Slop Stability, CU Triaxial vs Unconfined Compression test

[Draf7y]

I am trying to develop the drained and undrained soil parameters of borrow soils. Main question bolded.

I have been given the results from the CU triaxial tests and the results from the unconfined compression tests. Both tests were conducted on the same borrow soils. I am confident I used the CU traixial tests to develop my effective friction angle and effective cohesion correctly.

Here is my question. I am able to develop the undrained cohesion (Cu) and the undrained friction angle of the borrow soils using the CU traixial tests by graphing the undrained strength (Su) by the confining pressure. Is the unconfined compression test less or more reliable than the CU triaxial test when it comes to developing Su? After developing Su via the unconfined compression test, how do I develop the undrained cohesion and the undrained friction angle?

 

[irawanfirmansyah]

To get undrained strength you should use either UU Tx or Unconfined Compression Test, not from CU Tx. Undrained strength obtained from CU Tx will be stronger than those obtained from UU Tx and Unconfined Comp Test, as on CU Tx test, the soil specimens are consolidated first before being sheared.

Undrained strength obtained from CU Tx is normally intended for obtaining variation of Su with consolidation pressure (see attached figure) to be used in staged construction.

From unconfined comp test we get quu ( unconfineed compressive strength and Cu (undrained cohesion) = 1/2 quu. Undrained friction angle(phi u) is assumed equal to zero. Hence, Su = Cu