The value of compression index for a remoulded sample whose liquid limit is 50% is:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 2015
Detailed Explanation
Remoulding a clay (kneading it so its natural structure is destroyed) changes its compressibility. Skempton proposed a correlation for remoulded clays which has a smaller coefficient than the Terzaghi–Peck relation for undisturbed clays.
\[ C_c = 0.007\,(w_L - 10) = 0.007 \times (50 - 10) = 0.007 \times 40 = 0.28 \]
Why “0.28” Is Correct
The sample is remoulded, so the 0.007 coefficient applies, giving 0.28. Note that the liquid limit is used as a percentage (50), not as a fraction.
| Soil condition | Empirical relation for Cc | wL = 50% |
|---|---|---|
| Undisturbed (normally consolidated) clay — Terzaghi & Peck | Cc = 0.009 (wL − 10) | 0.36 |
| Remoulded clay — Skempton | Cc = 0.007 (wL − 10) | 0.28 |
wL is the liquid limit in percent. The remoulded value is lower because remoulding destroys the natural structure of the clay.
Why the Other Options Are Wrong
- A. 0.028: 0.028 is ten times too small; it results from a decimal slip.
- C. 0.36: 0.36 is the value for an undisturbed clay (coefficient 0.009), not a remoulded one.
- D. 0.036: 0.036 is a decimal slip on the undisturbed value.
Key Point
Remoulded: 0.007(wL − 10). Undisturbed: 0.009(wL − 10). The 0.009 one gives the bigger answer.
Key Concepts for Students
- The two formulas give values in the ratio 9 : 7 for the same liquid limit.
- Sample disturbance in the lab makes the e – log σ′ curve flatter and hides the preconsolidation pressure.
- Correlations are for preliminary design only; use oedometer results for final settlement calculations.
Practice more: this is question 15 in Soil Consolidation MCQs – Part 1. Try the whole set and check every answer instantly.