Triaxial Compression Test for Soils: UU, CU, and CD Types

What Is the Triaxial Test?

The triaxial compression test is the most comprehensive and widely used laboratory test for determining the shear strength and stress-strain behavior of soils. A cylindrical soil specimen is enclosed in a rubber membrane inside a pressurized cell — allowing independent control of confining pressure (σ₃) and axial stress (σ₁). This simulates actual field stress conditions far more realistically than the direct shear test.

Three Test Types

Equipment Components

• Triaxial Cell: Acrylic or stainless steel pressure chamber. Specimens typically 38 mm, 50 mm, or 76 mm diameter × 2× height.
• Cell Pressure System: Pressurized water or air maintains confining pressure (σ₃) precisely. Modern systems use pneumatic pressure controllers with digital readout.
• Back Pressure System: Saturates specimen by applying back pressure against cell pressure. B-value check confirms saturation (B ≥ 0.95 required).
• Axial Loading Frame: Motor-driven frame applies deviator stress at constant strain rate. Typically 50–100 kN capacity for standard 50 mm specimens.
• Load Cell: Submersible internal load cell (preferred) eliminates friction errors from the ram seal.
• Pore Pressure Transducer: Required for CU and CD tests to measure pore pressure development.
• LVDT / Displacement Transducer: Measures axial strain.
• Volume Change Device: Burette or digital volume gauge to measure drainage during CD consolidation and shearing.

Results: What You Get

A minimum of three specimens tested at different confining pressures yield three Mohr circles. The common tangent line defines the failure envelope — giving cohesion c and friction angle φ in terms of total or effective stresses.