Drained Shear strength from Undrained triaxial test

[rickos]

Is there a way to derive the ultimate drained shear strength of a saturated clay sample from the results of an undrained triaxial test with pore pressure measurements?
Thanks in advance.

 

[Focht3]

Both are effective stress concepts, so "the peak strength" can be taken directly from the undrained test. There are stress path differences that can't be avoided, and it would require a lot of work to recreate "the" drained path from an undrained test (although a fairly crude guesstimate would be relatively simple to develop using p-q' diagrams.) That is, assuming the stress path is a critically important part of your analysis, and you can't run any more lab tests (which is the preferred answer to your dilemma.) I know just enough about this topic to be dangerous; I'd have to do a lot of scratching my head and other parts of my anatomy to give you guidance on recreating the drained stress path.

What is your application? Soil type and stress history (in general terms)? Special considerations?



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[VAD]

Hello rickos:

I presume that when you mention ultimate strength you are inferring the residual strength or are you thinking of peak strength?. Knowing the pore water pressure you can determine the deviator stress and plot the drained stress stain curve for the test for each cell pressure yor samples have been subjected to. The resulting would provide the drained parameters. This type of test is generally used as it is much shorter than the conventional drained test.

I presume that you may be interested in the effective cohesion and effective friction angle. For ultimate if we assume that this is the same as residual then the triaxial test results are not the best to use. A shear box test is more preferred. However if one gets past about 20 % strain in the triaxial or if the soil is work softening then one can perhaps using some judgement use these values for the ultimate condition - experience required here. On the other hand phi can be taken as zero as well.

I would suggest that you read Chapter 28 of Lambe and Whitman's text Soil Mechanics for further enlightenment as it may provide you with the stress path concepts that Focht 3 was referring to and to better answer your own questions. You may have to read this section more than once or be able to reference this section when in doubt as the concepts are sometimes easy to forget.


Good Luck

 

[rickos]

Thanks all.

VAD. plot the drained stress stain curve for the test for each cell pressure yor samples have been subjected to
The problem is there was just one cell pressure.

 

[Focht3]

It sounds to me like some work with p-q' diagrams is in order. Let us know what you find -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[davecooper]

Rickos

If pore pressure is measured the effective stress can be determined. The difference as you probably already know is that the TX-CU-PP test is an undrained test. You can of course plot the effective stress path (pq) but the values may be different from a drained test due to the fact that the sample cannot drain which keeps any significant volume change from occurring. Come to think of it the stress path of a drained test is always linear because the confining pressure is constant. In an undrained test the pq can be linear for a while but can go all over the place due to the constantly changing effective confining pressure. Under fully saturated, fully drained conditions the cohesion should be zero, even in a fat clay. I know allot of people disagree with that but I am convinced of it. You can plot a total and effective strength envelope when pore-pressure is measured but there is still usually some cohesion in the effective strength envelope proving that it is not truly an effective stress envelope. I have gotten zero cohesion with the test on fat clay but rarely. The torsional ring shear has proven to be a great method for determining the peak drained stress in addition to the residual drained stress. There is not much research on undisturbed peak effective stress using the torsional ring shear but we use it all the time and the C is always zero. The results are more consevative than the triax test. The problem with the triax test and other strength tests are that full saturation and drainage are very hard to achieve. That is why there is so much apparent cohesion reported and used. We are in the process of accumulating undisturbed-peak-effective-stress data using the torsional ring shear. (That was a mouth full!) It is very promising. It is relatively quick and the samples are always fully saturated and drained proven by the typical zero cohesion. The problem with it is that gravel cannot be tested. Triax has to be used for that. To make a very long story short it is common practice to use the undrained triax with pore-pressure measurements to determine the effective stress. We run it all the time for our clients.

Best Regards,

David Cooper
Cooper Testing Labs