What the test can show
- A cylindrical specimen receives lateral pressure in a cell and axial load; stresses are controlled and, depending on instrumentation, pore pressure and volume change are measured.
- The test helps evaluate strength, deformation, and response under stress and drainage conditions relevant to geotechnical problems.
Common stages
- Prepare and mount the specimen with membrane, porous stones, and sealed connections; remove air from lines and transducers.
- Saturate when required and check the specified saturation criterion.
- Consolidate under the intended effective stress until volume or pressure response meets the completion criterion.
- Shear at a rate compatible with the drainage condition and measure load, axial deformation, pore pressure, and volume where applicable.
UU, CU, and CD
- UU: no consolidation and no drainage during shear; used to study rapid, short-term conditions.
- CU: consolidate first, then prevent drainage during shear; measuring pore pressure can provide total- and effective-stress responses.
- CD: consolidate and allow drainage during shear; the rate must be slow enough to dissipate pore pressure.
Analysis and control
- Relate principal stresses to strain; a series of specimens at different confining pressures can define a failure envelope.
- Select the test type for the duration of the problem and field conditions. Do not interchange total and effective parameters without checking the analysis.
- Durations in the source are illustrative: actual time depends on soil, drainage, specimen size, and the standard criterion.
Stress, corrected area, and parameters
- Axial strain: εa = ΔH/H0. With compression-positive convention and volumetric strain εv positive in contraction, A = A0·(1−εv)/(1−εa); under ideal undrained conditions, εv≈0.
- Deviator stress: q = F/A; σ1 = σ3 + q. If pore pressure u is measured, σ′1 = σ1 − u and σ′3 = σ3 − u.
- For axisymmetric triaxial compression, p′ = (σ′1 + 2σ′3)/3. Plot q–εa and, for CU tests with pore-pressure measurement, distinguish total and effective stresses.
- A Mohr-Coulomb envelope can be fitted as τf = c′ + σ′n·tan φ′. Use results from multiple confinements and state whether parameters are drained or undrained.
- Corrected area assumes an approximately uniform specimen shape. For barreling, cracking, or material loss, record failure mode and limit interpretation.
Educational reference guide. Check the current standard, calculation assumptions, and project specification before applying a procedure or acceptance criterion.