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"Simulating" an Su/P' with a Phi value in PCStabl

"Simulating" an Su/P' with a Phi value in PCStabl

"Simulating" an Su/P' with a Phi value in PCStabl

In newer stability programs, an undrained strength ratio (Su/P') can be assigned to a soil layer.  But since I have a lot of problems already set up in PCstabl files, I still use PCstabl (w/Sted) most of the time.

Now the question:

If I have a construction or other quick embankment loading on a fine grained foundation, I usually approximate the Su/P' in PCStabl by breaking the material into layers and zones with a separate C value assigned to each layer based on the P' calculated for the layer or zone.  But I have also encountered engineers who simply convert the Su/P' to an "equivalent" phi value (arctan Su/P') for the fine grained layer.  While not theoretically correct, the "equivalent phi strength" gives, and unless I'm missing something, should give, the same results.  Is this correct, or am I missing something important?

RE: "Simulating" an Su/P' with a Phi value in PCStabl

I answered my question.

The engineers' "treat as a phi" approach will produce erroneous, too-low results for the resisting reaction at the base of all slices in the analysis, except those with horizontal bases. At least their errors have been on the conservative side.

RE: "Simulating" an Su/P' with a Phi value in PCStabl

I don't think it is necessarily conservative for the slices on the passive side.  Those slices are in something more like plane-strain extension, and the pseudo-drained analysis can give them a normal stress that's higher than the vertical preconsolidation stress.

Refer to Chuck Ladd's Terzaghi Lecture: Ladd, C.C. (1991), "Stability Evaluation During Staged Construction," the 22nd Terzaghi lecture, ASCE Journal of Geotechnical Engineering, vol.117, no. 8, pp. 1540-615.  

That's probably the best general ref available on undrained shear strength.  Been a few years since I read it, but I do recall that there is a discussion of using Phi-cu = arctan (Su/p).  Su/p is not a constant - it varies with stress path: CIUC vs CAUC vs CAUE vs plane-strain extension, vs etc.

You work at MSHA with George G.?

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