Is my understanding of CU testing correct? 1

[harrohb]

Hi all, sorry for the wall of text but I was wondering if someone could “fact check” my understanding of what is happening in a CU test…

Sample is saturated with back pressure and B value confirmed with cell pressure (undrained). Sample is then consolidated, initial effective stress is 0 as the cell pressure is transmitted to the pore water. Drainage valves open and sample consolidates, the pore pressure dissipates and the cell pressure is instead taken up by grain to grain contacts and effective stress increases to cell pressure. Consolidation was isotropic so there is no deviatoric stress and therefore no shear stress.

Once the undrained shear stage starts, an axial pressure is applied to the sample. Deviatoric and shear stress increase. If we assume the soil behaves contractive then as the test is undrained the pore water will resist the contractive behaviour and the sample volume will remain constant (and void ratio). Pore pressure will increase. This in turn reduces the effective stress (the increased pore pressure is essentially “pushing” the grains apart, counteracting the effective stress forces. Despite the volume remaining the constant, the increased deviatoric and shear stress will cause the soil structure to destabilise and the grains to reorientate until the stress reaches a value where shear failure occurs.

The final Mohr circles for total and effective stress represent the amount of stress taken up by the grain to grain contact and pore pressure, and the amount of stress just taken up by the grain to grain contact, respectively (both at failure).

1/2 of the deviatoric stress represents the undrained shear strength. Any additional cell pressure AFTER the sample was consolidated would not cause any additional increase in undrained shear strength, as it would be taken up by the pods pressure. The shear stresses you’d need to apply for the soil to fail would remain the same regardless. The Su is purely defined by cohesion in this formula

The effective stress failure envelope is a line tangential to the effective stress Mohr circle and is defined by Phi and cohesion. It does not represent the soil in its current state at the end of the test (as excess pore pressures are in play). The mohr circle would shift to the “right” if the sample did drain as effective stress would increase with dissipation of pore pressure.

Am I on the right path here? I feel like I’m slightly misunderstanding what the effective stress Mohr envelope represents, I’d appreciate any feedback.

Another question I had was, if dilation and contraction is resisted by pore pressure during a Cu test and the volume remains the same, how is it that a sample can change from contractive to dilative behaviour, what is happening in the soil structure to allow this?

 

[DanielSgeo]

The questions your asking are typically answered in an entire quarter of graduate-level geotechnical engineering in lieu of that, I recommend reading Chapter 12 of Holtz & Kovacs (An Introduction to Geotechnical Engineering)

That said, I think you are on the right path with your understanding of Mohr's circle. There are few details in your description of the CU test that, if I'm understanding your writing correctly, that are not quite right. For example, after consolidation and the drain valves are closed, the cell pressure is constant in a CU test. Also, before consolidation, the effective stress is not zero, it is the cell pressure minus the back pressure. Even durng BP saturation, you typically maintin a positive effective stress.

However, I understand the use of a CU test to measure undrainded shear strenght (Su) is sort of a flawed approach due to strain rates (if you're running the test per ASTM) - although a well-run suite of CU tests with proper preconsolidation and consolidation stressing can yield excellend undrained strenght parameters (that is, total c & total phi). If your goal is Su, UU or UCS tests are a better approach. There is a good discussion of this in one of the FHWA manuals on susurface investigation - i believe the older Sabatini version. Also the H&K reference cited above.

 

[EireChch]

OP - I think you understand the CU test very well, better than most and probably better than me!

The OP said Sample is then consolidated, initial effective stress is 0 as the cell pressure is transmitted to the pore water..

DSgeo response is correct I believe as there is a certain amount of porepressure present in the soil due to back pressure application. I think the OP should have said Sample is then consolidated, initial excess effective stress is 0 as the cell pressure is transmitted to the pore water.

I really dont think people get Su/cu from CU tests. How do you do it, consolidate 3 samples to different pressures. One below your sigma'v and then two above it. You can plot 3 mohr circles with a failure line. Then go to your sigma'v and where that meets your failure line, go to your y axis and determine your undrained shear strength (or total stress c). I have never seen it discussed in a text book or lecture, I have checked the FWHA manual that DSgeo reference and couldnt find discussion on it. I have never seen anyone interpret cu from CU testing in practice.

In my experience, people use pocket penetrometers, vanes, CPT, UU, UCS (regrettably) and cu/sigma'v ratios / SHANSEP.

I would strongly discourage the use of UCS for determination of cu. The test does not apply any confining pressure so can not in anyway be considered representative of real world conditions.

 

[geomane]

Here is a screenshot from Soil Strength and Slope Stability discussing how to determine undrained shear strength from a CU test. We do this occasionally, if we have the budget.

 

[EireChch]

Geomane - thank you for that. I have looked so many times to find something similar!

I see this is from the 2nd addition, I only have the first addition so never seen it!