What is the Standard Penetration Test (SPT) and what does it measure?

SPT is an in-situ test performed during borehole drilling. A split-spoon sampler is driven 450mm into soil using a 63.5kg hammer falling 760mm. The N-value is blows required for last 300mm penetration. SPT provides: disturbed soil samples, relative density of granular soils (loose: N<10, dense: N>30), consistency of cohesive soils, and empirical correlations for bearing capacity and settlement. It's the most widely used field test due to simplicity and extensive experience.