OBJECT: To determine the shear strength of fine-grained soil by Tri-Axial Test (Demonstration only).
STANDARDS:
BS 1377: Part 7: 1990
OBJECTIVES:
- The Triaxial test is primarily designed to determine the shear strength parameters of a soil sample in terms of total stresses, i.e. the angle of shear resistance (j), the cohesion (c), and the undrained shear strength (cu). Or in terms of effective stresses, i.e. the angle of shear resistance () and the cohesion (c’).
- These values may be used to calculate the bearing capacity of soil and the stability of slopes.
THEORY:
The triaxial test is a laboratory test that mimics the in situ condition of the soil wherein the soil is undergoing stresses in the vertical and lateral directions. The confining pressure is an important parameter in deriving the shear strength of the soil. Unlike the direct shear test wherein failure is being forced on the horizontal plane, the triaxial test forces the soil in the weakest plane.
The consolidated undrained condition of the triaxial test as the name implies involves the application of loading on the soil forcing it to deform at a constant without drainage. This is similar in field conditions wherein the soil has been fully consolidated under a set of stresses and is subjected to a change in stress without time for further consolidation to take place.
The results of this experiment depend highly on the quality of undisturbed samples retrieved on the field, their handling, and preparation prior to the test itself. The experience of the geotechnical engineer is an important aspect in providing an assessment of the deviation between sample conditions and site conditions. The results of this laboratory test are useful for embankment analysis, earth pressure calculations, and foundation design.
APPARATUS:
- 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:
- Cell top plate of corrosion-resistant material fitted with an air bleed plug and close-fitting piston guide bushing.
- 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.
- The cylindrical cell body shall be removable for inserting the specimen and shall be adequately sealed to the top plate and base plate.
- Cell base of corrosion-resistant rigid material incorporating a connection port as shown.
- 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.
- The tubular membrane of high-density latex encloses the specimen and protects 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 caliper
- 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.
Step 1: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 use an alternative sample for preparing the test specimens.
Step 2:Protect the soil from loss of moisture during preparation.
Step 3:When a set of specimens is required for testing at different con-fining pressures, select the specimens so that they are similar. Record the location and orientation of each specimen within the block sample.
Step 4: 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%.
Step 5: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.
Step 6:After preparing the test specimens, break open the remainder of the sample and record a detailed description of the soil fabric.
Test Procedure
Step 1: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.
Step 2:Fit the membrane evenly on the stretcher.
Step 3:Place the membrane around the specimen while applying suction to the stretcher.
Step 4:Seal the membrane to the end caps by means of rubber O-rings (or the stretcher), without entrapping air.
Step 5: Place the specimen centrally on the base pedestal of the triaxial cell, ensuring that it is in the correct vertical alignment.
Step 6: 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.
Step 7:Fill the triaxial cell with water, ensuring that all the air is displaced through the air vent. Add some oil on top.
Step 8:Pressurize the triaxial cell and make final adjustments.
Step 9: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.
Step 10: 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.
Step 11: 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.
Step 12: 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.
Step 13: Start the test by switching on the machine.
Step 14: 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.
Step 15: Verify that the cell pressure remains constant.
Step 16: 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.
Step 17: Stop the test and remove the axial force.
Step 18: Drain the water from the cell, dismantle the cell, and remove the specimen.
Step 19: Remove the rubber membrane from the specimen and record the mode of failure with the aid of a sketch.
Step 20: Break open the specimen and record a description of the soil including its fabric.
Step 21: 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.
Review Questions
- What is pore pressure?
- What is function of O-ring in the test?
- Tri axial test is performed on which soil?
