In a standard penetration test, how is the N value determined?

• A. By averaging all increments of hammer blows
• B. By summing the first two increments (0"-6")
• C. By summing the second and third increments (6"-18")
• D. By calculating the total hammer weight

Answer: (C)

In a standard penetration test (SPT), the N value is determined by summing the blows counted during the test, specifically from the second and third increments of penetration, which are typically at depths of 6 to 18 inches. During the SPT, a split-spoon sampler is driven into the ground using a hammer, and the number of blows required to achieve certain increments of penetration is recorded.

The first increment (0 to 6 inches) is often not included in the N value calculation because it can be influenced by the initial resistance of the soil and friction at the top of the borehole. Therefore, by focusing on the second and third increments, the N value provides a clearer indication of the soil's resistance to penetration, minimizing the effects of initial drive conditions. This established method allows for a more standardized presentation of soil strength parameters for geotechnical engineering assessments.

Averaging all increments of hammer blows does not provide a consistent representation of soil behavior because early blows can be significantly affected by initial conditions. Similarly, summing only the first two increments would neglect the crucial data from the subsequent blows that yield more reliable information about soil properties. Lastly, calculating the total hammer weight is not relevant in determining the N value; rather,