According to the Mohr–Coulomb failure criterion in soils, which of the following statements are INCORRECT? A. Shear strength is attributable to the interlocking of soil particles and cohesion. B. Shear strength is only attributable to the internal friction between particles. C. The Y-intercept represents friction of soils. D. The Mohr–Coulomb failure envelope is a straight line.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 07/06/2024 (Morning)
Detailed Explanation
In the Mohr–Coulomb equation τ = c + σ tan φ, each term has a clear physical meaning. On a τ–σ graph, the envelope crosses the τ-axis (the y-axis) at height c and rises at an angle φ.
Why “B and C” Is Correct
A is correct: shear strength comes from cohesion (bonding between fine particles) and from friction and interlocking between grains.
B is incorrect: friction is only part of the strength; clays also have cohesion.
C is incorrect: the y-intercept is the cohesion c. Friction appears as the slope φ of the line.
D is correct within the Mohr–Coulomb criterion, which models the envelope as a straight line. So B and C are the incorrect statements.
Why the Other Options Are Wrong
- A. B and D: D is correct in the Mohr–Coulomb model; the envelope is taken as a straight line.
- C. A and C: A is correct: interlocking/friction and cohesion together give shear strength.
- D. A and D: Both A and D are correct statements, so they are not the incorrect pair.
Key Point
Intercept = c (cohesion). Slope angle = φ (friction). τ = c + σ tan φ.
Key Concepts for Students
- Sand: c ≈ 0, strength from friction and interlocking.
- Saturated clay, undrained: φ ≈ 0, strength = cu.
- Dense sand also gains strength from dilation (interlocking).
Practice more: this is question 2 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.