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Problem with effective stress path in CU test.

Problem with effective stress path in CU test.

Problem with effective stress path in CU test.


I'm doing CU tests but the results of the effective stress path are very funny. At first, it is parallel with the total stress path, then it moves fast to the right. I have changed new pore pressure sensor but the results are still the same. I did tests on many samples but the results are still the same.

Below is the method that I'm using to calculate the effective stress path:
strain = (initial record of LVDT - recorded measure of LVDT)/initial height of sample
Corrected area = initial sample area/(1-strain)
q (KPa) = axial load (KN)/corrected area (m2)
p (KPa) = q/3 + cell pressure
p' (KPa) = p - measured pore pressure (KPa).

Could anyone tell me what the problems are with my tests or samples? I did on many samples and the results are the same. However, other people have done this type of test on this materials and their results are the same as the ideal/theoretical shape of effective stress paths.

I attached my results in the attached file.

Thank you!

RE: Problem with effective stress path in CU test.

We really need to see the test results. There was no file attached.

Mike Lambert

RE: Problem with effective stress path in CU test.

Sounds like either it's strongly dilative, or the sign of the output from the u transducer has gotten flipped. What's the material, and what is its OCR, or relative density, percent compaction, or whatever other parameter might be applicable?

RE: Problem with effective stress path in CU test.

I have attached the file in the previous post but I do not know why it was not shown there. I tried to attach the link to download the file here Link. Hope that this time it works.

The material that I'm working on is overconsolidated peat since it is right on the surface. Specific Gravity = 1.5-1.7 depending on granular soil contamination. Unit weight = 0.9-1.1. Moisture content = 500-1000%.

@dgillete: Could you please explain more about the effect of dilation on the changes of u transducer? Thanks!

RE: Problem with effective stress path in CU test.

In inserted the link again: Link

RE: Problem with effective stress path in CU test.

For whatever reason, my employer's server forbids me to link to your data, so I can't look at it.

My point was that a strongly dilative material like a dense sand or a clay with OCR>5 causes negative excess pore-water pressure, so the stress path curves off to the right, instead of curving left like a contractive material that generates positive excess pore pressure, e.g., clay with low OCR. I know pretty close to zero about peat strength, but unless OCR is quite high, I would expect peat to be contractive.

The other explanation I suggested is that in the calculations, increase in u is somehow accidentally getting added to p, giving crazy p', instead of being subtracted from p. This could happen if, say, the transducer calibration constant (millivolts/kPa?) got put in as a positive when it should have been a negative number. I don't know your hardware or software, but it wouldn't be the first time such a thing happened.

Bon chance!

RE: Problem with effective stress path in CU test.

Your link just displays pages of numbers, not useful for figuring out your issue(s).

Mike Lambert

RE: Problem with effective stress path in CU test.

Thanks for the answer. I posted two links and only the second one works. You might want to download from the link that I posted right above your answer.

I agree with you that I do expect peat to contract. However, for almost all of the samples I tested on, the results are pretty the same. I did not do any oedometer tests on them so I'm not sure how much the OCR is. However, in accordance with some documents, peat is stated to be overconsolidated.

The reasons you gave out could happen. But my supervisor had done many experiments on the same materials at the same locations, and his effective stress path results are off to the left as the theory. That's why I was so confused to see the data like this happen to almost all of my tests.

About the calibration, I did all by myself so I can be sure that none of them has problem. I have also tested the u transducer with the cell full of water and it worked well.


RE: Problem with effective stress path in CU test.

@GeoPaveTraffic: The third button "DOWNLOAD" on the top right of the page to download the file to your computer. Then you can see all the graph and calculation inside by Excel. Thanks!

RE: Problem with effective stress path in CU test.

In that case, I've run out of ideas.

RE: Problem with effective stress path in CU test.

@dgillette: Thank you!

RE: Problem with effective stress path in CU test.

Is your peat amorphous or fibrous? That might have something to do with the triaxial test results. I did some testing many many years ago on organic silt and the test results gave quite high effective friction angles; of course, I was loathe to use the results.

RE: Problem with effective stress path in CU test.

@BigH: Those samples are all fibrous peat but with different degree of decomposition. However, fibers are still can be observed. Actually, because of the reinforcement effect of fibers, it could be understandable about the high effective friction angles of organic soils.

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