“SPT” is an abbreviation for the Standard Penetration Test, which is the most widely used in-situ soil characterization test world widely.
The SPT test involves driving a standard cylindrical sampler into the bottom of a borehole.
The total blows required from a hammer, over the interval 150 to 450 mm (6 to 18 inches) are summed to give the blow count N, in blows per foot.
The N-value is used as a basis for foundation design and as the primary index of liquefaction resistance.