What is the primary measurement obtained from the Standard Penetration Test (SPT)?

• A. Penetration resistance value, N
• B. Soil density
• C. Elastic modulus
• D. Shear strength

Answer: (A)

The primary measurement obtained from the Standard Penetration Test (SPT) is the penetration resistance value, commonly referred to as the "N value." During the SPT, a sampler is driven into the soil at the bottom of a borehole, and the number of blows required to drive the sampler for a specified distance (usually 12 inches) is recorded. This N value provides a direct indication of soil behavior and strength characteristics.

The N value is crucial for geotechnical engineers as it relates to the in-situ density of granular soils and gives insight into their relative strength. Higher N values typically indicate denser or stronger soil conditions, while lower values suggest weaker or looser soils. Therefore, this measurement is foundational for evaluating site conditions, determining bearing capacity, and making informed decisions on foundation design and other engineering assessments.

The other choices involve parameters that can be influenced or estimated using the SPT, but they are not the primary outcome of the test itself. Soil density may correlate with the N value but is not measured directly by the SPT. The elastic modulus and shear strength can also be derived from empirical correlations with the N value but are not the direct result of the SPT procedure. This makes the penetration resistance value the central focus