If the normal and shear stresses corresponding to failure are plotted, then a curve is obtained. The plot or curve is called the ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 11/10/2023 (Morning)
Detailed Explanation
To find a soil's strength parameters, several identical specimens are tested at different normal (or confining) stresses. For each test, the stress state at failure is plotted. The line or curve that joins these failure points — or that touches all the Mohr circles at failure — is called the strength envelope, also known as the failure envelope or Mohr envelope.
Why “strength envelope” Is Correct
‘Strength envelope’ is the standard name for this curve. It envelops (touches from above) all the Mohr circles at failure and separates possible stress states from impossible ones.
Why the Other Options Are Wrong
- A. failure line: ‘Failure line’ is not the standard term used in soil mechanics for this plot (though the idea is similar, the accepted name is strength or failure envelope).
- B. phreatic line: The phreatic line is the top flow line (free water surface) in seepage through an earth dam.
- D. parabolic curve: The envelope is not a parabola; it is close to a straight line.
Key Point
Strength envelope = failure envelope = Mohr envelope: τ = c + σ tan φ.
Key Concepts for Students
- At least three tests at different confining pressures are used to draw it.
- Its intercept is c and its slope angle is φ.
- Total-stress and effective-stress envelopes are drawn separately.
Practice more: this is question 10 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.