What is the N-value in the Standard Penetration Test?

The N-value in the Standard Penetration Test (SPT) specifically refers to the number of blows required to drive the sampler a certain distance into the soil, typically the last 12 inches of a 30-inch penetration. This value is crucial because it serves as an empirical measure of soil resistance and provides insight into soil strength and density. A higher N-value generally indicates a denser, stronger soil, while a lower N-value suggests looser, weaker soil. The N-value is instrumental in geotechnical engineering for estimating soil properties, making it a fundamental component of subsurface investigations.

The other options, while related to the characteristics of soil, do not define the N-value specifically. The measure of soil strength, although linked with the N-value, is more of an interpretation rather than a direct definition. The depth of penetration itself is not what the N-value measures; instead, it's about the effort required to achieve that depth. The soil's water content does influence soil behavior but is not represented by the N-value in the context of the SPT. Understanding the N-value is key for engineers when assessing soil for foundation design and other civil engineering applications.