Standard Penetration Test

Standard Penetration Test (SPT) is an in-situ test, used to determine the relative density and angle of shearing resistance of cohesionless soils. This test is also used to determine Unconfined Compressive Strength (qu) of cohesive soils.

The test employs a split-spoon sampler which consists of a driving shoe, a split-barrel of circular cross-section which is longitudinally split into two parts and a coupling.

SPT is done as per following procedure :

1. The borehole is advanced to the required depth and the bottom is cleaned.
2. The split-spoon sampler is driven into the soil by a drop hammer of 65kg falling through a height of 750 mm. [ Rate of blows = 30 blows/min]
3. The number of blows (N) for every 150 mm penetration is recorded.
4. The above step(3) is done for 2 more times.
5. The “N” value for 1^st penetration is discarded as it is a seating drive.
6. The total value of “N” for 300 mm penetration is Standard Penetration Value.

Corrections applied in SPT value :

1. Overburden Pressure Correction :

2. Dilatency Correction :

SPT test is halted, if :

1. 50 blows are required for any 150 mm penetration.
2. 100 blows are required for 300 mm penetration.
3. 10 successive blows produce no advance.