HOW TO CLASSIFY SOIL SHEAR TEST AS PER DRAINAGE CONDITION?

TYPES OF SOIL SHEAR TEST BASED ON DRAINAGE CONDITION

The shear strength parameters in the case of saturated soils depend very much upon the drainage conditions and therefore in the laboratory shear test, the drainage condition expected in the field for a particular problem should be simulated. Based on drainage condition the shear tests are classified as following 3 types.

1. Unconsolidated Undrained Test (UU test)
2. Consolidated Undrained Test (CU test)
3. Consolidated Drained Test (CD test)

1. UNCONSOLIDATED UNDRAINED TEST (UU)

Drainage is not permitted throughout the test. In the case of direct shear test drainage is not permitted during the application of both normal stress and shear stress. In the case of triaxial compression test drainage is not permitted during the application of both cell pressure and deviator stress. Since the test is conducted fast allowing no time for either consolidation of sample initially or dissipation of pore pressure in later stage, the test is also called quick test.

2. CONSOLIDATED UNDRAINED TEST (CU)

In this type of shear test the soil specimen is allowed to consolidate fully under initially applied stress and then sheared quickly without allowing dissipation of pore pressure. In the case of direct shear test the specimen is allowed to consolidate fully under applied normal stress and then sheared at high rate of strain to prevent dissipation of pore pressure during shearing. In the case of triaxial compression test the specimen is allowed to consolidate fully under the applied cell pressure and then the pore water outlet is closed and the specimen is subjected to increasing deviator stress at higher rate of strain.

3. CONSOLIDATED DRAINED TEST (CD)

In this type of shear test drainage is allowed throughout the test. The specimen is allowed to consolidate fully under the applied initial stress and then sheared at low rate of strain giving sufficient time for the pore water to drain out at all stages. The test may continue for several hours to several days.