The triaxial compression test is used to measure the shear strength of soils. [1] It involves placing a saturated soil specimen in a rubber membrane inside a triaxial cell. Cell pressure is applied to saturate the sample while maintaining drainage conditions. [2] Axial stress is then applied through a piston to induce shear failure while cell pressure is kept constant. There are three main types of triaxial tests based on drainage conditions during shear: consolidated-drained (CD), consolidated-undrained (CU), and unconsolidated-undrained (UU). [3] The test allows accurate measurement of stress-strain behavior and pore pressure changes in soil specimens under controlled laboratory conditions.

1. TRIAXIAL
COMPRESSION
TEST
BY :- ABHISHEK THAKKAR
1
Rocket Education

2. Contains
• Alternative tests
• Apparatus
• Test Procedure
• Advantages of triaxial test
• Disadvantages of triaxial test
• Reference
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3. Which tests are used ?
The following tests are used for measuring shear strength of a
soil.
1. Direct shear test (Box shear test)
2. Triaxial compression test.
3. Unconfined compression test.
4. Vane shear test.
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4. Apparatus
1. Axial Loading Piston.
2. Cell pressure gauge (P3).
3. Dial gauge measuring vertical displacement.
4. Specimen membrane.
5. Pore pressure gauge.
6. Pore fluid drainage valve (open / closed).
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5. Apparatus diagram
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[1]

6. Test Procedure
1. A saturated porous stone is placed on the pedestal and the
cylindrical soil specimen is placed on it.
2. The specimen is encased in a rubber membrane and sealed
to the pedestal at the bottom as well as to the loading cap at
the top by rubber O-ring the membrane is used to prevent
the water from penetrating in to the soil sample.
3. The triaxial cell is filled with water and pressure is applied to
the water, which in turn is transmitted to the soil specimen
all-round and at top. This pressure is called “Cell
Pressure”(Pc)
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7. Test Procedure cont...
1. With the cell pressure held is constant, additional axial
stress (Pd) is applied through the ram gradually until the
sample is fails. This additional axial stress is called
“Deviator Stress” (Pd)
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8. Phase 1 consolidation :
drainage conditions
• Cell pressure added, P3 increases
• Too possible drainage condition
 Drained
 Untrained
• Unconsolidated: symbol “U”
• Drainage valve closed
• No volume change
• Excess pore pressure will be generated.
• Consolidated: symbol “C”
• Drainage valve open
• Volume change allowed
• No excess pore pressure.
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9. Phase 1 Consolidation
• Hire, water filled in cell which create pressure P3
on the spacemen. As we see in fig.
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10. Phase 2 shear: Drainage conditions
• Drained : symbol “D”
• Drainage valve open
• No excess pore pressure.
• Untrained: symbol “U”
• Drainage valve closed
• No volume change
• Excess pore pressure will be generated..
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11. Phase 2 shear
• Hire, we are gradually increase presser at piston
until the spaceman e failed.
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12. Types of triaxial test
• UU: Unconsolidated- Undrained
• Also called Q- or quick-test
• CD: Consolidated- Drained
• Also called S- or slow-test
• CU: Consolidated- Undrained
• Also called R-test
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13. UU: Unconsolidated- Undrained
• P1f = maximum stress for soil failer.
• Su = pick shear stress measure undrain strength.
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14. Advantages of triaxial test
o The shear tests under all the three drainage conditions can be
preformed with complete control.
o Pore pressure and volumetric changes can be measured
directly.
o The stress distribution on the failure plane is uniform.
o Test is suitable for accurate research work.
o The specimen is free to fail on the weakest plane.
o The state of stress during any stage of the stage is known. The
Mohr circle can be drawn at ant stage of shear.
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15. Disadvantages of triaxial test
o In case of drained test, the test can take longer period as
compared with that in a drain shear test.
o The apparatus is bulky, costly and elaborate.
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17. Reference
• http://environment.uwe.ac.uk/geocal/SLOPES/SLOPSIT
E.HTM
• Geotechnical Engineering –I , Atul prakashan, Dr R. P.
Rethailya
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