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Triaxial tests by drainage; UCS vs cu

Short answer

Triaxial tests by drainage are UU (unconsolidated undrained), CU (consolidated undrained), and CD (consolidated drained). Unconfined compressive strength qu is the axial stress at failure with zero cell pressure, while undrained shear strength cu is the shear stress at failure; for saturated clay, cu = qu/2.

Drainage condition plays an important role in the measurement of shear strength of the soil. Write down the names of triaxial shear strength tests depending upon the drainage condition. Differentiate unconfined compressive strength from undrained shear strength for the unconfined compression test.

Triaxial tests by drainage condition

  1. Unconsolidated undrained (UU) test: no drainage during consolidation or shearing.
  2. Consolidated undrained (CU) test: drainage during consolidation, none during shearing (pore pressure may be measured).
  3. Consolidated drained (CD) test: drainage allowed during both consolidation and shearing.

Unconfined compressive strength vs undrained shear strength

PointUnconfined compressive strength quq_uUndrained shear strength cuc_u
MeaningMaximum axial stress at failure with zero cell pressureShear stress at failure under undrained conditions
Type of stressNormal (axial) stress, qu=σ1q_u = \sigma_1Shear stress (radius of the Mohr circle)
Relationqu=2cuq_u = 2c_ucu=qu/2c_u = q_u/2
Confining pressureσ3=0\sigma_3 = 0Value of σ3\sigma_3 does not matter (ϕu=0\phi_u = 0)
Used forQuick strength index of clayTotal stress design: bearing capacity and stability of clays

✓ Answer checked by Claude Opus 5.5 (high effort)

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