Determining Total Stress Parameters from CU Triaxial Results

[JHeisenberg]

If I receive CU triaxial lab results showing both effective and total stress strength envelopes, with c & phi provided, based on the angle formed from these envelopes, are the total stress parameters accurate?

GEC 5, Page 7-25, Section 7.4.3, 2nd Paragraph

" CU triaxial tests cannot be used to establish total stress strength envelopes in the sense described in Sections 7.2.6 and 7.5 because specimens are consolidated prior to shearing under undrained conditions. Total stress analyses are fundamentally predicated on the assumption that the effective stress in the laboratory is identical to the effective stress in the field and the act of consolidating specimens eliminates this fundamental requirement. Appropriate total stress strength envelopes can therefore only be acquired using UU tests."

Based on this, should a total stress envelope ever be provided from a CU triaxial test?

 

[fattdad]

The establishment of m-c strength parameters is excluded from the ASTM Standard for TXC testing.

Reliance on published m-c envelopes as provided by a laboratory is putting your professional reputation on the line. It's your job to obtain the strength parameters.

For each confinement you'll obtain an undrained failure. From those you'd get an Su/P.

Just do the determinations yourself!

f-d

ípapß gordo ainÆt no madre flaca!

 

[JHeisenberg]

fattdad, thank you for the reply.

I understand that as the engineer we should be determining these parameters, that is kind of what I was getting at.

I am wondering the correct way to determine the Total stress parameters. I have been seeing others using the same method to get total parameters as effective. Drawing Mohrs circle (total and effective) then drawing the failure envelope and getting your parameters from that.

From the text I quoted above, I am understanding this is not accurate for total parameters. Rather, you should do more like you referenced above and use the undrained failure of each test, plot out a line between these points and then use this relationship to determine su. I am trying to confirm (or not) that it is not accurate to take a clay like soil, plot a total stress failure envelope using Mohrs circle (based on a CU triax), then determine a c and phi for this material. Phi should be considered zero and and Su determined as said above.

 

[LRJ]

Whoever wrote that quote in the OP has, at best, not explained themselves clearly or, at worst, doesn't know what they are talking about . The consolidation stress applied to a CU triaxial is supposed to represent the in situ condition! CU triaxials are much more reliable than UU triaxials, particularly for the stiffness response, provided that the applied stresses are representative. UU triaxials can give reasonably representative results (at least for strength) if they are undertaken immediately after sampling (i.e. in the field), though re-consolidating the samples (i.e. making it a CU triaxial) can help to remove sample disturbance effects.

A single CU triaxial can be used to obtain both effective stress and total stress parameters provided that the pore-water pressure is measured and the sample sheared to a sufficient axial strain so as to reach the critical state line. Moreover, measuring pore-water pressure allows effective stress properties to be measured throughout shearing, so the effective stress path (either p'-q or s'-t) can be plotted throughout the shearing stage. Undrained shear strength (the only total stress parameter of relevance here?) is calculated in the usual way for a single test.

Unless you are dealing with unsaturated soils or some other specialist application, I'm not sure what relevance a total stress path is from a design perspective.