What is a Triaxial Shear Test?

The Triaxial Test is a widely used and flexible method in soil mechanics to measure soil shear strength parameters—cohesion (c) and internal friction angle (φ).

Key advantages

• Measures both total stress and effective stress parameters.
• Controls drainage conditions; precisely measures pore water pressure and volume change.
• No forced failure plane—failure develops naturally along weak planes, giving more representative data.

Three Main Types of Triaxial Tests and Their Applications

Unconsolidated Undrained (UU) Test

• Simple and fast.
• Total stress only; no drainage.
• Suited for short-term stability (during/immediately after construction).

Consolidated Undrained (CU) Test

• Most common; gives effective stress parameters (φ′, c′).
• Records excess pore water pressure during shearing.
• Allows faster shear rates than CD, saving time.

Consolidated Drained (CD) Test

• Simulates long-term soil response under sustained loads.
• Slow shearing to minimize newly generated pore pressure.
• Used when designs require effective stress strength (c′, φ′), e.g., foundations, slopes.

Test Procedure and Key Steps

1. Specimen Preparation (Specimen & System Setup)

   • Prepare specimens from soil samples (Shelby tubes/block samples for clays; molds for granular soils).
   • Cover with a rubber/latex membrane and place in the triaxial cell.
   • Connect pressure/volume controllers and transducers; set measurements.

2. Saturation

   • Apply partial vacuum and fill with de-aired water to ensure saturation.
   • Perform B-check (Skempton’s B value); confirm B ≥ 0.95.

3. Consolidation

   • Increase cell pressure while maintaining constant back pressure to reach target effective stress.
   • Monitor volume change until minimal; ensure ≥ 95% dissipation of excess pore pressure.

4. Shearing

   • Apply axial strain (εₐ) in compression or extension.
   • Control shear rate according to drainage condition.
   • Record deviator stress (q), εₐ, pore pressure, and volume change.
   • Continue until failure criteria: peak q, stabilized stress, or specified εₐ.

Data Analysis and Interpretation

Deviator Stress vs. Axial Strain

Plot deviator stress ( q = \sigma_1 – \sigma_3 ) against axial strain ( \varepsilon_a ) to assess stiffness, peak strength, and post-failure residual strength.

Pore Water Pressure Changes

In CU/UU tests, pore water pressure typically changes during shearing (excess u), indicating consolidation state, drainage behavior, and effective stress path.

Failure Envelope

Use Mohr’s circles for multiple confining pressures to draw the failure envelope and obtain c and φ—critical for slope stability and foundation bearing capacity assessments.

Volume Changes

In CD tests, compression vs. dilation trends reflect compressibility and dilatancy, informing deformation behavior during shearing.

Conclusion

The Triaxial Test—via UU, CU, CD—offers a powerful, controlled framework to capture strength (c, φ) and deformation traits across soil types. A standardized workflow (preparation, saturation, consolidation, shearing) plus careful analysis (stress–strain, pore pressure, Mohr’s circles, failure envelope) supports reliable design for foundations, slopes, and bearing capacity.