A triaxial test is performed on a cylindrical core soil sample to determine its shear strength. The triaxial test attempts to replicate the in-situ stresses (stresses in the original place the soil sample was taken) on the core soil sample. The triaxial test is primarily designed to determine the shear strength parameters of a soil sample either in terms of total stresses, i.e. the angle of shear resistance (φ), the cohesion (c), and the undrained shear strength (cu). Or in terms of effective stresses, i.e. the angle of shear resistance (φ’) and the cohesion (c’). These values may be used to calculate the bearing capacity of a soil and the stability of slopes.
The described test is an undrained test without measurement of pore pressure. This method covers the determination of the undrained shear strength (cu), the Cohesion (c), and the Angle of internal friction (φ) of a specimen of cohesive soil when it is subjected to a constant confining pressure and to strain-controlled axial loading when no change in total moisture content is allowed. Tests are usually carried out on a set of 3 similar specimens, subjected to different confining pressures.
This test is carried out in the triaxial apparatus on specimens in the form of cylinders of height approximately equal to twice the diameter. Specimen’s diameter range from 38 mm to about 110 mm. In this test, the specimen is confined in an impervious membrane between impervious end caps in a triaxial cell which can be pressurized by water. The axial load is increased by applying a constant rate of strain until the specimen fails, normally within a period of 5 min. to 15 min.
Required equipment
• Triaxial cell, of dimensions appropriate to the size of the test specimen, suitable for use with water at internal working pressures required to perform the test. (A gas shall not be used for pressurizing the cell.)
The main features of the cell are as follows:
(a) Cell top plate of corrosion-resistant material fitted with an air bleed plug and close-fitting piston guide bushing.
(b) Loading piston for applying axial compressive force to the specimen. Lateral bending of the piston during a test shall be negligible. Friction between the piston or seal and its bushing shall be small enough to allow the piston to slide freely under its own weight when the cell is empty. The clearance between the piston and its bushing or seal shall minimize leakage from the cell.
(c) Cylindrical cell body which shall be removable for inserting the specimen, and shall be adequately sealed to the top plate and base plate.
(d) Cell base of corrosion-resistant rigid material incorporating a connection port.
• Apparatus for applying and maintaining the desired pressure on water within the cell to an accuracy of ± 5 kPa with a gauge of test grade for measuring the pressure.
• Machine capable of applying axial compression at a uniform rate to the specimen at a convenient speed within the range 0.05 mm/min to 4 mm/min. The machine shall be capable of applying an axial deformation of about one-third the height of the specimen tested.
• Means of measuring the axial deformation of the specimen, readable to 0.01 mm.
• Calibrated loading ring, supported by the crosshead of the compression machine so as to prevent its own weight from being transferred to the test specimen.
• Rigid corrosion-resistant or plastic end caps of the same diameter as the test specimen. A self-aligning seating shall be provided between the top end cap and the loading ram.
• Tubular membrane of high-density latex to enclose the specimen and provide protection against leakage from the cell fluid.
• Membrane stretcher, to suit the size of the specimen.
• Two rubber O-rings, for sealing each end of the membrane onto the top cap and base pedestal.
• Extruder for vertical extrusion of sample from U-100 tubes
• Sample tubes 38 mm internal diameter and about 230 mm long, with sharp cutting edges and cap
• Trimming knife, wire saw, spatula
• Steel rule
• Vernier calipers
• Apparatus for Moisture Content determination.
Sample preparation
The specimen shall have a height equal to about twice the diameter, with plane ends normal to the axis. The size of the largest soil particle shall not be greater than one-fifth of the specimen diameter.
- Remove the soil from its sampling tube or container and make a careful inspection to ascertain the condition. Report any indication of local softening, disturbance, presence of large particles, or other non-uniformity. If these features cannot be avoided using an alternative sample for preparing the test specimens.
- Protect the soil from loss of moisture during preparation.
- When a set of specimens is required for testing at different confining pressures, select the specimens so that they are similar. Record the location and orientation of each specimen within the block sample.
- Measure the length L0 (in mm), diameter D0 (in mm), and mass m (in g) of each prepared specimen with sufficient accuracy to enable the bulk density to be calculated to an accuracy of ± 1 -2%.
- Place the specimen that is to be tested first between end caps in the membrane as quickly as possible to prevent loss of moisture. Seal the specimens that are not to be tested immediately to prevent loss of moisture.
