Question

Both isotropically consolidated undrained (CIU) triaxial test and an isotropically consolidated drained (CID) triaxial test are conducted on a loose sand. Without calculation which test will have a higher deviator stress at failure?

Answer 1

Solution:

Note: Deviator stress is the difference of the major and minor principal stresses.

In a consolidated undrained triaxial test, the water cannot drain out of the soil Because there is no arrangement in the undrained triaxial test to let water move out during the test. As a result, throughout the test , a pore water pressure acts on the soil sample . When the axial load is increased ( major principal stress ) the pore water pressure ( minor principal stress) also acts on the soil sample and therefore, the Deviator stress is reduced.

( As deviator stress = major principal stress - minor principal stress).

On the other hand, in case of consolidated drained test, water is allowed to move out with a help of a porous disk placed on the pedestal. Due to this there is negligible or no minor stress acting on the soil sample due to the water ( as there is no water present) . As mentioned above, Deviator stress is the difference of principal stress. Here, only axial pressure ( major principal stress) acts and therefore the Deviator stress gets equal to the major principal stress and there is no extra pressure to subtract from it.

So therefore, the isotropically consolidated drained ( CID ) trial tests will have higher Deviator stress at failure.