Triaxial compression test:
Triaxial compression test is another laboratory method for determining shear strength of soil. It is no doubt complicated as compared to other shear test. In Triaxial compression test height to diameter ratio of a cylindrical soil sample is 2 to 3 which is loaded in all three dimensions although as a result of lateral stresses the analysis is reduced to 2 dimensions, as being equal in all the directions.
The soil sample is taken carefully and en wrapped in a rubber membrane which generally has porous platens on each end. It is then placed in water perspex cell. Now water is pumped into the cell to raise its pressure to cell pressure which act in all direction. By using proving rings a vertical load is applied and recorded, until the shear failure occurs. As the cell pressure was acting in all the directions an additional vertical stress will occur which causes the failure of the sample this additional stress is known as deviator stress.
When a soil sample is removed from a soil mass, all of the horizontal and vertical stresses acting on the soil are removed. So, when the soil is tested in the laboratory to determine strength, the test should be conducted under test conditions similar to that of field.
The triaxial compression test is a procedure that permits different horizontal and vertical stresses to be applied to the soil specimen. This procedure will closely duplicate the expected field conditions.
The concept of triaxial compression test is that an all around equal pressure is applied to the soil sample in the form of confining pressure.
The confining pressure is obtained by imposing a compressive stress on a fluid that completely surrounds the soil specimen. The fluids most commonly used are glycerin and water. Compressed air is also used. The confining or chamber pressure is also called the minor principal stress.
Usually, the principal change in stress experienced by the soil is a change in vertical stress due to the construction of the building, dam or other structure above the supporting soil. It would be logical to test the soil specimen in a configuration that permits the specimen to be placed in the testing device and loaded in a manner that duplicates what will happen in the field.
Thus, soil specimens are tested in the triaxial compression test by applying a vertical loading to the specimen.
The vertical load being applied to the soil specimen causes it to feel the vertical stress. This vertical stress increases than the confining pressure.
This increase in vertical pressure is called deviator stress.(Δσ)
When the deviator stress is being applied, σ1 the major principal stress is no longer equal to σ 3 .
Thus σ1 becomes
σ1 = σ 3 + (Δσ)
Triaxial compression test results are analysed by plotting mohr circles for thee stress conditions of each sample when failure occurs. By plotting mohr circles, strength parameters of the soil such as cohesion C and the angle of internal friction Φ can be determined. These parameters are used to calculate the shear strength of the soil.
The triaxial compression test can be applied to both cohesion and cohesion less soils.