at] geo-slope [dot] com with your question, and attach your .gsz file and a screen capture of the error message.

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