Unconsolidated Undrained vs Consolidated Undrained triaxial tests 6

[Muckshifter]

First post and hopefully someone can help!

I'm really struggling to understand WHY UU and CU tests show different results. The main thing I can't get my head around is why the UU test produces a flat line (ie no phi value) while the CU value produces a phi value.

In both tests the samples are sheared with no drainage permitted ie undrained so why would the deviator stresses increase in the CU test such that a line can be drawn throught he mohr circles to get a phi value. Just can't get my head around why the UU mohr circles are always the same size whereas the CU mohr circles get bigger?! OK so the CU samples are consolidated to a particlaur in-situ stress but why when you do that on three different samples does the mohr circle get bigger with each stress increase increment while the UU test stays the same.

Anyone able to explain this in simple terms - been reading text books all day and nothing explains it in lay mans terms.

Thanks in advance to anyone who can help

 

[moe333]

It basically comes down to the difference between total and effective stress.

In simplified terms, shear strength is proportional to effective stress (not total stress). Higher effective stress will yield higher shear strength. In the CU test you are consolidating samples to different effective stresses so the sample consolidated to the higher effective stress will give you a higher shear strength, and a line drawn between the Mohr circles will give you a friction angle.

With the UU test, no consolidation is occurring so you are not increasing the effective stress. The increase in confining stress is taken by the water rather than the soil. You are adding total stress rather than effective stress, so the shear strength doesn’t increase with an increase in confining stress.

There are many nuances but this is the basic idea.

 

[Muckshifter]

Thanks moe333

So basically it is governed by effective stress and the fact that you consolidate the material in a CU test but not in a UU test.

 

[molerat2210]

Muck,

The consolidation phase is not the only difference between CU and UU. The CU test includes pore pressure measurements during the shearing phase. Therefore, the effective stresses can be calculated. The UU test is a glorified unconfined compressive strength test. The CU test can model a proposed long term loading condition. The UU provided an estimate of short term shear strength at the existing condition.

 

[motorbiketocrank]

Maybe this analogy will help:

Consider a sealed bag of popcorn. Take the bag of loose popcorn and shear it. Then, add a weight on the bag and shear it again. You will get the same result because the connection between the kernels of popcorn hasn’t changed. All you did by adding weight on the bag was increase the pressure in the trapped air. This is UU.

Then, put a small hole in the bag, add some weight and let the air seep out (i.e. consolidate it). Tape the hole up and then shear it. You will get a higher result because you have compressed the kernels closer together and they have a higher strength. This is CU. You are basically testing the strenghts of “different” materials (loose popcorn/dense popcorn).

The rationale is similar for a cohesive soil where particles are like the popcorn and the water in the soil is like the air in the bag. You have to be careful in how the results are applied to behavior in the real world. The phi angle really isn’t the same as you would consider in modeling the behavior of a sand. It’s more that you are taking the plots of “different” materials (or really the same material at different densities) and then just showing them on the same graph….and the “phi” angle helps you project how the material behaves the more is is consolidated.