When does “short term” become “long term”

I have a slope which is stable using undrained parameters and fails using drained parameters. If the slope (cutting) is to remain unsupported for a week as opposed to a year (no time frame mentioned yet) how do decide what is long term or not?

My slope keeps failing using drained parameters as opposed to undrained parameters:

  • clay undrained: density 19kn/m3 Cu 55kpa
    2nd clay undraind: 19.40kn/m3 cu 170kpa

  • Clay drained: 19kn/m3 phi’ 25
    2nd clay draind: 19.40kn/m3 phi’ 24


If it’s a cut slope in over consolidated clay, drained strength can be lower than undrained due to dissipation of negative pore water pressure.
Refer to Soil Strength and Slope Stability by Duncan, Wright and Brandon for a discussion on when to use undrained or drained strengths.

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