The observed N-value of an SPT test is 21. The N-value after correcting for dilatancy is:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 23/03/2021 (Evening)
Detailed Explanation
In very fine or silty sand below the water table, rapid driving makes the dense sand try to dilate. Negative pore pressure develops and the soil appears stronger than it is, so the observed N is too high. A dilatancy correction is applied when the observed (or overburden-corrected) N is greater than 15.
\[ N_2 = 15 + \tfrac{1}{2}\,(N_1 - 15) \]
\[ N_2 = 15 + \tfrac{1}{2}\,(21 - 15) = 15 + 3 = 18 \]
Why “18” Is Correct
Applying the correction to N1 = 21 gives a corrected value of 18.
Why the Other Options Are Wrong
- A. 19: 19 does not follow from the correction formula.
- C. 15: 15 would mean the whole excess above 15 is removed; only half is removed.
- D. 21: 21 is the uncorrected value; since it exceeds 15 the correction applies.
Key Point
Dilatancy correction (fine/silty sand below water table, N > 15): N = 15 + ½(N′ − 15).
Key Concepts for Students
- Overburden correction is applied first, then dilatancy.
- If N ≤ 15, no dilatancy correction is needed.
- Corrected N is used to estimate φ and allowable pressure of sands.
Practice more: this is question 3 in Soil Exploration MCQs – Part 1. Try the whole set and check every answer instantly.