Unconfined compression Test vs. UU Triaxial Test

[Okiryu]

I heard that when dealing with soft clays, in order to obtain undrained shear strength (Su) values, it is best to conduct triaxial tests (UU) rather than unconfined compression tests.
Is there a good reason for this? Is this apply also for stiff clays?

 

[fattdad]

There may be regional responses as in some parts of the US, the UC test is widely used. So, I'm in Virginia and don't see too much UU testing of clay.

If you take a Shelby Tube sample of clay and it's below the water table, is it saturated? Maybe not. . . We all sort of know this 'cause we know that to run a CU-bar we have to use backpressure to saturate the sample. How rare is it to get a clay sample and find the B-value is at 0.97 or such on the first application of back pressure? Pretty rare, unless it's some normally-consolidate muck, which wouldn't be suited for UU testing either way.

I work for the highway department. When our consultants use UU testing, we actually require that they do UU testing at three confinements. You see with each confinement, you'll get a different level of saturation, 'cause the cell pressure will telegraph to backpressure if the sample is not saturated. This will lead to some measure of "consolidation" in the undrained membrane.

When a sample is 100 percent saturated, the results will show phi=0 conditions. When the sample is not, you'll see a curved envelope that approaches zero.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

I typically specify that soft clays or silty soils be tested using the UU test instead of unconfined compression. While fattdad is correct that it really shouldn't make any pratical difference, I often find that by appling a little confining pressure helps with sample distubance and yields better results.

Mike Lambert

 

[Mccoy]

I totally ruled out unconfined compression from my shoplist when I realized after a few instances that the values of Su resulting from the test were by far too low (based on experience not on comparative analysis with triaxial tests). This may be a local phenomenon but some authors (like Paul Mayne) tend to confirm that unconfined compression often yields too conservative Su values. The material I consulted was a pretty exhaustive presentation from Paul Mayne and others, it was available online.

 

[Okiryu]

Thank you all for your responses. Here in Japan, the correlation that is normally recommended between SPT N-values and C (Su) is C=6.25xN in KPa units. I think that a similar correlation is recommended also by Terzaghi & Peck. Anyway, I found this correlation conservative and decided to do more testing, so then I started doing unconfined compression tests, however in my current project UCT results showed values close to the above correlation. For this project, when checking settlements based on the allowable bearing pressure provided by the UCT, settlements were very small and since generally settlement controls for clay soils I was planning to increase the recommended bearing pressures but then the concern is about that the soil can have a shear failure.
I might try next time with triaxial tests.
Mccoy, the material that you consulted is from Professor Mayne's website at Georgia Tech?

 

[Mccoy]

Okiryu yes, as you know he's one of the eminent recognized experts of soil investigation, this is one of such presentations, pls look up at table 1.1 and successive discussion on Su

http://geosystems.ce.gatech.edu/Faculty/Mayne/papers/SOA-1_at_17th_ICSMGE_2009_Egypt.pdf