Standard Penetration Test (SPT) is an in-situ dynamic penetration test widely used in geotechnical investigation to calculate the relative density and angle of shearing resistance of cohesionless soil. This test also used to determine the strength of stiff cohesive soils.
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Required Apparatus and Tools for Standard Penetration Test:
The following tools are required to perform tests on-site.
- Drop hammer
- Tripod head
- Guiding rod
- Standard split spoon sampler
- Pulley
- Drilling rod
- Heavy-duty helical auger
- Sand bailer
- Manila rope
- Casing pipes and coupling
- Casing clamps
- Measuring tapes
- A straight edge (50 cm)
- Toolbox
Test Procedure of Standard Penetration Test:
- The test utilizes a thick-walled test tube, with an external dia. of 50.8 mm and an inside dia. of 35 mm, and a length of around 650 mm.
- This is crashed into the ground at the lower part of a borehole by blows from a slide hammer with a mass of 63.5 kg (140 lb) falling through a distance of 760 mm (30 in).
- The sample tube is driven 150 mm into the ground and afterward the quantity of blows required for the cylinder to enter each 150 mm (6 in) up to a profundity of 450 mm (18 in) is recorded.
- The amount of the number of blows needed for the second and third 6 crawls of the entrance is named the “standard infiltration obstruction” or the “N-esteem”.
- In situations where 50 blows are deficient to propel it through a 150 mm (6 in) stretch the infiltration after 50 blows are recorded. The blow check gives a sign of the thickness of the ground, and it is utilized in numerous experimental geotechnical designing formulae.
- The SPT values are presented either in terms of the table or in the form of bore log data.
N value in SPT test:
The primary aim of the standard penetration test is to obtain penetration resistance. This penetration resistance is called as N value.
N value is recorded based on the blow count to advance through a 150-millimeter interval of soil.