Select the correct option with regard to the following statements (S1, S2) pertaining to consolidation in soils. S1: A soil is termed as normally consolidated if the present effective overburden pressure is less than the maximum to which the soil was ever subjected in the past. S2: The larger the value of the coefficient of consolidation, the shorter it takes for full consolidation to occur.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 23/03/2021 (Morning)
Detailed Explanation
The stress history of a clay is described by comparing its present effective overburden pressure σ′0 with the maximum effective stress it has ever carried, the preconsolidation pressure σ′c.
- Normally consolidated (NC): σ′0 = σ′c, so OCR = 1.
- Overconsolidated (OC): σ′0 < σ′c, so OCR > 1 (for example, after erosion of overlying soil or melting of an ice sheet).
\[ \text{OCR} = \frac{\sigma'_c}{\sigma'_0} \]
Time for a given degree of consolidation:
\[ T_v = \frac{c_v\,t}{d^2} \quad\Rightarrow\quad t = \frac{T_v\,d^2}{c_v} \]
Why “S1 is false and S2 is true” Is Correct
S1 is false: a present stress lower than the past maximum is the definition of an overconsolidated soil, not a normally consolidated one.
S2 is true: for the same layer and the same degree of consolidation (same Tv and d), time is inversely proportional to cv. A larger coefficient of consolidation means the soil drains faster and consolidation finishes sooner.
Why the Other Options Are Wrong
- A. S1 is true and S2 is false: S1 is the definition of an overconsolidated soil, so it cannot be true for a normally consolidated soil.
- B. Both S1 and S2 are false: S2 is correct: t is inversely proportional to cv.
- C. Both S1 and S2 are true: S1 is false, so ‘both true’ is wrong.
Key Point
NC: present stress = past maximum (OCR = 1). OC: present stress < past maximum (OCR > 1). Higher cv → faster consolidation.
Key Concepts for Students
- The preconsolidation pressure is found from the e – log σ′ curve by Casagrande's construction.
- Overconsolidated clays are much less compressible under loads below σ′c.
- cv = k / (mv·γw): higher permeability gives higher cv and faster consolidation.
Practice more: this is question 7 in Soil Consolidation MCQs – Part 1. Try the whole set and check every answer instantly.