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.

 

[GeoPaveTraffic]

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

 

[LRJ]

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.

 

[fattdad]

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!

 

[HENRYZAU]

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

 

[GeoPaveTraffic]

So Henryzau, what would you run instead?

Mike Lambert

 

[HENRYZAU]

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

 

[Okiryu]

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

 

[HENRYZAU]

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

 

[LRJ]

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.

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.

 

[HENRYZAU]

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

 

[GeoPaveTraffic]

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

 

[HENRYZAU]

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.

 

[GeoPaveTraffic]

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

 

[HENRYZAU]

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.

 

[LRJ]

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.

 

[HENRYZAU]

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.

 

[LRJ]

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. N[sub]k[/sub] typically comes from correlation q[sub]c[/sub] and s[sub]u[/sub] 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.

 

[fattdad]

In response to LRJ, we disagree.

f-d

ípapß gordo ainÆt no madre flaca!

 

[HENRYZAU]

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.