TRI-AXIAL COMPRESSION TEST

TRI-AXIAL TEST

OBJECTIVE: 

 To determine the shear strength characteristics of a given sample of compacted/ undisturbed soil.

APPARATUS USED:

•    The Tri-axial compression test apparatus with accessories.

•     Sampling tubes, 1 ¹/2^'' internal diameter.

•     Sample extruder

•     Split mould

•     Proctor mould for preparing compacted soil.

THEORY:

            If a cylindrical specimen of  soil is subjected to normal stresses along three principal planes ( two of which are equal) , the sample will fail by shear either along a diagonal plane or by lateral bulging (i.e, failure along innumerable planes), when the shearing stress exceeds the shearing strength i.e the Mohr’s circle of stress at failure touches the strength envelope. Hence if a number of samples are tested to failure with different lateral pressures, Mohr’s circle at tangent to all the circles will represent the strength envelope, the slope of which gives the angle of internal friction Ø and the intercept of Y-axis gives the cohesion.

PROCEDURE:

1. Collect the soil from the Proctor’s mould of 4^'' dia. or sampling tube to three 1¹/2'' dia. Samples are collected. The sampling tubes are pushing into the soil slowly by means of hydraulic jack.

             

2. From a sampling tube transfer the soil into a split mould whose internal dimensions are the same as the sample to be tested.

3. Place the solid bottom plate on the pedestal of the tri axial cell and place the membrane stretcher with the rubber membrane attached to its inner surface.

 4. Apply a suction at the membrane stretcher and transfer the soil from the split mould to the space formed inside the membrane.

 5. Place the top cap in proper position. Release the suction and fasten the membrane with the top cap and the pedestal by ‘o’ rings or rubber bands after removing the membrane stretcher.

6. Fix the Perspex cylinder in proper position and place the cell, on the loading platform.

7.Fill the cell completely with water. While filling the cell, the air vent valve should be kept open.
 

8. When water comes out through the air vent, close the valve and build up a desired pressure in the cell which can be read from the pressure gauge.

9. Adjust stress and strain dial gauges to proper positions and apply deviator load by operating the machine at a constant rate of strain (0.05 in. Per min.).

10. Note stress dial gauge readings corresponding to a series of strain dial readings until the sample fails.

11. Dismantle the cell, take out the sample and clean the cell.

12. Repeat steps 3 through 11 for the other two samples at different cell pressures.

OBSERVATIONS:

Sample height.....................................                        Moisture content................................

Sample diameter.................................                        Bulk density.......................................

Strain dial readings    (divs)	Axial deformation  (in.)	Axial strain, ɛ (%)	Area dia.  Ac	Stress dial readings (divs)	Axial load  (kg)

Deviator Stress at Failure = ............................................... ..……………..(from stress-strain graph).

RESULTS:   

1 Draw deviator stress vs. axial strain curve of each of the sample and from the curve find deviator stress at failure.

2. Draw Mohr’s Circles with deviator stress found from each sample at failure. From the common tangent, find C and Ø.