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Standard Penetration Test

In this article, we briefly Explain the standard penetration test for measuring the penetration resistance of the sell so read the whole article without skip and share it with friends

Standard Penetration Test

Standard Penetration Test is an in situ test that are particularly useful for cohesionless soil (i.e. sand) because undisturbed sampling of cohesionless soil is difficult.

Standard Penetration Test is used to assess the bearing capacity, shear strength, the angle of internal friction of the soil. It can also be used to determine the unconfined compressive strength of cohesive soils.

  • Standard Penetration Test is conducted in a borehole using a split spoon sampler. The borehole is drilled to the required depth and the sampler is lowered to the bottom of the borehole.
  • The sampler is then driven into the soil by a hammer of 63.5 kg mass falling from a height of 750 mm at the rate of 30 blows per minute.
  • The number of hammer blows required to drive the sampler 150 mm through the ground is counted.
  • The sampler is again driven further by 150 mm and the number of blows is counted.
  • Once again the sampler is driven further 150 mm into the ground and the number of blows is counted.
  • The number of blows for the first 150 mm is discarded.
  • The number of blows for the last two 150 mm is added which gives the standard penetration number (N).
  • Thus standard penetration number is equal to the number of blows required for 300 mm penetration beyond the first 150 mm penetration.
  • If the number of blows for the first 150 mm penetration exceeds 50 then the test results are discarded and the test is stopped.

This standard penetration number (N) is corrected for dilatancy and overburden pressure.

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Correction for Dilatancy

Very fine silty sand and also the fine sand develop pore water pressure which does not get dissipated easily.

This pore water pressure increases the resistance of soil against the hammer blow and hence standard penetration number (N) gets increased which otherwise should be less than this value.

When N > 15, the corrected N value for dilatancy is,

Nc = 15 + 0.5(N - 15)

where N = Recorded N value.

If N ≤ 15 then,

Nc = N

Correction for Overburden Pressure

The overburden pressure on soil affects the penetration resistance of soil, particularly in cohesionless soils.

The confining pressure in cohesionless soil increases with depth and thus the penetration resistance measured at shallow depths gets underestimated and at greater depths, it gets overestimated.

Thus N value obtained in the field under various overburden pressures is corrected to a standard effective overburden pressure.

Thus corrected N value or overburden pressure is given by,

Nc = 350N/(σ + 70)

Where Sigma = Effective overburden pressure (in kN/m2) ≤ 280 kN/m2

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