Standard Penetration - GATE Civil Foundation Engineering Free MCQ Test

According to Indian standard, the two commonly used penetration tests are static cone penetration test and standard penetration test.

Standard Penetration Test (SPT):

• It is specially used for cohesionless soils. It is used to determine 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 standard penetration number is equal to the number of blows required for 300 mm of penetration beyond a seating drive of 150 mm.
• If the number of blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued.
• Hence the standard penetration test is useful to measure the angle of shearing resistance of cohesionless soils, i.e Shear strength of sands.

Important Points
Some of the other methods for in-situ site exploration are

• Pressure Meter Test (PMT)
• Static Cone Penetration Test (SCPT)
• Vane Shear Test (VST)

The penetration test are useful for general exploration of erratic soil profiles, for finding depth to bed rock or hard stratum, and to have an approximation indication of the strength and other properties of soils.

For a constant density index, the N value increase with increasing effective overburden pressure for which correction have been proposed by Gibbs and Holtz, peck, Thornburn, Whitman and others.

In standard penetration test, the first 15 cm of drive may be considered to be a seating drive. The total blows required for the second and third 15 cm of penetration is termed as the penetration resistance N.

The cone test is considered very useful in determining the bearing capacity of pits in cohesion less soils, particularly in fine sands of varying density.

The N-value for cohesion less soil shall be corrected for overburden and the values N0 obtained after overburden correction is corrected further for dilatancy.

The split tube sampler, used in a test for standard penetration is commonly known as split spoon sampler resting on the bottom of the bore hole which is allowed to sink under its own weight.

In 1986, Lio and Whitman gave the following expression for
Normalizing factor C n:
C n = √ (100/σ’).

For silts, sandy silts, slightly cohesive silt sand mixture type of soil, the cone resistance q c is 2.