Understanding SPT-N Values and Soil Correlations

The Standard Penetration Test (SPT) is a common in-situ test used in geotechnical exploration to determine the subsurface profile and estimate soil engineering properties. The result of the test is typically expressed as the SPT-N value, which is the number of blows required to drive the sampler the final 6 inches (150 mm) of a 12-inch (300 mm) penetration.

SPT-N values are empirical and are commonly correlated with various soil properties, particularly for granular soils (sands and silts). These correlations are widely used in foundation design.

Correlation of SPT-N Values with Soil Properties

Let's examine how SPT-N values correlate with the properties listed in the options:

• Angle of Internal Friction ($\phi$): For cohesionless soils like sands, there are well-established empirical correlations between the SPT-N value and the angle of internal friction ($\phi$). Higher N-values generally indicate denser sand with a higher angle of internal friction. Examples include correlations proposed by Peck, Hanson, and Thornburn or Meyerhof.
• Relative Density ($D_r$): For cohesionless soils, the SPT-N value is a good indicator of relative density ($D_r$). Denser sands offer higher resistance to penetration, resulting in higher N-values. There are numerous empirical correlations between SPT-N and relative density.
• Unit Weight ($\gamma$): Unit weight is a fundamental soil property. While soil density influences the SPT-N value, the N-value itself is not typically used in standard empirical correlations to directly estimate the unit weight of the soil. Unit weight is often determined through laboratory tests on soil samples or estimated based on soil type and moisture content using other methods.
• Sensitivity ($S_t$): Sensitivity is a property specific to cohesive soils (clays). It is defined as the ratio of the undisturbed shear strength to the remolded shear strength ($S_t = \tau_{undisturbed} / \tau_{remolded}$). While SPT can be performed in cohesive soils and N-values can be empirically correlated with undrained shear strength or unconfined compressive strength ($q_u$) for undisturbed clays, there is no standard, widely accepted direct correlation between SPT-N values and the sensitivity ($S_t$) of clay. Determining sensitivity requires knowledge of both undisturbed and remolded strength, and the SPT N-value primarily reflects the undisturbed condition.

Identifying the INCORRECT Correlation

Based on common geotechnical engineering practice and the established empirical relationships, SPT-N values are reliably correlated with relative density and angle of internal friction for cohesionless soils. They are also correlated with the strength of cohesive soils (like unconfined compressive strength).

Among the given options, sensitivity is a property for which standard, direct empirical correlations with SPT-N values are not commonly used or well-established compared to relative density or angle of internal friction. While SPT can be used in clays, its correlation is typically with strength, not the sensitivity ratio itself.

Similarly, unit weight is not typically estimated directly from SPT-N values using standard correlations.

However, comparing sensitivity and unit weight as less directly correlated properties, sensitivity (a property of cohesive soils related to the ratio of strengths) is often cited as a property not reliably estimated from the granular-soil-centric SPT test via direct correlation, unlike the correlations for relative density and angle of internal friction which are fundamental uses of the SPT in sands.

Therefore, the property with which SPT-N values are INCORRECTLY correlated, or least directly and reliably correlated using standard empirical methods among the given options, is sensitivity.