The Standard Penetration Test (SPT) is a common in-situ test used in geotechnical engineering to determine the strength and density characteristics of soil. The test involves driving a standard split spoon sampler into the ground using a 63.5 kg hammer falling from a height of 750 mm. The number of blows required to achieve a penetration of 300 mm, after an initial seating drive of 150 mm, is recorded as the N-value.

The N-value provides an indication of the soil's relative density for cohesionless soils (like sand) and consistency for cohesive soils (like clay). For sandy soils, a higher N-value generally indicates a denser soil, while a lower N-value suggests a looser soil.

N-Value Ranges for Sandy Soils

The N-value is crucial for classifying sandy soils based on their relative density. Different ranges of N-values correspond to different densities of sandy soil. Here’s a typical classification:

From the table, it is clear that for a dense sandy soil, the N-value typically falls within the range of 31 – 50 blows per 300mm. This indicates a soil that is compact and offers significant resistance to penetration during the Standard Penetration Test.

Standard Penetration Test Significance

The N-value obtained from the Standard Penetration Test (SPT) is widely used for various geotechnical design purposes, including:

• Estimating the bearing capacity of shallow and deep foundations.
• Predicting settlement of structures.
• Assessing the liquefaction potential of sandy soils during earthquakes.
• Correlating with other soil properties like angle of internal friction for sands.

It's important to note that the N-value might need corrections for overburden pressure and dilatancy, especially when dealing with fine sands below the water table, to get a corrected N-value (N'). However, for the general classification of dense sandy soil, the raw N-value range of 31-50 is standard.