Multi-Stage UU Triaxial Testing
Multi-Stage UU Triaxial Testing
(OP)
I was wondering if someone can help clarify something for me. I previously worked for an offshore GI contractor and designer and have fairly recently moved to onshore work.
I have noticed since moving onshore that many GI contractors adopt the performance of a multi-stage unconsolidated undrained triaxial test with varying cell pressure. My question is, what are the perceived benefits of performing this test over a single-stage test. In my experience I have only ever specified single stage UU tests with cell pressure = in situ total stress, and sets of 3 consolidated triaxials for stress dependence.
As I understand, theoretically the deviator stress at failure should be equal regardless of cell pressure (though in practice this is not always the case, I anticipate this is due to imperfect initial saturation / possible thixotropic effects of shear surfaces).
Can anyone provide guidance as to the rationale for assigning/performing these tests and any advantages the additional stages bring? The issue I have is that these tests are being summarised in this case by apparent cohesion and friction angle values, which I believe are indicative neither of total or effective stress shearing.
Thanks.
I have noticed since moving onshore that many GI contractors adopt the performance of a multi-stage unconsolidated undrained triaxial test with varying cell pressure. My question is, what are the perceived benefits of performing this test over a single-stage test. In my experience I have only ever specified single stage UU tests with cell pressure = in situ total stress, and sets of 3 consolidated triaxials for stress dependence.
As I understand, theoretically the deviator stress at failure should be equal regardless of cell pressure (though in practice this is not always the case, I anticipate this is due to imperfect initial saturation / possible thixotropic effects of shear surfaces).
Can anyone provide guidance as to the rationale for assigning/performing these tests and any advantages the additional stages bring? The issue I have is that these tests are being summarised in this case by apparent cohesion and friction angle values, which I believe are indicative neither of total or effective stress shearing.
Thanks.





RE: Multi-Stage UU Triaxial Testing
Multi-stage CU tests are often ran. The rational is cost of the test and getting more out of the samples that you have.
Mike Lambert
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
UU tests are cheap and the project cost to take one Shelby tube and run 3 UU tests is trivial.
I would also agree in commercial practice, it's not a typical consideration.
I work for the state DOT.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
Mike Lambert
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
If the samples are permanently below the water table they should be fully saturated. If they are not saturated by the time they get to the laboratory then it is likely they have lost moisture during the sampling process. Furthermore, for overconsolidated intact samples, it is likely that the samples will need higher confining pressures to get the Skempton B-value to come in to an acceptable value (which can be less than 0.95). It's much more preferable to use the higher stresses for effective stress testing of overconsolidated soils than back pressuring the samples as back pressuring is much more likely to destroy/weaken the inherent structure of the soil.
The SHANSEP equation isn't appropriate for all soils.
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
I can't agree with that approach.
If you want a Su strength, run UU tests.
Mike Lambert
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
How are you using the results of the test or the correlation? Stability studies, foundations, what?
Mike Lambert
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
Correlations are useful, but the correlations to select should be based on test data. UU tests aren't great, but they will tell you something which an empirical correlation can't. Furthermore, if your foundation were to fail, you wouldn't have a leg to stand on if you had gone with a correlation over test data if the test data design line was more conservative.
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
I agree that it is better to spend the money on more reliable testing. However, more reliable testing is more expensive, so there is a balance to be struck.
RE: Multi-Stage UU Triaxial Testing
f-d
ípapß gordo ainÆt no madre flaca!
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
RE: Multi-Stage UU Triaxial Testing
Mostly on very deep soft soils. For example, 20m soft clay with continious 40-50 kPa undrained shear strength. These sites are mostly previous river beds. This just represents two or three examples. Others, I cannot recall at the moment.
But, OF COURSE, I had chance to observe the validity in some cases too! I usually correlate with depth rather than using Shansep equation. Do you always have the required lab results to calculate Shansep coefficients? I do not. If you do not, what do you use? Recommended values? I remember some values are recommended by Ladd.
About UUC discussions: I do not wish to contribute them. This are really detailed discussions and I do not find capacity to comment on them. But I will continue to use UUC even if they disagree. Also, if UUC is not reliable, my comments on Shansep are not worth reading, since most of them depends on it.
RE: Multi-Stage UU Triaxial Testing
"If one runs UUC tests on high quality samples, the Su can be too high (unsafe) by more than 25 to 50%. And UUC strengths from low quality samples can easily be 25 to 50% too low.
In essence, UUC tests are generally a waste of time and money and have little advantage (except within crusts) over less costly strength index tests like the Torvane, lab vane and fall cone. The cost savings will be better spent on consolidation tests and Atterberg Limits, which can then be used with a Level C (Ladd 1991) estimate of S and m in order to directly calculate Su or to check strengths estimated from in situ vane or piezocone tests."
RE: Multi-Stage UU Triaxial Testing