The static cone penetration test and the standard penetration test are performed on a soil at a certain depth. The value of static cone resistance is 8 MPa and the N value is 20. The soil met at that depth is:
Try it first: pick an option, then read the explanation.
Asked in SSC JE (previous year paper)
Detailed Explanation
The ratio of cone resistance qc (in kg/cm²) to SPT N value depends mainly on the soil's grain size, so it can be used to identify the soil. Typical values:
\[ q_c = 8\ \text{MPa} = 8\ \text{N/mm}^2 \approx 80\ \text{kg/cm}^2 \qquad \frac{q_c}{N} = \frac{80}{20} = 4 \]
Why “medium dense sand” Is Correct
A ratio of 4 lies between the values for clean fine-to-medium sands (3.5) and coarse sands (5), so the soil is a sand. N = 20 (between 10 and 30) indicates medium density. Hence medium dense sand.
| Soil type | qc/N (qc in kg/cm²) |
|---|---|
| Silts, sandy silts, slightly cohesive silt–sand mixtures | 2 |
| Clean fine to medium sands, slightly silty sands | 3.5 |
| Coarse sands and sands with a little gravel | 5 |
| Sandy gravel and gravel | 6 |
SPT density classification for sands: N 4–10 loose, 10–30 medium dense, 30–50 dense, > 50 very dense.
Why the Other Options Are Wrong
- A. sandy silt: Sandy silt gives qc/N ≈ 2, much lower than 4.
- B. clay silt mixture: Clay–silt mixtures have even lower ratios.
- C. sand and gravel mixture: Sand–gravel mixtures give qc/N ≈ 6.
Key Point
qc/N: silt ≈ 2, sand ≈ 3.5–5, gravel ≈ 6. N 10–30 = medium dense.
Key Concepts for Students
- 1 MPa ≈ 10 kg/cm².
- CPT gives a continuous profile; SPT gives a value every 1.5 m or so.
- Correlations like these are approximate and site-specific.
Practice more: this is question 55 in Shear Strength of Soils MCQs – Part 4. Try the whole set and check every answer instantly.