Standard Penetration test

• Introduction

It is the measurement of resistance to penetration of a sampling spoon ,a cone or other shaped tools under dynamic or static loading.by this test we get a penetration value (N) which helps in finding out the other properties of soil,particularly for cohesive soil.

The properties which we can find -

1. Angle of internal friction.

2. Density index.

3. Unconfined compressive Strength.

4. Consistency of soil.

• Specification of  Test Equipments  

1. A clean hole of  55mm to 150mm in diameter.

2. Split spoon sampler outer dia-50.8mm and inner dia-35mm.

3. A hamer of 63.5 kg.

• Procedure

1. The split tube sampler commonly known as split spoon  sampler,resting on the bottom of the bore hole is allowed to sink under its own weight. 

2. It is then seated 15 cm with the blows of the hammer falling through a height of 15 cm,there after the split spoon is further driven by 30cm .

3. The no of blows required after each 15 cm penetration is recorded.The first 15cm of drive may be considered to be Seating Drive. 

4. The total blows required for the second and tird 15cm of penetration is termed as the Penetration resistance N.   

If the split spoon sampler is driven less than 45 cm (total),then the penetration resistance

shall be for the last 30 cm of penetration.   

• Correction's Required 

After finding out the penetration value ( N ) we need to do two correction

1. Overburden Correction.

2. Dilatancy Correction.

          1.Overburden Correction-

The penetration resistance ( N ) in granular soil influenced by overburden pressure,after correcting the value,N value is given by N'.

           2. Dilatancy Correction-

Silty fine sands & fine sands below the water table developed pore water pressure which is not easily dissipated .The pore pressure increase the resistance to the soil & hence penetration.

So a correction is needed,we have to correct the value of  N' again and after correction N' is given by N''.