Factors of safety

[Lovi]

What would a negative factor of safety for a slope signify?

 

[BigH]

How much greater the reserve strength the slope has vs the "failure" strength.

 

[Lovi]

Thank you BigH.

im being told by colleague that the negative has no significance and that i should simply ignore the negative sign. what do think of this? im not completely convinced

in my case, im using fos= restoring moment/disturbing moment

and im getting a negative restoring moment due to the water pressure being more than the weight of the slices

does this not mean that the restoring moment is acting in the opposite direction(same direction as the disturbing moment) and that what i have is basically, disturbing moment/disturbing moment? and a very unstable slope?

thanks

 

[BigH]

restoring (or "strength" cannot act in same direction as driving force. I think you might have a "sign" wrong.

 

[dgillette]

In addition to the effect of the water (which I don't understand - see below), there are other things that can cause you to calculate negative FS. If you have are doing a limit-equilibrium analysis that involves shearing through concrete in the upper part of the slope, some programs can produce negative interslice forces so large that FS can actually get negative. You might also see this happen if the exit is much steeper than 45-phi/2, which can cause spurious negative base force. Go through all of your base forces and interslice forces looking for weird ones.

I'm puzzled as to how the water pressure can exceed the weight of the slice, assuming you are putting the TOTAL unit weight in for the soil.

Can you post a sketch?