Question

The standard penetration resistance value obtained in a deep deposit of sand at a depth of 6.0 m was 28. The unit weight of sand is 18.0 kN/m\(^3\). What is the corrected value of number of blows for overburden pressure?

• A. 60
• B. 57
• C. 59
• D. 55

Hint:

- Overburden pressure correction for SPT is calculated using: \[ N_c = N \times \frac{350}{\sigma' + 70} \] - Ensure proper depth and unit weight values are used.

Answer 1

The Correct Option is C

Step 1: Apply the overburden pressure correction formula: \[ N_c = N \times \frac{350}{\sigma' + 70} \] where, \( N = 28 \) (measured SPT value), \( \gamma = 18.0 \) kN/m\(^3\) (unit weight of sand), \( D = 6.0 \) m (depth). Step 2: Calculate overburden pressure \( \sigma' \): \[ \sigma' = \gamma \times D = 18.0 \times 6 = 108 { kN/m}^2 \] Step 3: Substitute values in the correction formula: \[ N_c = 28 \times \frac{350}{108 + 70} \] Step 4: Compute corrected \( N \) value: \[ N_c = 28 \times \frac{350}{178} = 28 \times 1.966 = 59 \] Thus, the corrected number of blows is \( 59 \), making the correct answer (C) 59.