slope analysis methods

[72shortbox]

I have been using PCSTABLE6 to determine the stability of a clay slope. The slope is totally submerged under 45 feet of water. I would like to understand in more depth how the program is determining the stability to make sure I am analyzing the stability in the best manner. I was using the Bishop Circle method.

Any feedback or comentary on the following is greatly appriciated.

1) Could anyone explain briefly what interslice forces are and how they effect the stability results when neglected?

2) Is it recommended to exclude the piezometic surface and replace it with some type of pressure line along the slope surface?

3) I set up the geometry with the piezometric surface as a strait line at the crest of the slope so everything was submerged. It is my understanding at the stability results are independent of the water surface elevation as long as the entire slope is submerged. Is this correct?

4) When should the Janbu method be used over the Bishop Circle. When using the Janbu method how does the the correction factor to adjust for inerslice forces come into play. Do you consider the results to be overly conservative when using the correction facotors?

 

[Mccoy]

Basford,
I'll try to answer, at least partially, to some of your questions:

1)Interslice forces are simply due to interactions between slices. In simplified methods, by neglecting them or assuming zero interslice shear forces you introduce an error in the final evaluation of FS. As far as I remember, textbooks typically report a few tens % error for Fellenius, a 5% error for simplified Bishop, and variable error for simplified Jambu, with respect to exact methods such as Morgenstern and Price. Probably it all goes back to the classical study by Fredlund and Krahn, which also resulted in smaller error by jambu simplified and corrected than by rigouros Jambu. I cannot say if there were recent developments in this field.
2)and 3)unfortunately I cannot answer because I never worked w/ submerged slopes, nor I ever tried PCSTABLE6, but a brief discussion about this topic appears in the following thread, started a while back by yourself: "Underwater dredged slope".
4)The Jambu method is suitable when you suspect a non-circular slipping surface, usually suggested by pre-existing surfaces, site investigations, inclinometers, stratigraphy, geotechnical judgment... As Fredlund and Krahn reported in their comparative study (the example was pretty simple though) corrected simplified Jambu performed better than rigorous Jambu. So I would say you'd better always apply the correction equations. Jambu underextimates FS with respect to Morgenstern-Price (by a slight amount if rigorous or corrected).