Multi-Stage UU Triaxial Testing

[jcm1890]

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.

 

[bdbd]

HENRYZAU, I have seen tens of sites that do not correlate well with SHANSEP. You do not see increasing cu with increasing depth. Please, do not make Bible out of studies.

 

[HENRYZAU]

bdbd, I would like to know more details of your cases that you think SHANSEP doesn't work well. E.g. Geology, ground profile, any recent backfilling/reclamation? The big argument is whether UUC is a reliable test method to define undrained shear strength of clay, what's your opinion?

 

[bdbd]

Henryzau,

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.

 

[HENRYZAU]

bdbd, a bit disappointed as I expected some convincing edifence from you to challenge Prof Ladd's SHANSEP Equation, published in his Terzaghi Lecture in 1991. For those too busy to down load Ladd's 2003 paper or too busy to have a good read,here is some extract
"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."

 

[bdbd]

Bible goes on. Good job.