Standard Penetration Test (SPT) is a simple and low-cost testing procedure widely used in geotechnical investigation to determine the relative density and angle of shearing resistance of cohesionless soils and also the strength of stiff cohesive soils.
For this test, a borehole has to be drilled to the desired sampling depth. The split-spoon sampler that is attached to the drill rod is placed at the testing point. A hammer of 63.5 kg (140 lbs) is dropped repeatedly from a height of 76 cm (30 inches) driving the sampler into the ground until reaching a depth of 15 cm (6 inches). The number of the required blows is recorded. This procedure is repeated two more times until a total penetration of 45 cm (18 inches) is achieved. The number of blows required to penetrate the first 15 cm is called “seating drive” and the total number of blows required to penetrate the remaining 30 cm depth is known as the “standard penetration resistance”, or otherwise, the “N-value”. If the N-value exceeds 50 then the test is discontinued and is called a “refusal”. The interpreted results, with several corrections, are used to estimate the geotechnical engineering properties of the soil.