Experiment:

To determine the shear strength parameters (c and) for a given soil sample using the triaxial compression test.

Apparatus:

1. Triaxial compression device2. Triaxial cell3. Specimen mould4. Rubber membrane 5. Membrane stretcher

Theory:

1. Triaxial test is more reliable because we can measure both drained and undrained shear strength.

2. Generally 1.5” diameter (3” tall) or 3” diameter (6” tall) specimen is used.3. Specimen is encased by a thin rubber membrane and set into a plastic cylindrical

chamber.4. Cell pressure is applied in the chamber (which represents σ3’) by pressurizing the cell

fluid (generally water).5. Vertical stress is increased by loading the specimen (by raising the platen in strain

controlled test and by adding loads directly in stress controlled test, but strain controlled test is more common) until shear failure occurs. Total vertical stress, which is σ1’ is equal to the sum of σ3’ and deviator stress (σd).

6. Measurement of σd, axial deformation, pore pressure, and sample volume change are recorded.

7. Depending on the nature of loading and drainage condition, triaxial tests are conducted in three different ways.

UU Triaxial testCU Triaxial testCD Triaxial testIn this lab, we will conduct UU triaxial test.

Unconsolidated Undrained Triaxial Test (UU Triaxial Test)

As drainage is not permitted and consolidation is not necessary, this test is very quick, and also referred as Q-test, pore pressure increases right after the application of σ3’ as well as after the application of σd. This test is common in clayey soils.

ApplicationUU triaxial test gives shear strength of soil at different confining stresses. Shear strength is important in all types of geotechnical designs and analyses.

Procedure:

1. Measure diameter, length, and initial mass of the specimen.2. Set a soil specimen in a triaxial chamber after putting it in the rubber membrane.3. Increase the cell pressure to a desired value (10 psi for the first case and 20 psi in the

second case).4. Shear the specimen at the rate of 1%/min.5. Record ΔL, and σd in every 10 seconds.6. Continue the test until the deviator stress shows ultimate value or 20% axial strain. 7. After completion of the test, release the cell pressure to 0, vent the pressure and bring the

cell down by bring the lower platen down, drain the cell, and clean the porous stone and the assembly.

8. Sketch the mode of failure.9. Measure the weight of the soil specimen again, and put the specimen into the oven.10. Repeat the test for the second and third specimen (20 psi of cell pressure and third

specimen 30 psi of cell pressure).11. Calculate axial strain. εa 12. Calculate vertical load on the specimen.13. Calculate corrected area of the specimen (Ac)14. Plot σd versus axial strain separately for three tests.15. Plot σd vs εa for three tests in the same plot.16. Plot Mohr circle based on σ1 and σ3 at failure. They should give the same σd value.17. Calculate the moisture content of the specimen after the test.

Observations and Calculations:

Sample ht. 3 inchDiametr 1.5 inch

Area 1.77 inch2

Cell pressure

Dial guauge reading

strain

load dial guage reading

Load (lbs) 1- A/1-

stress (psi)

                                                                                                                                                      

Axial Strain (%)

Dev

iato

r S

tres

s (p

si)

0 20 40 60 80 100 120 1400

10

20

30

40

50

60

Principal Stress, (psi)

She

ar S

tres

s, (

psi)