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Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

(OP)
Can someone please tell me where in this document (or maybe another) to find a discussion on what the confining pressure should be in a triaxial test? One of our senior engineers told me the confining pressure in a UU 1-point triaxial test should be equivalent to the total overburden at the depth the sample was taken. I assume no earth pressure coefficient should be applied? I also have skimmed the posts on this site regarding this topic and I have read different recommendations?

I have read through quite a few different references and manuals and can't find the answer I am searching for.

RE: Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

Supposedly it shouldn't matter for a total stress test such as a UU. However, I don't see any reason not to use the in situ total vertical stress, just in case.

K0 comes into it when you are reconsolidating a test (e.g. CIU, CID, CAU, CAD, etc.). The confining pressure in a UU test is just the cell pressure - there is no axial component since you are not consolidating it (Unconsolidated Undrained).

RE: Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

(OP)
LRJ, I understand that for a saturated clay, the undrained shear strength is independent of the cell pressure. The mohrs circle are the same size and the only way a soil can gain strength is by increasing the effective stress, which is not applicable in an undrained test.

So in theory, the cell pressure should not make a difference if the sample is saturated. If the spil is partially saturated that is a different story. I just wasn't sure if the total overburden was correct to use. I will continue to use it however.

RE: Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

(OP)
I should have said "not applicable in an unconsolidated test"

RE: Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

One reason for using the overburden stress or slightly higher it to help reduce the effect of sample disturbance.

Mike Lambert

RE: Bishop and Henkel, The Measurement of Soil Properties in the Triaxial Test

We'd take one Shelby tube and run three UU tests. Take a clay sample that's 30 ft below the water table (12.5 psi). We'd run one UU at 7 psi, one at 12.5 psi and another at an even greater cell pressure (e.g., like to replicate the load of the proposed engineering improvements - like the embankment fill).

According to theory, they'd all be the same - i.e., if they're all saturated. However, there is no guarantee that a clay sample below the water table is saturated. (I recognize that in another thread we have a reader who doesn't agree with this claim.) So, if you use three different confinements on a UU test, you may find that the undrained shear strength is not phi=0.

Not to get too impractical that is. . .

f-d

ípapß gordo ainÆt no madre flaca!

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