Standard Penetration Test

Standard Penetration Test: Standard Penetration Test can be used to determine-

1.  Angle of shearing resistance.

2.  Relative density or density index.

3. Allowable bearing pressure on the basis of settlement criteria.

4. Bearing capacity on the basis of shear criteria.

5. Unconfined compressive strength of clays.

6. Ultimate pile load capacity.

Apparatus of Standard Penetration Test-

1. Drilling Rig

2. Split Spoon

3. Drop Hammer (65 kg)

4. Driving Head (anvil)

5. Guiding Rod

6. Tripod

7. Extension Rod

Procedure of Standard Penetration Test-

1.  Erect the tripod over the test hole and assemble the unit.

2. Allow the spoon to rest on the bottom of the hole.

3. Drive the spoon with blows from the hammer falling from 75cm until 100 blows.

4. Count number of blows required to effect each 5 inches of penetration.

5. The number of blows required for second and third 6 inches of drive added is recorded as penetration value N.

The split spoon sampler is driver by dynamic mechanism of hammer. Sampler is placed over soil on a bore hole. The test is conducted either at every 2 to 5 m interval or at the change of stratum.The weight of hammer is 65 kg and height of fall of hammer is 75 cm.

The observed vales of N is subjected to following 2 corrections applied in sequence.

1.      Overburden pressure correction- Let N number is observed value of SPT.

Let N₁ is corrected value for overburden pressure.

N₁ = Nx (350/σ+70)

Where σ = effective overburden pressure at the test level in kN/m²

2.      Water table- If water table is present at or above the test level then water table correction is required. If water table below test level then it is not required. The pore water pressure is developed due to presence of water table and in fine soils during dynamic actions of hammer. The corrected value for overburden pressure is further corrected for presence of water table.

Let N₂ is corrected value for water table then it is given by

  N₂  = 15 + (1/2)x (N₁ -15)

When N₁<15 then water table is not required.

The test is conducted at different level and corrected value is averaged that is final Standard Penetration Test number is average of corrected value of Standard Penetration Test.

Let N_A, N_B, N_C  and N_d  are corrected  Standard Penetration Test number for test conducted at four different levels, then final Standard Penetration Test number is-

Final N= (N_A+N_B+N_C + N_d)/4

NOTE- If any of the corrected values N_A, N_B, N_C  and N_d   more than 50% variation with average value (N), then it means that value is not correct and represent the error in test.

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