Tuesday, 31 May 2016

Standard Penetration Test (SPT)


One of the most common in-situ tests is the standard penetration test or SPT. This test which was originally developed in the late 1920s.

SPT is most commonly used in situ test, especially for cohesionless soils which cannot be easily samples. the test is extremely useful for determining the relative density and the angle of shearing resistance of  for cohesionless soils.

it can also used to determine the unconfined compressive strength of cohesive soils.

The standard penetration test is conducted in a borehole using a standard split-spoon sampler. 

When the borehole (55 to 150 mm in dia) has been drilled to the desired depth, the drilling tools are removed and the split-spoon sampler, attached to standard drill rods of required length is lowered to the bottom of the borehole and rested at the bottom.

The split-spoon sampler is then driven into the soil for a distance of 450 mm in three stages of 150 mm each by blows of a drop hammer of 63.5 kg mass falling vertically and freely through a height of 750 mm at the rate of 30 blows per minute (IS 2131 – 1981). The number of blows required to penetrate every 150-mm is recorded while driving the sampler. If full penetration is obtained, the blows for the first 150 mm is retained for reference purposes, but not used to compute the SPT value because the bottom of the boring is likely to be disturbed by the drilling process and may be covered with loose soil that may fall from the sides of the boring. The number of blows required for the next 300 mm of penetration is recorded as the SPT value. The number of blows is designated as the “Standard Penetration Value” or “Number” N.

The slit-spoon sampler is then withdrawn and is detached from the drill rods. The split barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split-barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory. 

Precautions

The drill rods should be of standard specification and should not be in bent condition

The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear

 The drop hammer must be of right weight and the fall should be free, frictionless and vertical.

 The height of fall must be exactly 750 mm. Any change in this will seriously affect the N value.

 

The standard penetration number is corrected for dilatancy correction and overburdon correction.

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