Which laboratory test determines the compactability of soil?

The Standard Proctor test is specifically designed to determine the compactability of soil, making it the correct choice for this question. This test evaluates the relationship between the moisture content and the density of the soil when subjected to compaction. By using a specific amount of energy applied to a sample of soil, this test helps to establish the optimum moisture content at which the maximum dry density can be achieved.

This information is vital for various engineering applications, especially in construction and foundation work, as it ensures that the soil will have sufficient stability and strength when compacted to the correct density. The results from the Standard Proctor test provide engineers with critical data needed to assess the soil's behavior under load, which is essential for designing safe and effective structures.

The other tests listed serve different purposes. The California bearing ratio test evaluates the bearing capacity of subgrade soils for pavement design, the consolidation test measures the soil's volume change under load, and the unconfined compressive strength test assesses the strength of a cohesive soil without lateral support. Each of these tests is significant in its own right, but none specifically focus on compactability as the Standard Proctor test does.