- After preparing the test specimens, break open the remainder of the sample and record a detailed description of the soil fabric.
Test Procedure
- Place the specimen on the base end cap and place the top cap on the specimen. Filter stones may be used on top and bottom of the specimen.
- Fit the membrane evenly on the stretcher.
- Place the membrane around the specimen while applying suction to the stretcher.
- Seal the membrane to the end caps by means of rubber O-rings (or the stretcher), without entrapping air.
- Place the specimen centrally on the base pedestal of the triaxial cell, ensuring that it is in the correct vertical alignment.
- Assemble the cell body with the loading piston well clear of the specimen top cap. Check the alignment by allowing the piston to slide down slowly until it makes contact with the bearing surface on the top cap, then retract the piston. If necessary remove the cell body and correct any eccentricity.
- Fill the triaxial cell with water, ensuring that all the air is displaced through the air vent. Add some oil on top.
- Pressurize the triaxial cell and make final adjustments.
- Raise the water pressure in the cell to the desired value with the loading piston restrained by the load frame or force-measuring device. The pressure should be kept on for about ½ hour before proceeding with the test. The cell pressure shall be determined by the Engineer.
- Adjust the loading machine to bring the loading piston to within a few mm of its seating on the specimen top cap. Record the reading of the force-measuring device during steady motion as the initial reading.
- Adjust the machine further to bring the loading piston just in contact with the seating of the top cap. Record the reading of the axial deformation gauge.
- Select a rate of axial deformation such that failure is produced within a period of 5 min to 15 min. Engage the appropriate gear on the compression machine. The rate of axial deformation shall be decided by the Engineer.
- Start the test by switching on the machine.
- Record readings of the force-measuring device and the deformation gauge at regular intervals of the latter, so that at least 15 sets of readings are recorded up to the point of failure.
- Verify that the cell pressure remains constant.
- Continue the test until the maximum value of the axial stress has been passed and the peak is clearly defined, or until an axial strain of 20 % has been reached.
- Stop the test and remove the axial force.
- Drain the water from the cell, dismantle the cell and remove the specimen.
- Remove the rubber membrane from the specimen and record the mode of failure with the aid of a sketch.
- Break open the specimen and record a description of the soil including its fabric.
- Determine the moisture content of the whole specimen, or of representative portions. If there are surfaces of failure, moisture content specimens should be taken from zones adjacent to them.
Plotting and Calculations
1) From each set of readings calculate the axial force, P (N), applied to the specimen by multiplying the difference between that reading and the initial reading of the gauge on the force-measuring device by its calibration factor (in N per division).
2) Calculate the cross-sectional area, A (mm), of the specimen, on the assumption that it deforms as a right cylinder, from the equation:
Where
A0 is the initial cross-sectional area of the specimen (in mm2) calculated from the initial diameter D0
ε is the axial strain, equal to ΔL/L0
Where
L0 is the initial length of the specimen (in mm)
ΔL is the change in length measured by the axial deformation gauge (in mm).
3) Calculate the Principal Stress difference, i.e. the Deviator Stress (in kPa = kN/m2): (𝜎1 – 𝜎3) = P/A x 1000 [ kPa = N/mm2 x 1000 = kN/m2]
4) Plot the stress-strain relationship for each specimen, i.e. the deviator stress against axial strain. Curves for all three specimens may be plotted on the same graph.
5) The values at failure should be tabulated as shown below:
| Specimen No. | Deviator stress (𝜎1 – 𝜎3) | Cell Pressure 𝜎3 | 𝜎1= (𝜎1 – 𝜎3)+ 𝜎3 |
| A | |||
| B | |||
| C |
6) Using these values of 𝜎3 and 𝜎1, the Mohr circle at failure for each specimen can be drawn on the same graphical plot. The scale on the vertical axis (shear stress axis) and the horizontal axis (principal stress axis) must be the same. Draw the best line fit to touch the circles. This tangential line is the Mohr-Coulomb envelope representing failure. The angle of inclination (f degrees) of the envelope to the horizontal is measured, and the intercept (c) on the shear stress axis is read off.
7) Calculate the value of the Undrained Shear Strength, cu (in kPa), from the equation:
cu = ½ (𝜎1- 𝜎3)
8) Calculate the Bulk Density of the specimen ρ, from the equation:
Where
m0 is the mass of the specimen (in g).
