What does the direct shear test primarily assess?

The direct shear test primarily assesses shear strength related to consolidation stress in the context of soil mechanics. This test evaluates the ability of soil to resist sliding along a defined failure plane under a controlled shear stress condition. During the procedure, a soil specimen is placed in a shear box and subjected to horizontal shear stress while simultaneously controlling vertical stress, which simulates the consolidation pressure that the soil experiences in the field.

The results from a direct shear test provide valuable information on the shear strength parameters of the soil, including the cohesion and friction angle. These parameters are crucial for understanding soil behavior under load conditions, particularly in foundation and slope stability analyses. The test can be performed under drained or undrained conditions, but its fundamental goal is to determine how soil strength is influenced by the effective stress at which it is consolidated.

In contrast, the other options address different aspects of soil properties and strength testing. For instance, assessing the compression strength of cohesive soils refers to vertical load-bearing capacity, which is not the central focus of the direct shear test. Similarly, while drainage properties of saturated soils and viscous properties of partially saturated soils pertain to different soil behaviors and characteristics, they do not directly relate to the shear strength measurement that the direct shear test aims to provide.