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# Negative Cohesion "c" CU Triaxial 3 Stage?

## Negative Cohesion "c" CU Triaxial 3 Stage?

(OP)
Hello, we run 3-stage CU tests, and had one that created a very steep failure envelope tangent to the Mohr circles that crossed the Y axis below 0. That would give us a cohesion value that is negative. Is that even possible? Would you just call the cohesion 0 and plot the failure envelope through the 0 point? Thanks.

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

From a practical standpoint you can't have negative cohesion.

Have you looked at the p-q plots? What do they indicate?

Mike Lambert

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

Just to agree with Mike. The p-q can tell a lot!

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

yep its not possible , i am guessing that the last sample was messed up or the test was conducted in a wrong way

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

(OP)
We do these almost every week and this was a rare one. When plotting the failure envelop on the first two circles the line was closer to zero, but barely negative -0.2psi. When adding the third circle it increased the angle to have the cohesion to be around -3.5psi. I went with the first two, omitted the third and plotted the envelop through zero. Could it be from the first two stages being cut off too soon before failure? We do a 3 stage on one sample.

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

Again, you need to look at the p-q plots. There is a lot of info on the test in those plots.

Mike Lambert

### RE: Negative Cohesion "c" CU Triaxial 3 Stage?

Hi DLN, did you consolidate every stage with increasing cell pressure?

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