## Problem with effective stress path in CU test.

## Problem with effective stress path in CU test.

(OP)

Hi,

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!

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.

Mike Lambert

## RE: Problem with effective stress path in CU test.

## RE: Problem with effective stress path in CU test.

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.

## RE: Problem with effective stress path in CU test.

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.

Mike Lambert

## RE: Problem with effective stress path in CU test.

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.

Thanks!

## RE: Problem with effective stress path in CU test.

## RE: Problem with effective stress path in CU test.

## RE: Problem with effective stress path in CU test.

## RE: Problem with effective stress path in CU test.

## RE: Problem with effective stress path in CU test.