Friday, October 26, 2012

Standard Penetration Test - SITE EXPLORATION.

The test (IS : 2131—1963) is performed in a clean hole, 55 to 150 mm in diameter. A casing or drilling mud may be used to support the sides of the hole. A thick wall split tube sampler, 50.8 mm outer dia. and 35 mm internal dia. is driven into the undisturbed soil at the bottom of the hole under the blows of 65 kg drive weight with 75 cm free fall: The minimum open Iength of the sampler should be 60 cm. The sampler is first driven through 15 cm as a seating drive. It is further driven through 30 cm and the number of blows required for this are counted. This number of blows is termed as penetrarion resistance N.

In very fine, or sitly, saturated sand, an apparent increase in resistance occurs. Terzaghi and Peck have recommended the use an equivalent penetration resistance Ne, in place of the actually observed value of N, when N is greater than 15. Ne is given by the following relation:

Terzaghi and Peck’s empirical charts for determining nct bearing pressure qp for footing on sand depend on B and N value, to limit maximum settlement of individual footing to 2.5 cm and differential settlement of 2 cm, assuming that a differential settlement of 2 cm can be tolerated by most of the ordinary structures. The empirical relations are represented by the following equation:

The standard penetration test is very useful for the design of rafts. The safe bearing value for rafts may be taken as smaller of the values of q1 and q2 given bellow:

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