Soil borings completed for a subsurface investigation...

Soil borings completed for a subsurface investigation indicate the site is underlain by a deporit of sasdy soils. At one boring location, the blow count logged for the standard penetration test, $\mathrm{N}_{60}$ while sampling at a depth of $7 \mathrm{~m}$ was 24 blows. The sand has a unit weight of $16.5 \mathrm{kN} / \mathrm{m}^3$. Using criteria that relates SPT results and the soil angle of intermal friction, estimate the value of $\phi$ for the sand at the sample depth.

Key Concepts

Standard Penetration Test (SPT)

The Standard Penetration Test is a widely used in-situ testing method in geotechnical engineering that involves driving a sampler into the soil and recording the number of blows required to advance it a specified distance. This test provides an indirect measure of the soil’s density and strength characteristics, aiding in the classification of soil types and supporting foundation design.

Blow Count (N-value)

The blow count, or N-value, is the number of hammer blows recorded during a standard penetration test to achieve a given penetration increment in the soil. It serves as a practical indicator of the soil resistance and relative density, and empirical relationships are often used to correlate this value with soil strength parameters.

Soil Angle of Internal Friction (phi)

The soil angle of internal friction, denoted as phi, is a fundamental parameter in soil mechanics representing the shear resistance of granular materials. It reflects the ability of the soil to resist sliding and plays a critical role in assessing the stability of slopes, earth structures, and foundations.

Empirical Correlations in Geotechnical Engineering

Empirical correlations are relationships developed from field and laboratory data that link easily measured parameters, such as SPT blow counts, to more directly needed soil properties like the angle of internal friction. These correlations provide practical estimates and are valuable tools in situations where direct measurement of soil strength parameters is difficult.

Soil Unit Weight

The unit weight of soil is a measure of its density, expressed as weight per unit volume, and is a key factor in calculating in-situ stresses and assessing the overall stability of soil layers. It influences many aspects of geotechnical design, including the evaluation of soil bearing capacity and the estimation of settlement behavior under load.

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