For which soil type is the Standard Penetration Test most effective?

The Standard Penetration Test (SPT) is primarily designed to assess the properties of granular soils, such as sand and gravel. Its effectiveness in these soil types can be attributed to how the test measures resistance to penetration. In granular soils, the SPT can provide straightforward data regarding density, which is essential for evaluating the soil's bearing capacity and stability.

Granular soils have a more consistent structure that allows the test to yield reliable results. The test involves driving a sampler into the ground using a standard weight, and the number of blows required to achieve penetration correlates well with the soil's density and cohesion in these materials. This association helps engineers and geotechnical specialists accurately determine the soil's load-bearing characteristics.

In contrast, clayey, rocky, or peaty soils often present complications during the SPT due to their cohesive nature or the presence of larger rock fragments. The results in these materials can be less reliable because the test may not penetrate effectively or may be affected by the inherent properties of these soil types.