How Many Types of Triaxial Tests Are There?

Triaxial tests are a core part of geotechnical engineering, helping us understand how soil behaves under pressure. But did you know there isn’t just one kind of triaxial test? In fact, there are three main types, each tailored to simulate different field conditions. These variations—UU, CU, and CD—are chosen based on how water moves through the soil during testing. Beyond that, specialized and advanced methods have been developed to explore soil behavior even further. Let’s break them down.

Unconsolidated Undrained Test (UU)

Often called the quick test, the UU test is the simplest and fastest triaxial method. It’s typically used for soft clays¹ and preliminary site investigations².

Key Characteristics:

• No drainage allowed at any stage.
• Sample is tested immediately after applying confining pressure—no consolidation period.
• Tests are completed quickly, typically within an hour.

What it tells you:

• Total stress parameters only (not effective stress).
• Good for short-term stability analysis³ (e.g., undrained loading right after construction).

This test is common for low-cost, quick assessments, but it’s limited in terms of data detail.

Consolidated Undrained Test (CU)

The CU test includes a consolidation phase⁴, making it more realistic for long-term load simulations. However, it still does not allow drainage during shearing.

Key Characteristics:

• Drainage allowed during consolidation only.
• Pore pressure is monitored during undrained shearing.
• Can derive both total and effective stress parameters⁵.

What it tells you:

• More accurate understanding of saturated soil behavior⁶.
• Used in critical projects like dams, foundations, and retaining structures.

CU is widely used due to its balance of realism and data richness.

Consolidated Drained Test (CD)

The CD test is the most time-consuming, but it provides the clearest picture of effective stress behavior in soils, especially for long-term load scenarios.

Key Characteristics:

• Drainage is allowed throughout the entire test.
• Pore pressure remains zero, or close to it, by design.
• Long duration, especially for low-permeability soils like clay.

What it tells you:

• Direct access to effective stress parameters.
• Ideal for evaluating long-term stability and slow loading conditions (e.g., slopes, embankments).

CD tests are crucial when precision matters, especially in high-risk, high-investment projects.

Special Variants and Advanced Testing

Beyond the standard trio, engineers also employ specialized versions of triaxial tests to simulate complex field conditions or gather advanced data.

Advanced Types Include:

• Stress Path Triaxial Tests: Simulate actual field stress paths to observe response under complex loading.
• Cyclic Triaxial Tests: Used for evaluating liquefaction potential during earthquakes.
• True Triaxial Tests: Allow independent control of all three principal stresses—not limited to axial symmetry.
• Bender Element Triaxial Tests: Measure shear wave velocity for stiffness characterization.
• Temperature-Controlled Triaxial Tests: Examine thermo-mechanical behavior, useful in permafrost or nuclear waste studies.

These tests are often used in research, advanced design, or high-risk infrastructure.

Conclusion

There are three primary types of triaxial tests—UU, CU, and CD—each simulating different real-world conditions through drainage control. Beyond them, a range of advanced variants help engineers explore soil behavior in greater depth. Choosing the right triaxial test means aligning your lab strategy with project goals, soil type, and time constraints, ensuring safety and performance from the ground up.