Triaxial Test

What Is a Triaxial Test and Why It Matters

The Triaxial Test is the gold-standard laboratory method for measuring soil shear strength and deformation under controlled stress paths. By applying an all-around confining pressure and an axial load to a cylindrical specimen, engineers can quantify peak and residual strength, stress–strain behavior, pore-pressure response, and drained vs. undrained performance—critical inputs for Bearing Capacity, Settlement Analysis, Slope Stability, and Retaining Wall Design.

Unlike index tests, triaxial testing lets you choose drainage and loading rate to simulate the field condition. You can run undrained tests for short-term behavior of clays, drained tests for long-term stability or sands, and consolidated tests to account for in-situ effective stresses and overconsolidation ratios. For adjacent fundamentals see Soil Mechanics and our overview of Geotechnical Soil Testing.

Enduring external references that rarely change include ASTM International (D4767, D2850), FHWA geotechnical manuals, and USACE guidance. These organizations provide long-lived standards and procedures that remain stable across years.

Choose the triaxial mode to match the field drainage condition—then your parameters will be design-ready, not just lab-ready.

Test Variants: UU, CU, and CD at a Glance

Triaxial tests are grouped by whether the specimen is consolidated before shearing and whether drainage is allowed during shearing:

• UU (Unconsolidated-Undrained): No consolidation; rapid undrained shear with drainage closed. Delivers total stress parameters (often c_u or s_u). Good for short-term stability checks in low-permeability clays.
• CU (Consolidated-Undrained): Consolidate specimen to desired effective stress (often K₀ or isotropic), then shear undrained while measuring pore pressure. Gives both total and effective parameters via pore-pressure correction. Preferred for most clay design because it honors in-situ stress history.
• CD (Consolidated-Drained): Consolidate and shear slowly with drainage open; pore pressures dissipate. Suitable for sands and long-term clay behavior; outputs effective stress strength (c′, φ′) and stiffness under drained conditions.

Apparatus & Specimen Preparation

A standard setup includes a triaxial cell with confining fluid, loading frame with axial displacement control, pressure/volume controllers for cell and back-pressure, load cell, axial deformation transducer, and a pore-pressure transducer for CU/CD. Specimens are typically 38–50 mm diameter with height-to-diameter ratio ≈ 2.0.

• Specimens: Undisturbed tubes (Shelby/piston) for clays; recompacted or pluviated reconstituted specimens for sands to target density/void ratio.
• Membranes & end platens: Latex membrane with O-rings; use lubricated porous stones and, for CU/CD, bender elements or local strain gauges if available.
• Back-pressure saturation: Aim for B ≥ 0.95 (Skempton parameter) to ensure high saturation before shearing.

Step-by-Step Procedure

1. Mount & seal: Place specimen between porous stones, apply membrane and O-rings, ensure no leaks.
2. Saturation: Apply back-pressure incrementally while maintaining effective stress; verify B-value ≥ 0.95.
3. Consolidation: For CU/CD, load isotropically or anisotropically (e.g., K₀) to target σ′₃. Record volume change and end-of-primary consolidation.
4. Shearing: Increase axial load at strain rate appropriate to drainage condition (fast for UU, slow for CD). Log deviator stress, axial strain, and pore pressure (CU/CD).
5. Post-peak/residual: For clays and some sands, continue to large strains to identify strain-softening and residual strength if design requires it.
6. Repeat at multiple confining stresses: Plot Mohr-Coulomb failure envelope and/or p′–q parameters.

Stress Paths & Strength Parameters

Results are interpreted in total or effective stress space. For effective stress, the Mohr–Coulomb criterion defines peak strength via cohesion intercept c′ and friction angle φ′. Modern practice also uses invariants: mean effective stress p′ and deviator stress q to describe paths and hardening behavior.

Build an envelope by testing at least three confining stresses and regressing peak points. For clays, capture strain rate effects and note whether the structure is sensitive or overconsolidated. For sands, peak strength depends on relative density and confining stress; consider dilation at low confining pressures.

Pore Pressure Response & Effective Stress

In CU tests, pore pressure measurements convert total to effective stresses during shear, unlocking realistic design parameters. Skempton’s parameters relate pore pressure change to stress increments in the undrained condition.

Positive Δu during shearing indicates contractive behavior (typical of normally consolidated clays or loose sands), while negative Δu indicates dilation (dense sands, overconsolidated clays). These tendencies are central to Liquefaction screening and post-peak stability.

