Safety Factor of Limit Equilibrium Method

[geotechman]

When I use Limit equilibrium method to analyse the slope stability, such as the Slide or Slope/W, I find the safety factor is not unique even if the the same method is adpoted, such as Bishop or others. The key question is the the selection of "slip surface grid" and "slip surface radus". The different location will lead to different result but no any errors occur.

I am wondering which location is correct? whether these results are all correct?

I wish get help especially those users who use Slope/W or Slide software.

 

[VAD]

I am not sure of the programs you are using. There are such things as interslice forces which are taken or not taken into consideration by some methods which can have an influence on results.

After a while, depending on the problem you are analysing, you will think about other issues related to slope stability that your judgement would be the best guide on what to rely on. All are probably correct based on their limitations.

Despite the above statement, it is of interest to know why results vary and wish you luck in your pursuit. You will probably need to read the literature on the development of the various methods and as well the premise of the programs.

Good Luck

 

[Focht3]

geotechman:

Your main question seemed to be, The key question is the the selection of "slip surface grid" and "slip surface radus". The different location will lead to different result but no any errors occur.

You are correct. If you use circular failure surfaces, you will get lots of values; your job is to find the smallest reasonable value. If you use an irregular failure surface, the grid spacing becomes important. My comment is the same about irregular surfaces as it is about circular ones: your job is to find the smallest reasonable value.

Good luck!

 

[geotechman]

Focht3:

You got it. I indeed get lots of different values using circular failure surfaces while adopting different "slip surface grid" and "slip surface radus" location or increment. According to your point of view, I should compare many such results by changing these parameters to get the smallest SF. However, I think such methods are bit little countless and aimless. The maunal of Slope/W and Slide seem not explain explicitly the detailed methodology.

although these results using different parameters are little different, such as 1.089 and 1.101 or others, how to think which SF is smallest and reasonable result.

I tried some examples supplied by software manual but using different "slip surface grid" and "slip surface radus" location or increment from the manual, of course, the results are also different from the "standard" answer supplied by the manual. How to evaluate my result?

 

[Focht3]

Well, let me give you one suggestion on finding the "minimum values" for circular surfaces. I don't have Slide or Slope/W so I can't give you specific guidance on those programs. Do a lot of analyses - keep a record of the X,Y centers and the calculated factor of safety of each circle. Use a contouring package like Surfer to estimate the X,Y location with the minimum Factor of Safety. You will likely see more than one minimum -

For irregular surfaces, it's a trial-and-error approach.