Identify the state of sand if the number of blows in the SPT test is more than 50.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 15/11/2022 (Shift 3)
Detailed Explanation
In sands, the SPT N-value is a good indicator of density. Higher N means the sand is more tightly packed, which also means a higher relative density and angle of internal friction.
Why “Very dense” Is Correct
Using the standard correlation (Terzaghi and Peck), an N-value above 50 indicates very dense sand, with relative density above about 85% and a friction angle above about 41°.
| SPT N (blows / 300 mm) | State of sand | Relative density (%) | φ (approx.) |
|---|---|---|---|
| 0 – 4 | Very loose | 0 – 15 | < 28° |
| 4 – 10 | Loose | 15 – 35 | 28° – 30° |
| 10 – 30 | Medium dense | 35 – 65 | 30° – 36° |
| 30 – 50 | Dense | 65 – 85 | 36° – 41° |
| > 50 | Very dense | > 85 | > 41° |
Typical correlation (Terzaghi and Peck; Meyerhof). Exact boundaries vary slightly between books.
Why the Other Options Are Wrong
- A. Loose: Loose sand has N of about 4 to 10.
- B. Medium dense: Medium dense sand has N of about 10 to 30.
- C. Dense: Dense sand has N of about 30 to 50.
Key Point
Sand N: very loose < 4, loose 4–10, medium 10–30, dense 30–50, very dense > 50.
Key Concepts for Students
- N-values in sand are corrected for overburden before using such tables.
- Very dense sands show general shear failure under footings.
- Relative density I_D = (e_max − e)/(e_max − e_min) × 100.
Practice more: this is question 1 in Miscellaneous on Soil Mechanics MCQs – Part 1. Try the whole set and check every answer instantly.