In a triaxial test, the cell pressure is increased from 0.1 N/m² to 0.26 N/m² and the pore water pressure from 0.07 N/m² to 0.15 N/m². Skempton's pore pressure parameter is given by:
Try it first: pick an option, then read the explanation.
Correct answer: C. 0.5
Asked in SSC JE 23/01/2018 (Morning)
Detailed Explanation
Skempton's pore pressure parameter B tells how much of an all-round (cell) pressure increase is taken by the pore water:
\[ B = \frac{\Delta u}{\Delta\sigma_3} \]
\(\Delta\sigma_3 = 0.26 - 0.10 = 0.16\), \(\Delta u = 0.15 - 0.07 = 0.08\)
\[ B = \frac{0.08}{0.16} = 0.5 \]
Why “0.5” Is Correct
Half of the increase in cell pressure appeared as pore pressure, so B = 0.5. That indicates a partially saturated specimen; a fully saturated soil gives B ≈ 1.
Why the Other Options Are Wrong
- A. −3: B cannot be negative for an increase in cell pressure; −3 has the ratio inverted and the sign wrong.
- B. −0.5: The sign is wrong: both pressures increased.
- D. 2: 2 is the inverse ratio (Δσ₃/Δu).
Key Point
B = Δu/Δσ₃. Saturated: B ≈ 1. Dry: B = 0.
Key Concepts for Students
- The A parameter relates pore pressure to the deviator stress: Δu = B[Δσ₃ + A(Δσ₁ − Δσ₃)].
- Labs check saturation with a ‘B-check’ before shearing (B ≥ 0.95).
- A is positive for NC clays and can be negative for heavily OC clays.
Practice more: this is question 22 in Shear Strength of Soils MCQs – Part 2. Try the whole set and check every answer instantly.