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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.

RE: Multi-Stage UU Triaxial Testing

I have never seen a multi-stage UU test. I see no way that this could result in representative results.

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

I believe this relates to the saturation of the samples. Offshore, where all samples are assumed to be fully saturated, you will get no difference because it is a total stress test. Onshore, for samples above the water table, there is void space with air in, so you will get a gradient for different confining stresses as the air void space is compressed.

RE: Multi-Stage UU Triaxial Testing

There is no guarantee the clays below the water table are saturated. We know this from conducting CU tests, where backpressure beyond the in-situ pore-pressure levels are required to attain saturation. If a UU sample is not saturated, multi-stage UU tests will show a curved failure envelope. As cell pressure increases, you will ultimately attain a phi=zero envelope, but maybe not in the stress range of your proposed engineering improvements.

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

UU triaxial is simply a waste. Refer to Ladd 2003 paper.

RE: Multi-Stage UU Triaxial Testing

So Henryzau, what would you run instead?

Mike Lambert

RE: Multi-Stage UU Triaxial Testing

Mike, I would run CK0U triaxial or do level C estimate Cu = SxSigma'v0x(OCR)^m

RE: Multi-Stage UU Triaxial Testing

Henryzau, can you share with us that Ladd's paper?

RE: Multi-Stage UU Triaxial Testing

It's a free download, just google Ladd and DeGroot 2003 soft ground

RE: Multi-Stage UU Triaxial Testing

Quote (fatdad)

There is no guarantee the clays below the water table are saturated. We know this from conducting CU tests, where backpressure beyond the in-situ pore-pressure levels are required to attain saturation.

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.

Quote (HENRYZAU)

Mike, I would run CK0U triaxial or do level C estimate Cu = SxSigma'v0x(OCR)^m

The SHANSEP equation isn't appropriate for all soils.

RE: Multi-Stage UU Triaxial Testing

LRJ, for what soils in your opinion SHANSEP Equation is not applicable?

RE: Multi-Stage UU Triaxial Testing

So you would you a correlation instead of a test result?

I can't agree with that approach.

If you want a Su strength, run UU tests.

Mike Lambert

RE: Multi-Stage UU Triaxial Testing

Mike, why SHANSEP Equation is better than conventional UUC is well explained in Section 7.2 of Ladd and DeGroot (2003). Have a good read, it's a free download.

RE: Multi-Stage UU Triaxial Testing

I will read, but I don't expect that any paper is going to convince me that a correlation is better than a test result.

How are you using the results of the test or the correlation? Stability studies, foundations, what?

Mike Lambert

RE: Multi-Stage UU Triaxial Testing

Mike, I would trust the correlation more than UUC. The undrained strength is normally required for short term slope stability check for an embankment on soft ground or a bearing check for a pad/crane footing.

RE: Multi-Stage UU Triaxial Testing

SHANSEP is not appropriate for e.g. glacial tills. I'm sure there are other soil types too.

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

LRJ, there are other quick,cheap and more reliable in-situ tests (such as CPTu, DMT and vane shear) than UUC; I am not saying purely relying on correlations to do a foundation/slope design, rather UUC is unreliable as explained in Ladd's paper. The money saved from doing UUC is better to spend on other more reliable tests.

RE: Multi-Stage UU Triaxial Testing

The unit price for a UU test is less than a CPT, at least in the markets I have operated in (Europe and Australia), so it depends how many UU tests you want. Furthermore, the in situ tests are reliant upon correlation with lab testing (e.g. Nk typically comes from correlation qc and su from UU tests).

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

In response to LRJ, we disagree.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Multi-Stage UU Triaxial Testing

Nkt can come from correlation with vane shear test, DMT not UUC, you two appear to ignore Ladd's suggestions on UUC. I have no further comments let's stop here.

RE: Multi-Stage UU Triaxial Testing

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.

RE: Multi-Stage UU Triaxial Testing

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?

RE: Multi-Stage UU Triaxial Testing

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.

RE: Multi-Stage UU Triaxial Testing

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."

RE: Multi-Stage UU Triaxial Testing

Bible goes on. Good job.

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