Triaxial loading of soils is more common than biaxial loading (as in the direct shear loading). The triaxial shear test or triaxial compression test is the laboratory test that better simulates field loading.
Apparatus Required for Triaxial Shear Test
The test apparatus consists of a different items mentioned below :
- strain controlled loading frame
- triaxial cell (for 38 mm or 100 mm dia. specimen)
- pressure chambers
- proving ring sample former
- sample trimmer
- pore pressure apparatus
- volume change measuring device etc.
Procedure
- A cylindrical sample is fitted between rigid caps and coated with latex membrane in the typical triaxial test.
- After that, it’s put in a water-filled perpex cell.
- By adding pressure to the water in the cell, the sample is confiningly compressed.
- By loading the sample through a ram, an additional (deviator) stress (σd = σ1 – σ3) is applied, which is gradually increased until the specimen fails.
- The outlet valve can be left open to simulate a drained condition or closed to simulate an undrained condition during triaxial loading.
- The drainage outlet is connected to either the pore pressure apparatus or the volume measuring unit, which measures the change in volume during drained filling.
Also Read : Vane Shear Test [ IS 2720 (Part XXX) 1980 ]
Principle & Principal Stresses of triaxial test
Principal of Triaxial Test
The Mohr diagram was used to determine the relationship between the major and minor principal stresses. The equation for failure condition is described as :
There are two unknowns in this equation (c, Φ). At least two tests on identical soil specimens are needed to determine these.
Principal Stresses in Triaxial Test
The triaxial test can be divided into two stages. The specimen is subjected to cell water pressure (σ3) in the first stage, and deviator stress (σd = σ1 – σ3) in the second stage. This process is done till the specimen fails.
Therefore,
σ1 = σd + σ3
= (σ1 – σ3) + σ3
Observation Table
- Initial Water Content :
- Proving Ring Constant :
- Sample Size : 38 mm x 76 mm (dia)
Also Read : California Bearing Ratio Test【CBR Test】of Soil
Result and Calculation
- The area (A) of the specimen at any stage of the stage is determined by the following equation :
Where,
Ao = original area of specimen
Lo = original length of specimen
L = length of specimen for which area A is determined.
- Divide the axial load by the area A to get the deviator stress.
- A plot between (σ1 – σ3) and ε can be made for each test.
- The shear parameters are calculated from a plot of mohr circles that includes the aim peak value of deviator stress and cell press.
Characteristics
Following are the various characteristics of triaxial shear test :
- The drainage conditions can be fully controlled.
- On the failure plane, the stress distribution is uniform.
- Direct measurements of volumetric and pore pressure changes are possible.
- On the weakest plane, the specimen is free to fail.
- The test is appropriate for research purposes.
- On the failure plane, the stress distribution is uniform.
- Since the state of stress at all intermediate stages up to failure is established, the Mohr circle can be drawn at any stage of shear.
- It is the only test that can accurately determine shear parameters for all types of soils and drainage conditions.
- The belief that the specimen would stay cylindrical is incorrect.
- The problem in the field is usually three-dimensional, but the test only activates the axis-symmetrical problem.
- As compared to the direct shear test, the drained test takes longer.
- Consolidation in the field is usually anisotropic, while consolidation in the test is isotropic.