The unconsolidated undrained (UU) triaxial shear test is a widely used laboratory method for determining the shear strength parameters of soil under rapid loading where no drainage occurs. Performed per IS 2720 Part 11, this test subjects a cylindrical soil specimen to confining pressure while applying axial load until failure. Unlike drained tests, the UU variant prevents dissipation of pore water pressure, simulating immediate loading scenarios such as rapid construction fills and foundation loading on saturated clays. Engineers obtain the undrained shear strength (Su) and Mohr-Coulomb parameters cohesion (c) and friction angle (φ). For a broader overview, refer to the Triaxial Shear Test On Soil Procedureadvantages.
What Is the Unconsolidated Undrained Triaxial Test
The UU triaxial test, also called the quick triaxial test, measures soil shear strength under undrained loading conditions that replicate sudden construction events. The specimen is subjected to a confining pressure (σ₃) from pressurised water within a triaxial cell, and vertical deviator stress (σ₁ − σ₃) is applied incrementally until shear failure occurs. No drainage from soil pores is permitted at any stage. The drainage valves remain closed throughout, so pore water pressure generated during loading cannot dissipate. This condition directly simulates field situations where loading happens too rapidly for pore water to drain out of the soil mass. Saturated clays under rapid construction loading behave exactly in this undrained manner. Three identical or near-identical specimens from the same soil are tested under confining pressures of 0.5, 1.0, and 1.5 kg/cm², and the resulting failure stresses are used to construct Mohr’s circles for deriving shear strength parameters. For comparison, study the Direct Shear Test Of Soil Procedure As Per Is 2720 Part 13 1986, which uses a simpler shear box approach with a horizontal shear plane.
Required Apparatus and Specimen Preparation
The UU triaxial test requires a triaxial testing machine with a triaxial cell, a water pressure unit with a hand pump for generating and maintaining confining pressure, a proving ring for axial load measurement, and a dial gauge for recording axial deformation. Rubber membranes with a membrane stretcher enclose the specimen, while a sample trimming apparatus prepares the soil to the correct dimensions. Bins for moisture content determination, a precision balance, and a drying oven complete the apparatus list. Each proving ring must be calibrated to convert dial readings into load values accurately. The soil specimen must measure 76.2 mm in length and 38.1 mm in diameter. For undisturbed samples, it is trimmed from a sampling tube using a universal extractor frame. For compacted samples, compaction follows the standard Proctor method at optimum moisture content or any moisture content relevant to the field conditions. Length and diameter are measured at three or more locations, and the average values are recorded for computation. The initial weight (W₁) is noted before assembly. A relevant technical discussion examines Why Is Shear Box Test Not A Better Alternative To Triaxial Test In Determining Shear Strengths Of Soils.
Step-by-Step Testing Procedure
The UU test follows a methodical sequence of specimen mounting, cell assembly, pressurisation, and loading.
Specimen Mounting and Sealing
- Enclose the trimmed specimen inside a 38.1 mm rubber membrane using the membrane stretcher. Apply suction, slide over the specimen, release suction, and unroll the membrane ends.
- Place non-porous discs on both specimen ends to prevent drainage of air or water.
- Remove the porous cylinder base. Clean the pedestal and roll an O-ring to its bottom.
- Centre the specimen with non-porous plates over the pedestal. Roll the O-ring over the membrane to seal the base.
- Place the top cap and seal the membrane top edge with a second O-ring. Verify vertical alignment within the chamber.
Cell Assembly and Pressurisation
- Position the chamber and plunger without disturbing the specimen. The loading frame should just touch the top cap. Position the compression dial gauge and tighten the cylinder to the base.
- Close both drain valves, keep the air lock nut open, fill the water cylinder, build pressure with the hand pump, and open the connecting valve. Once water emerges from the air lock, close it and adjust confining pressure to the target value.
- Lower the plunger to contact the top cap (indicated by a proving ring deflection). Zero the deformation dial gauge and record initial readings.
Application of Axial Load
- Start the motor to apply vertical load at a constant strain rate. The proving ring indicates load; the deformation gauge records axial compression.
