Which lab test is primarily used for testing slightly cohesive to cohesive soils?

The Standard Proctor Test is primarily utilized for determining the optimal moisture content at which a soil type will reach its maximum dry density when compacted. This test is especially relevant for slightly cohesive to cohesive soils, as it assesses how these soils behave under compaction efforts.

When performing the Standard Proctor Test, soil samples are compacted in a mold under controlled conditions, and the moisture content is adjusted to find the point where the soil achieves its maximum dry density. This test provides crucial insights into the soil’s density, which is vital for engineering and construction projects, particularly when dealing with foundations, roadways, and earthworks involving cohesive soils.

While the California Bearing Ratio Test evaluates the strength of the subgrade for pavement design and is more concerned with the shear strength of soils, the Atterberg Limits Test assesses the plastic and liquid limits of cohesive soils, providing information about their consistency but not their compaction behavior. The Compaction Test is a broader term that could encompass various methods, including the Standard Proctor Test, but doesn't specifically denote the same established procedure tailored for cohesive soils as the Standard Proctor Test does. Thus, the focus on optimal compaction moisture and dry density makes the Standard Proctor Test the right choice for this context.