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Standard Penetration Test

Published: December 20, 2018 | Last updated: July 5, 2023

What Does Standard Penetration Test Mean?

A standard penetration test (SPT) is carried out in soils where it isn't possible to bring out an undisturbed sample, such as in weak rock, silt, clay, sand and gravel. The test provides an indication of the relative density of the granular deposit and its approximate shear strength. A standard penetration test can also be used for cohesive soils to determine its unconfined compressive strength. The procedure is simple and is inexpensive, making it a widespread test method for geotechnical subsurface soil investigation.

Trenchless technology utilizes methods such as horizontal directional drilling (HDD) and pipe ramming to install pipes below ground surface and hence these tests are an integral part of soil investigation prior to carrying out the trenchless procedure.

Trenchlesspedia Explains Standard Penetration Test

An SPT is carried out on site and is done at the bottom of a borehole of desired depth. A thick tube is lowered into the borehole and a slide hammer is used to drive the tube into the ground. The slide hammer is of standard weight and is suspended at a standard falling distance.

The tube is allowed to penetrate the first 150 mm and thereafter the number of blows required to penetrate further 150 mm until it reaches a recorded depth of 450 mm. The sum of the blows required to penetrate the second and third 150 mm penetration is known as the SPT resistance or the N value.

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