UU from CU

[moe333]

I have a 3-point CU w/PP TX on a remolded clay sample. The lowest confining stress is 1500 psf and I have a decent total stress c-phi. Is there a good way to estimate the corresponding remolded UU strength from the 3-point CU? The CU was back pressure saturated but the UU strength I want would be remolded wet of optimum but not completely saturated.

I think taking the UU strength as the c-intercept of the CU would be underestimating the UU strength.

Thanks

 

[BigH]

I'm don't see how you can estimate a remoulded strength from the CU w/pwp - or any test that determines the strengths based on maximum shear stress. What is the sensitivity of the clay? Is it somewhat sensitive or massively so like the Leda Clays of Canada? I think I see where you are going, though, as the sample strains more and more, it would "travel down" the failure line in the q-p' plot - but the c' intercept would be zero unless you set some arbitrary strain. If I remember correctly, the direct shear test would be better at determining the remoulded strengths.

That's my thought off the top of my head - I'll dream on it in a few minutes . . .

 

[moe333]

BigH, I should have said compacted UU strength rather than remolded. I'm trying to estimate UU strength at 95% of ASTM 698 from CU test also compacted to 95% of ASTM 698.

 

[fattdad]

The undrained shear strength would be a horizontal line from the total strength failure circle as determined from each of the consolidation states. As the void ratio varies by each consolidation step, pick the state of consolidation that relates to your 95 percent relative compaction.

Bear in mind that the actual field condition may not be as you are testing as compacted clay fill rarely becomes fully saturated, even if it becomes below the water table and also as it is often not placed in an environment where it comes below the water table.

(you can jam a pocket penetrometer into the Proctor mold also to cross check.)

Hope I'm helping, it's been awhile. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

The difference between Su strengths and Effective strengths is that in the former you don't know the pwp to apply at particular stress states. You do know the pore pressure in your test - so develop a q-p plot instead of a q-p' plot. Shouldn't that tell you the Su value?

 

[moe333]

The application is evaluation of end of construction stability. When placing a high compacted clay fill the end of construction strength is conservatively estimated by the UU strength of the clay compacted to the anticipated field M&D. The lowest CU strength I have is 1500 psf (saturated and consolidated test). I also have two tests at higher confining pressures. I'm trying to find a way to estimate the unconsolidated strength UU from the CU. The difference between the two is the UU will not be saturated (at least at low confining pressures) and the UU has not been consolidated.

 

[dgillette]

In theory, you could estimate the UU envelope from CU, but it would require really good volume and pressure measurements during the consolidation phase, so you could back out p' vs p total.