From Triaxial Results to Design: Strength & Stiffness

Convert raw data into parameters used by structural and civil design. Report both peak and relevant post-peak/residual values with the corresponding strains. Provide modulus in the strain range important to the project (initial E₅₀ or tangent E at service strain) and select undrained strength s_u or effective c′–φ′ consistent with field drainage.

• Undrained strength (clays): UU or CU (total stress) for short-term; for long-term, use CU-p′ or CD (effective stress).
• Drained strength (sands): CD with target density; characterize dilation angle if relevant to peak strength.
• Stiffness: Report E₅₀, E_secant to a design strain, and, if measured, small-strain shear modulus G₀ (from bender elements) for settlement modeling.
• Design soil profile: Summarize at depths/strata and link to consistent use across Geotechnical Design Software.

Use these parameters directly in downstream checks: footing capacity (Shallow Foundations), pile shaft/base behavior (Deep Foundations), wall backfill strength/friction (Earth Retaining Structures), and settlement (Soil Consolidation).

Special Soils & Advanced Triaxial Options

• Dense sands & dilatancy: Control volume change and measure dilation angle; use CD at low strain rate and consider constant-p′ paths if available.
• Soft/structured clays: High sensitivity (ratio of peak to remolded s_u)—document post-peak carefully for stability analyses.
• Partially saturated materials: Measure suction (u_a − u_w) if unsaturated mechanics matters (e.g., Expansive Soils).
• Stress path control: CAU/CID with automated pressure–volume controllers; anisotropic consolidation to simulate in-situ K₀.
• Cyclic triaxial: For seismic/fatigue loading and Liquefaction triggering assessments.

QA/QC, Reporting & Troubleshooting

High-quality triaxial data depend on saturation, rate control, and precise measurements. Calibrate load cells and pressure transducers, verify membrane integrity, and log corrections (area change with strain). Report consolidation curves, stress–strain, pore pressure, and volumetric strain with interpretation notes.

• Saturation check: B ≥ 0.95 before shearing; if not, extend back-pressure or re-trim.
• Rate control: For CD, choose strain rates that keep drainage open (pore pressure ≈ 0). For UU, run fast to maintain undrained conditions.
• End restraint: Lubricate end platens; apply area correction \(A = \frac{A_0}{1-\epsilon_a}\).
• Repeatability: Duplicate tests at a confining level to show scatter; reconcile outliers and specimen variability.
• Data management: Centralize raw and interpreted data for cross-project reuse—see Geotechnical Data Analysis and Geotechnical Reporting.

FAQs: Quick Answers on Triaxial Testing

How do I choose UU vs. CU vs. CD?

Match the test to field drainage: short-term loading of clays (excavation, end-of-construction) → UU/CU (total); long-term slopes and foundations → CU (effective) or CD. Sands are usually tested in CD.

How many confining stresses are required?

At least three to define a linear Mohr-Coulomb envelope; more if curvature is expected or if you need residual strength at large strains.

What about anisotropy and K₀ consolidation?

If in-situ stress is non-isotropic (typical), consolidate anisotropically (K₀) for CU/CD to better reflect field conditions, especially for overconsolidated clays and layered soils encountered in Site Characterization.

Can I use triaxial for rockfill or highly gravelly soils?

Large-scale triaxial apparatus are needed, or use alternative tests/empiricism. For fills, combine with Standard Proctor Test, gradation control (Sieve Analysis), and field Compaction Test.

Which external references should I cite?

Use long-lived standards at ASTM (e.g., D4767 for CU/CD and D2850 for UU), plus methodology and QC guidance from FHWA and USACE.

Conclusion

The Triaxial Test transforms high-quality specimens into actionable strength and stiffness parameters under realistic drainage and stress paths. Select the appropriate mode (UU, CU, CD), achieve robust saturation, consolidate to representative effective stresses, and shear at a rate consistent with your drainage assumption. Interpret results in total and/or effective stress space using Mohr–Coulomb, p′–q plots, pore-pressure trends, and modulus over design strain ranges. Then carry those parameters directly into design models for Shallow Foundations, Deep Foundations, Earth Retaining Structures, and Slope Stability. Anchor your procedures to enduring standards at ASTM International and agency guidance from FHWA and USACE. With disciplined QA/QC and clear reporting, your triaxial program will produce parameters that are defensible, repeatable, and ready for high-stakes decisions.