What specific shear strength parameter for cohesive soils is directly determined from an Unconfined Compression Test (UCT)?

The specific shear strength parameter for cohesive soils that is directly determined from an Unconfined Compression Test (UCT) is the undrained shear strength (cu).

The Unconfined Compression Test (UCT) is a laboratory procedure conducted on a cylindrical specimen of cohesive soil, such as clay, without any lateral confining pressure applied to its sides. The test involves applying a gradually increasing axial compressive load to the specimen until it fails, which is typically identified by a sudden decrease in load resistance, a distinct yield point, or excessive deformation. The maximum axial compressive stress at which the specimen fails is defined as its unconfined compressive strength (qu). This value, qu, represents the direct measurement from the test.

For saturated cohesive soils tested under undrained conditions—meaning no water can enter or leave the soil specimen during the rapid loading, simulating real-world rapid loading scenarios—the angle of internal friction (φu) is considered to be zero. In this specific scenario, the Mohr's circle representing the stress state at failure in a UCT has its center on the normal stress axis at qu/2 and passes through the origin. The radius of this Mohr's circle is equivalent to the undrained shear strength (cu). Therefore, the undrained shear strength (cu) is directly calculated as one-half of the unconfined compressive strength (qu). This relationship is expressed as cu = qu / 2. This makes the undrained shear strength (cu) the direct and fundamental shear strength parameter derived from the UCT, representing the soil's resistance to shearing under undrained conditions, which is crucial for short-term stability analyses of structures on cohesive soils.