Question

Drainage is not permitted during application of cell pressure (before application of deviator stress) in:

(1) Unconsolidated undrained test

(2) Consolidated undrained test

(3) Consolidated drained test

Out of these statements:

1. 1 and 2 are correct
2. 1 and 3 are correct
3. 2 and 3 are correct
4. Only 1 correct

Answer 1

Correct Answer - Option 4 : Only 1 correct

Concepts:

Unconsolidated undrained(UU) test:

• In the UU test, excess pore water pressure develops in both stages so as there is no drainage, effective stress on the soil will not increase in both stages. i.e. during cell pressure application as well as in the 2nd stage of loading when additional axial stress is applied.
• It is quick to test and maybe complete in 5-10 minutes.
• Such tests are suitable for low permeable soil such as clays with fast loading.
• U test is carried out for evaluation of short-term stability of the structure.

Consolidated Undrained Test (CU test):

• During the first stage of confining pressure, drainage is allowed from the soil sample.
• Hence consolidation will take place. But during vertical shear loading, drainage is not permitted.
• Example: stability analysis of earthen dam during the sudden drawdown.

Consolidation Drain test (CD test):

• Drainage is permitted during both the self-pressure (confining pressure) stage and the shear stage.
• This test is the most time taking and for some soils may take several weeks. So, it is also called ‘slow test;
• CD test is carried out for evaluation of long-term stability of the structure