CivilLab · Study Aid

Mohr-Coulomb Shear Strength

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What am I seeing?
  • The circle is the stress state and the line is the strength. Failure is not a stress level, it is the moment the circle grows out to touch the line.
  • Pore pressure slides the circle to the left without changing its size. That is why a saturated soil can fail under a total stress that the same soil carries safely when drained.
  • In a drained test the total and effective circles are the same one. In an undrained test the gap between them is exactly u.
  • The failure plane sits at 45 + φ/2 from the plane σ1 acts on, because that is the point where the circle first touches the envelope, at 2θ = 90 + φ.
  • With φu = 0 the undrained strength ignores the cell pressure, so every UU circle has the same radius cu and the deviator at failure is always 2cu.