- Record proving ring readings at strain intervals of 0.5%, 1.0%, 1.5%, 2.0%, then every 1.0% until failure or 20% strain.
- Maintain constant confining pressure by monitoring the gauge and adjusting the hand pump as needed.
Post-Test Steps
- At failure or 20% strain, stop the motor, close the chamber valve, open the air lock, and drain the cylinder. Remove the cell carefully.
- Record the failure pattern, shear plane angle, and final dimensions. Weigh the membrane (W₂ should equal W₁), remove it, and collect a sample from the shear zone for moisture content.
- Repeat with three specimens under confining pressures of 0.5, 1.0, and 1.5 kg/cm² (50, 100, and 150 kPa respectively).
Similar controlled loading principles apply in other geotechnical tests, such as the Load Test On Piles Methods Of Pile Load Test.
Calculations and Area Correction
Two key calculations are performed on the raw data: axial strain and stress intensity with area correction.
Axial strain e = ΔL / L₀ = (change in length) / (initial length), expressed as a percentage for convenience in plotting.
Vertical stress is the axial load P divided by the instantaneous cross-sectional area at that strain level. Because the specimen is sealed and no drainage is permitted, its volume remains constant. As the specimen shortens under compression, the cross-sectional area increases uniformly. The corrected area is:
A = A₀ / (1 − e)
where A₀ is the initial area and e is the axial strain as a decimal. The deviator stress (σ₁ − σ₃) = P/A. Corrected deviator stress is plotted against axial strain for each confining pressure to generate stress-strain curves. Similar area-based stress computation is used in the Double Shear Test On Mild Steel for metallic specimens.
Interpreting Results with Mohr’s Circle
From the stress-strain curve for each confining pressure, the deviator stress at failure (peak stress or stress at 20% strain) is identified. This failure deviator stress (σ₁ − σ₃)ₓ is added to σ₃ to obtain the major principal stress σ₁. The confining pressure serves as the minor principal stress σ₃.
| Confining Pressure σ₃ | Equivalent Stress | Typical Application |
|---|---|---|
| 0.5 kg/cm² | 50 kPa (7.25 psi) | Shallow foundation loading |
| 1.0 kg/cm² | 100 kPa (14.5 psi) | Medium-depth embankment |
| 1.5 kg/cm² | 150 kPa (21.75 psi) | Deep foundation loads |
For each confining pressure, a Mohr’s circle is plotted on shear stress versus normal stress axes. The circle centre is located at (σ₁ + σ₃)/2 on the normal stress axis, and the radius equals (σ₁ − σ₃)/2, representing the maximum shear stress at failure. After plotting all three circles, a common tangent line – the Mohr-Coulomb failure envelope – is drawn touching all circles. The intercept of this envelope on the shear stress axis gives the cohesion intercept (c), and the slope angle of the envelope gives the internal friction angle (φ). In saturated UU tests on fine-grained soils, the confining pressure often has negligible influence on the deviator stress at failure, resulting in a horizontal failure envelope with φ ≈ 0° and undrained shear strength Su = (σ₁ − σ₃)/2.
The modulus of elasticity (E) can be estimated from the stress-strain slope at half the ultimate deviator stress. These parameters feed into bearing capacity, slope stability, and earth pressure computations. Graphical interpretation of test data is similarly used in the Marshall Stability Test Flow Test On Bitumen for bituminous mixtures.
Conclusion
The UU triaxial shear test per IS 2720 Part 11 remains essential for evaluating undrained shear strength in saturated fine-grained deposits under rapid loading. By maintaining constant volume through drainage prevention, the test replicates the undrained response governing short-term stability in many field situations. From specimen preparation at 76.2 mm by 38.1 mm dimensions, through membrane sealing and O-ring confinement, to axial loading under three confining pressures, each phase contributes to reliable strength parameters. The area-corrected calculations and Mohr-Coulomb graphical analysis yield cohesion and friction angle values that underpin foundation design, slope stability, and earth-retaining structure assessment. Selecting the appropriate testing and verification method is equally critical in other contexts, such as choosing between Air Test Vs Water Test For Gravity Pipeline Leakage Selecting The Right Testing Method based on site and service conditions.