The following results were obtained from a consolidated-undrained test on a normally consolidated clay. The principal effective stresses of the sample in kN/m² are Soil: Clay | Cell pressure: 250 | Deviator stress: 152 | Pore water pressure: 120
- Aσ1 = 370, σ3 = 152
- Bσ1 = 402, σ3 = 120
- Cσ1 = 250, σ3 = 120
- Dσ1 = 282, σ3 = 130Correct
Solution
Concept
Effective principal stresses
Formula
σ3′ = σ3 − u; σ1′ = σ3′ + σd
Step-by-step solution
1. σ3′ = 250 − 120 = 130 kN/m². 2. σ1′ = 130 + 152 = 282 kN/m².
Explanation
Effective stress = total − pore pressure.
Why the other options are wrong
Other options do not subtract the pore pressure correctly.
Shortcut
σ3′ = cell − u; σ1′ = σ3′ + σd.
Common mistake
Forgetting to subtract u.
Correct answer: D) σ1 = 282, σ3 = 130