Standard penetration resistance in very stiff clays lies between:
Try it first: pick an option, then read the explanation.
Correct answer: D. 15 and 30
Asked in SSC JE 2010
Detailed Explanation
The SPT N-value gives a rough idea of the consistency of a clay. Terzaghi and Peck's table links N to consistency and unconfined compressive strength.
Why “15 and 30” Is Correct
From the table, very stiff clay has N between 15 and 30 (qu about 200–400 kN/m²). 2–4 is soft, 4–8 medium and 8–15 stiff clay.
| SPT N-value | Consistency of clay | Unconfined strength qu (kN/m²) |
|---|---|---|
| < 2 | Very soft | < 25 |
| 2 – 4 | Soft | 25 – 50 |
| 4 – 8 | Medium | 50 – 100 |
| 8 – 15 | Stiff | 100 – 200 |
| 15 – 30 | Very stiff | 200 – 400 |
| > 30 | Hard | > 400 |
Terzaghi and Peck's correlation. It is rough for clays; SPT is more reliable in sands.
Why the Other Options Are Wrong
- A. 2 and 4: 2 to 4 corresponds to soft clay.
- B. 4 and 8: 4 to 8 corresponds to medium clay.
- C. 8 and 15: 8 to 15 corresponds to stiff clay.
Key Point
Clay N: soft 2–4, medium 4–8, stiff 8–15, very stiff 15–30, hard > 30.
Key Concepts for Students
- For clays, vane shear or unconfined compression tests are more reliable than SPT.
- q_u (kN/m²) ≈ 12.5 N is a rough correlation for clays.
- The first 150 mm of driving is not counted (seating drive).
Practice more: this is question 4 in Soil Exploration MCQs – Part 1. Try the whole set and check every answer instantly.