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# infinite slope analysis

## infinite slope analysis

(OP)
what is the rule of thumb in selecting the depth at wich a slide will occur (z) for an infinite slope analysis

using FS = C/(unit weight * z *(1/2 sin 2*slope angle)

is it a function of slope angle or material property or jsut an estimate.

### RE: infinite slope analysis

You vary z until you find the minimum FS.  It's generally trial-and-error.  At least that's what my profs made us do...

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### RE: infinite slope analysis

The infinite slope will tend to the "thin" slope which is really like erosion.  This is not practical - what is needed is basically a wedge analysis - what we have done in practice is to "require" a significant mass of movement - usually this was taken as either a thickness of about 10 ft or 10 ft behind crest "breakout".  Of course, significant might be slightly different for each case - e.g., footing near slope, etc.

### RE: infinite slope analysis

basford:

In an infinite slope analysis, the state of stress on slide plane parallel to the slope is dependent only upon the unit weight of the material, the depth in the slope and the flow net for water in the slope.  You can find the solution graphically by ploting the locus of points (shear stress and normal stress on the slide plane, which is a radial line from the origin on a shear stress/normal stress plot) and the Mohr Coulomb failure envelope.  Where these two intersect corresponds to the depth that you are looking for.

Glen

### RE: infinite slope analysis

(OP)
Let me restate my question
When does the infinite slope formula no loger apply.  For example given a slope and soil strength using the depth of slide to be say 5' I get a FS.  If my total slope height/length is 100ft it think the infinite slope holds, but what about if the slope is only 10' heigh my depth of slide is 50% if the total hieght so I would think the infinite slope assumptions no longer apply.  What is the cut off z=20% of height, 30%, 10%????

### RE: infinite slope analysis

basford:

I think that I understand better your question.  I have seen very large slopes where the failure mode has been a shallow surface slide (amenable to an infinite slope analysis) and I have seen very small slopes where the failure mode has been a circular arc or wedge (amenable to limit equilibrium slope stability analyses).  I don't believe that there is a fast rule about height of slope, depth of failure and which analysis technique will control.

If there is the potential for either, you should analyze both and use the lowest factor of safety.  If the foundation and embankment soils are fairly strong, then the most likely failure mode is probably a shallow failure when the slope surface becomes saturated and seepage occurs parallel to the slope.  The infinite slope theory would say that the stable slope angle with seepage parallel to the slope is roughly one-half of the angle of repose.

I hope this helps.

Glen

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