What is the relation between the unconfined shear strength and the undrained shear strength measured from UU and Cu triaxial compression tests?
The correct answer and explanation is :
The relationship between unconfined shear strength (τ_uc) and the undrained shear strength (C_u) measured in triaxial compression tests (such as UU tests) can be understood through the principles of soil mechanics. In simple terms, the unconfined compressive strength (qu) of a soil sample, which is typically measured using the unconfined compression test (UCT), is the maximum axial stress that a sample can withstand without lateral confinement. It is used primarily for cohesive soils and is expressed as:
[
\tau_{uc} = \sigma_1 = C_u
]
Where:
- τ_uc is the unconfined shear strength.
- C_u is the undrained shear strength from triaxial tests (typically measured under undrained conditions).
- σ_1 is the axial stress at failure in the triaxial test.
In an Undrained Triaxial Compression Test (UU Test), the soil specimen is subjected to a triaxial stress state, where the confining pressure (σ_3) is held constant, and no drainage is allowed during the test. This simulates rapid loading conditions, as in foundation or slope failures where drainage cannot occur. The undrained shear strength (C_u) from this test is determined at failure based on the relationship between the axial stress (σ_1) and the confining pressure (σ_3).
In general, for cohesive soils (like clays), the unconfined shear strength (τ_uc) measured directly in a laboratory test is equal to the undrained shear strength (C_u) derived from the UU triaxial compression test, because both are directly related to the failure behavior of the soil under undrained conditions. The unconfined shear strength is a simpler and more straightforward way to estimate the strength of the soil when no drainage is permitted, whereas the undrained shear strength from triaxial tests provides more detailed insights into the stress-strain behavior under controlled conditions.
In summary, for undrained conditions, τ_uc = C_u, meaning the unconfined shear strength and the undrained shear strength obtained from a UU triaxial test are typically the same in cohesive soils under undrained loading.