In tri axial test, the major principle stress σ1 is applied in the vertical direction, and the other two principal stresses σ and σ are applied in the horizontal direction by the fluid pressure round the specimen.

When the soil is in the state of stress defined by the equation σ1’= σ3’tan2 α’ + 2c’ tan α’ as principal stress relationship, it is said to be in plastic equilibrium.

The deviator stress σd, is given as the ratio of additional axial load to the area A2 at failure or during at any stage of the test.

Pore pressure developed in the specimen during the test can be measured with the help of a separate pore pressure measuring equipment such as Bishop’s pore pressure apparatus developed by Bishop in 1950, 1961.

In tri axial test, precise measurements of the pore pressure and volume change during the test are possible.

The vertical stress applied by the loading frame, through the proving ring is equal to (σ1 – σ3); this stress difference is called the deviator stress.

The tri axial compression test was first introduced in U.S.A by A. casagrande and Karl Terzaghi in 1936-37.

Bishop’s apparatus mainly consists of the null indicator, the control cylinder, pressure gauge, mercury manometer and burette.

Three outlet connections are generally provided through the base of the test apparatus: cell fluid test, pore water outlet from the bottom of the specimen and the drainage outlet from the bottom of the specimen.

As a shear test can be performed under all three drainage condition, tri axial test is most commonly used in research laboratory.