Unconfined compression (UC) test — correct interpretation: Which statement best describes the unconfined compression test in relation to other shear strength tests on soils?

Introduction / Context:
The unconfined compression (UC) test is a rapid laboratory test widely used for saturated cohesive soils (primarily clays). It helps estimate undrained shear strength and is conceptually linked to the triaxial compression framework.

Given Data / Assumptions:

• Specimen is cylindrical and saturated.
• No lateral (cell) pressure is applied: sigma_3 = 0.
• Undrained loading conditions typically apply.

Concept / Approach:
In a triaxial test, a cylindrical specimen is subjected to a confining pressure sigma_3 and an increasing axial stress until failure. The UC test is the limiting case where sigma_3 = 0. Hence, it is a special case of the triaxial test, not of the direct shear test. The undrained shear strength su is often taken as half the unconfined compressive strength qu for saturated clays: su = qu / 2.

Step-by-Step Solution:

Verification / Alternative check:
Compare UC results with consolidated undrained (CU) triaxial with pore-pressure measurement; trends should be consistent, though magnitudes differ due to drainage and consolidation history.

Why Other Options Are Wrong:

• Direct shear test uses a different apparatus and boundary conditions; UC is not a special case of it.
• “Confining pressure is maximum” is incorrect; in UC it is zero.
• Option D is descriptive but incomplete and could mislead; the key relationship is its identity as a triaxial special case.

Common Pitfalls:
Using UC for non-cohesive soils; interpreting qu directly as shear strength without dividing by 2 for clays.

Final Answer:
An unconfined compression test is a special case of the triaxial compression test