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

[geomane]

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.

 

[LRJ]

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).

 

[geomane]

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.

 

[geomane]

I should have said "not applicable in an unconsolidated test"

 

[GeoPaveTraffic]

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



Mike Lambert

 

[fattdad]

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!