In a triaxial test, the intermediate principal stress is:
Try it first: pick an option, then read the explanation.
Correct answer: B. Equal to the confining pressure
Asked in SSC JE 2005
Detailed Explanation
The triaxial specimen is surrounded by water at the cell pressure, which acts equally in both horizontal directions. The vertical direction gets the extra deviator stress.
Why “Equal to the confining pressure” Is Correct
So the major principal stress σ₁ is vertical, and the two horizontal stresses, the intermediate σ₂ and the minor σ₃, are both equal to the confining pressure.
Why the Other Options Are Wrong
- A. Equal to zero: σ₂ is zero only in the unconfined compression test, where the cell pressure itself is zero.
- C. Equal to the deviator stress: The deviator stress is σ₁ − σ₃, an extra vertical stress, not σ₂.
- D. Equal to the deviator stress minus the confining stress: This has no physical meaning in the test.
Key Point
Triaxial compression: σ₁ = σ₃ + σd; σ₂ = σ₃.
Key Concepts for Students
- Mohr–Coulomb ignores σ₂ anyway.
- In UCS tests σ₂ = σ₃ = 0.
- Same as Q309.
Practice more: this is question 50 in Shear Strength of Soils MCQs – Part 4. Try the whole set and check every answer instantly.