What is a standard penetration test?
​The SPT is a common sampling and testing method used for subsurface explorations extending through soil and relatively low-strength rock. Perform the SPT in accordance with ASTM D1586.

The standard penetration test is an in-situ test that is coming under the category of penetrometer tests. The standard penetration tests are carried out in borehole. The test will measure the resistance of the soil strata to the penetration undergone. A penetration empirical correlation is derived between the soil properties and the penetration resistance.
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The test is extremely useful for determining the relative density and the angle of shearing resistance of cohesionless soils. It can also be used to determine the unconfined compressive strength of cohesive soils.
The advantages of standard penetration test are that the test is simple and economical, and the test provides representative samples for visual inspection, classification tests and for moisture content. Actual soil behavior is obtained through SPT values. The method helps to penetrate dense layers and fills. Test can be applied for variety of soil conditions.​

How is a standard penetration test performed?
​Tools requirements to conduct SPT are Standard Split Spoon Sampler, Drop Hammer weighing 63.5kg, Guiding rod, and a Drilling Rig Driving head (anvil).
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The split-spoon sampler used in the SPT is driven into the soil at the bottom of the borehole with a 140-lb hammer falling 30 inches. Unless refusal conditions are encountered, the sampler is driven a minimum of 18 inches into the soil. The number of blows to cause each 6 inches of penetration should be recorded on a boring log. If refusal conditions are encountered (i.e. more than 50 blows to cause 6 inches or less of penetration), the number of blows and corresponding penetration (in inches) should be recorded on the log and the test can be terminated, The number of blows to cause the last 12 inches of penetration (termed the standard penetration resistance or N-value) should be determined by adding the number of blows from the 2nd and 3rd 6-inch increment and recorded on the Geological Boring Log. The number of blows for the first increment should be ignored.