Unconfined compression (UC) test — correct interpretation: Which statement best describes the unconfined compression test in relation to other shear strength tests on soils?
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AAn unconfined compression test is a special case of the triaxial compression test
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BAn unconfined compression test is a special case of the direct shear test
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CThe confining pressure is maximum during a UC test
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DIn a UC test, a cylindrical specimen is loaded in major principal stress until failure occurs along a shear plane (with zero cell pressure)
Answer
Correct Answer: An unconfined compression test is a special case of the triaxial compression test
Explanation
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:
Recognize UC as triaxial with sigma_3 = 0 (no confinement).Apply axial load until failure; record peak axial stress qu.Compute undrained shear strength su = qu / 2 (clay, undrained).Interpret results within triaxial Mohr–Coulomb framework, with phi_u ≈ 0 often assumed for normally consolidated clays.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