Q.59 If the normal and shear stress corresponding the failure are plotted, then a curve is obtained. The plot or curve is called the
- Aphreatic line
- Bstrength envelopeCorrect
- Cparabolic curve
- Dfailure line
Solution
Concept
The curve joining normal and shear stress at failure is the strength (failure) envelope.
Formula
τf = c + σ tan φ
Step-by-step solution
1. Do several shear tests on the soil and find the stresses at failure. 2. Plot shear stress τ against normal stress σ for each failure point. 3. Join these points (or draw a line touching the Mohr circles). 4. This curve is called the strength envelope (Mohr failure envelope).
Explanation
When we plot normal stress and shear stress at failure, we get the strength envelope. Mohr called it the failure envelope. Coulomb took it as a straight line: τf = c + σ tan φ.
Why the other options are wrong
A: The phreatic line is the top seepage line in an earth dam, not a stress plot. C: 'Parabolic curve' only describes a shape, not this standard term. D: 'Failure line' is not the standard name; the accepted term is strength (failure) envelope.
Shortcut
Stress at failure plotted as σ vs τ gives the strength envelope.
Common mistake
Choosing 'failure line' because the envelope is often a straight line.
Correct answer: B) strength envelope