Standard Penetration Test

Explain standard penetration test of measuring the penetration resistance of the soil.

Standard Penetration Test

 It is in-situ that is particularly useful for cohesion less soil (i.e. sand) because undisturbed sampling of cohesion less soil is difficult. This 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.

•  This test is conducted in a bore hole using a split spoon sampler. The bore hole is drilled to the required depth and sampler is lowered to the bottom of the bore hole.

•  The sampler is then driven into soil by a hammer of 63.5 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 number of blows is counted.

•  The number of blows for the first 15l0 mm is discarded.

•  The number of blows for the last two 150 mm are 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.

Correction of 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 sol against the hammer blow and hence standard penetration number (N) gets increases 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 cohesion less soils. The confining pressure in cohesion less soil increases with depth and thus he 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 pressure is given by, to a standard effective overburden pressure. Thus corrected N Value for overburden pressure is given by, to a standard effective overburden pressure. Thus corrected N value for overburden pressure is given by,

Nc = 350N (𝜎𝑜+70)

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