Read Microsoft Word - Triaxial Compression Test 1 Procedure.doc text version

Revised 2011, WKS

Datasheet No.10



The Triaxial Compression Test

Unconsolidated Undrained Test on a Cohesive Soil

EQUIPMENT: Compression machine (strain controlled) Triaxial cell Rubber membrane O-Rings Top and bottom caps Membrane stretcher Calipers PROCEDURE: 1. As in the unconfined compression test check with the instructor that the rate of feed on the machine to be used is correct. If necessary, carefully strip the wax from your sample. Trim the ends square and to a length of between 100 and 104 mm. Determine its average diameter, height and mass. Based on the measured specimen length, determine the strain dial reading that would correspond to 15% strain and highlight on DS 10. 2. Fit a rubber membrane smoothly into the membrane stretcher folding the ends of the membrane over the stretcher. Attach the tube on the stretcher to a vacuum source and apply a vacuum. 3. Insert the sample into the membrane and attach caps using the O-Rings. 4. Remove the specimen from the stretcher and insert in the triaxial cell.


5. Assemble the cell and place in the compression machine. Bring the cell piston into contact with the proving ring until a load just flickers on the dial. 6. Apply a chamber pressure of 300 kPa using the air water cylinder to fill the cell and zero the dial readings. 7. Turn on the machine and take simultaneous load and deformation readings at each 0.5 mm of deformation. Take dial readings until the loads fall off in two successive readings or two readings beyond the 15% strain value. Observe the chamber pressure gauge throughout the test. The chamber pressure should not vary more than ±3 kPa. 8. After the sample fails, or 15% strain plus 2 readings occurs, shut off machine, release the cell pressure by opening the bleed valve on the air water cylinder. Then open the air bleed on the top of the cell. When the cell has drained, disconnect and remove it. Close the "Clinger" valve on the air water cylinder to prevent water siphoning from the cylinder. 9. Remove the membrane from the sample and obtain Torvane and pocket penetrometer readings as in the unconfined compression test. 10. Carefully gather any pieces dislodged by the Torvane test, or re-weigh the sample, and place it in the oven to obtain a moisture content. COMPUTATIONS AND REPORT: 1. Calculate axial strain, the corrected area A' and the load P as in the unconfined compression test and place in the appropriate columns of the data sheet. (Note the proving ring load constant). 2. Compute the deviator stress,


P x 1000 A'



= deviator stress in kPa P = load in N A' = corrected area in mm2

3. Plot a curve of deviator stress kPa (y-axis) vs. percent strain (x-axis) and obtain the deviator stress at failure (maximum if before 15% strain or that at 15% strain). 4. Compute the major principal stress, 1 for the test using the deviator stress at failure, f.

1 = 3 + f


3 = the cell pressure

5. Convert the deviator stress at failure from the test and the average approximate unconfined stress from the "Pocket Penetrometer" (in kPa) to shear strengths by dividing by 2. Compare these with "Torvane" results. 6. Calculate the degree of saturation of the specimen assuming a specific gravity of 2.750. If this assumption appears to produce a negative volume of air in the specimen then report the specific gravity value obtained by assuming saturation. 7. Using the deviator stress at failure, f estimate 1 for cell pressures of 150 kPa and 450 kPa and plot the corresponding Mohr's circles and failure envelope. 8. In the conclusions of the report, compare the shear and unconfined compressive strengths from the triaxial test, Pocket Penetrometer and Torvane with the results obtained from the unconfined compression lab. Include a comparison of the water contents, dry densities and saturation rates for the two samples.



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Microsoft Word - Triaxial Compression Test 1 Procedure.doc