Understanding SPT Blow Count and Sand Density

The Standard Penetration Test (SPT) is a common in-situ test used in geotechnical engineering to determine the dynamic resistance of the soil or the penetration resistance of the ground at the sampling depth. It provides valuable information about the soil's characteristics, particularly its relative density in granular soils like sand and silt, and its consistency in cohesive soils like clay.

The result of the SPT test is typically expressed as the 'N' value, which is the number of blows required by a standard hammer to drive the split-spoon sampler the final 12 inches (30 cm) of penetration. The initial 6 inches (15 cm) of penetration is considered seating drive.

SPT N Value Correlation with Sand Density

For sandy soils, the SPT N value is directly related to the relative density. Higher N values indicate denser sand. There are established correlations that classify the density state of sand based on the measured N value. These correlations can vary slightly depending on the source or local practice, but a widely used classification is as follows:

Analyzing the Given Condition: N > 50 Blows

The question asks to identify the state of sand if the number of blows in the SPT test is more than 50. According to the correlation table above:

• If the N value is greater than (>) 50, the density state of the sand is classified as Very Dense.

Therefore, based on the Standard Penetration Test (SPT) results showing more than 50 blows, the sand is considered to be in a very dense state.

Revision Table: Key SPT Concepts for Sand Density

Additional Information: Factors Affecting SPT and Sand Density

It is important to note that the SPT N value can be affected by several factors, including the overburden pressure, the type of hammer used, borehole diameter, and the presence of fines in the sand. Corrections are often applied to the measured N value (N_m) to obtain a corrected N value (N₁₍₆₀₎ or similar) that is more representative of the soil's intrinsic properties and comparable across different sites.

• Overburden Pressure Correction: Applied because N value increases with depth due to higher confining pressure.
• Energy Correction: Applied to account for variations in the energy delivered by different hammer types and systems.
• Fines Correction: Sometimes applied for silty sands, as the presence of fines can increase resistance.

The correlation between SPT N value and density state is a useful guide, but a complete geotechnical investigation involves considering multiple test results and site-specific conditions.