Triaxial Compresstion Test (part 3) - Determination of soil strength parameters

From the before blog (part 1) we talked about, three types of triaxial testing methods. which is commonly assign by 2 letters.

Consolidated Drained Test (CD)

This test is used to obtain the effective shear strength parameters C' and phi'. Basically during both consolidation stage and shearing valves should be open.

During consolidation stage
u = 0
During shearing stage
u = 0 + 0
u = 0


Time consumption is high (nearly 01 week) therefore high cost, therefore called as the "slow test".

The CD test simulates the condition where a soil has been consolidated due to some prior loading, and then loaded to failure very slowly so that pore water pressure will not develop or will dissipate quickly.

In this case, the soil strength parameters needed for design are C' and phi' and the analysis is referred to as the effective stress analysis or long term stress analysis.

Particle example is an embankment constructed slowly subsequent to consolidation under it's original weight.

Consolidated Undrained Test (CU)

This test is an alternative quicker way of obtaining the effective shear strength parameters C' and phi'.

Basically, during consolidation stage valves should be open and during shearing stage valves should be closed.

Pore water pressure increment,
during consolidation stage,
u = 0,

during shearing stage,
u = B*sigma 3 + A*sigma d (B*sigma3 = 0)
u = A*sigma d

Unconsolidated Undrained Test (UU)

For this test during both consolidation and shearing stages we need to close the valves.

Pore water pressure increment,
during consolidation stage,
u = B*sigma 3,

during shearing stage,
u = B*sigma 3 + A*sigma d

Time consumption is very low (10 - 20 mins) therefore called as "quick test".

Advantages of triaxial test

1) Pore water pressure can be measured (also controlled)
2) We can simulated field conditions
3) We can apply uniform stress and strain