Which one of the following statement is valid for arriving at standardized value of (N) in a standard penetration test?
- AFor a constant density index, the (N) value increases with an increase in overburden pressureCorrect
- BDensity index decreases with depth
- CDensity index increases with depth
- DAngle of shearing resistance decreases with depth
Solution
Concept
Overburden correction of the SPT N-value
Formula
Ncorrected = Nobserved × CN, where CN depends on the effective overburden pressure
Step-by-step solution
1. For the same density (relative density), the SPT N-value is higher at greater overburden pressure. 2. So the observed N must be corrected to a standard overburden to compare soils at different depths.
Explanation
The SPT resistance of a sand depends on both the relative density and the effective overburden pressure. At constant relative density, N rises with the overburden pressure, so a correction is applied to give a standardised value.
Shortcut
Same density, deeper sand → higher N. Hence a correction factor CN is used.
Common mistake
Assuming that a higher N always means a denser sand, regardless of depth.
Correct answer: A) For a constant density index, the (N) value increases with an increase in overburden pressure