Advantages of standard penetration test
The standard penetration test is mostly utilized in-situ test. It is ideal for cohesionless soils as it can not be sampled smoothly. The test is conducted to find out the pertinent density as well as the angle of shearing resistance of cohesionless soils. It is also useful to find out the unrestrained compressive strength of cohesive soils.
A standard split spoon sampler is applied to conduct the standard penetration test in a bore hole. When the bore hole is drilled to the required depth, the drilling tools are eliminated and the sampler is reduced to the bottom of the hole. With the help of a drop hammer having 63.5 kg mass, the sampler is driven into the soil. The hammer is dropped through a height of 750mm at the rate of 30 blows per minute (IS – 2131: 1963).
The number of hammer blows necessary to push 150mm of the sample is counted. The sampler is again pushed by 150 mm and the number of blows recorded. Similarly, the sampler is again pushed by 150mm and the records are maintained for number of blows. The number of blows recorded for the first 150mm is ignored.
The number of blows recorded for last two 150mm intervals are taken into consideration to provide the standard penetration number (N). Alternatively, the standard penetration number is equivalent to the number of blows necessary for 150mm penetration apart seating drive of 150mm.
If the number of blows for 150mm drive surpasses 50, it is treated as denial and the test is suspended. The standard penetration number is rectified for dilatancy amendment and overburden amendment.
(a) Dilatancy Amendment: silty fine sands and fine sands underneath the water table form pore water pressure that can’t be easily restrained. The pore pressure raises the durability of the soil as well as the penetration number (N).
Terzaghi and Peck (1967) suggested the following rectification in the case of silty fine sands if the determined value is N and surpasses 15.
The rectified penetration number
Where NR defines the recorded value and NC is the rectified value.
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