Relationship Between Drained and Undrained parameters 1

[gaurav33]

Hello there, I have a question to for you guys out there, how do we work out the Effective c' and phi' from a undrained triaxial with PWP. Is there a formula i can use.

Thanks

 

[BigH]

See Chapter 21 of Lambe and Whitman's Soil Mechanics.
sin(phi)' = tan b where b is angle of line of q-p' plot(two dimensional - sigma1 and sigma3)
c' = a/cos(phi)' where a is intercept of q-p' plot.
Octahedral stresses (sigma1, sigma3, sigma2) is different.

 

[gaurav33]

Guys i have a soil sample which is a Clayey Material and i have got results from the CUPP of c'=63kPa and phi'=9.5 deg. This to me is a bit weird a not to sure what to make of it??

 

[BigH]

It sounds a bit like I used to see in Indonesia back in the 80s - they run the test too fast and the pore pressures never equalized - i.e., they ran the test in 20 minutes like a quick triaxial - measured the pwp from the transducers - but the readings were bogus. . . .

 

[gaurav33]

Guys I just do not understand it have 2 ore test come back with s similar result but the thing is that they are not even from the same site and I am getting similar results from the Triaxials, i am thinking there is more to this, any thoughts what we can do

 

[BigH]

Who is doing your lab testing? What country ar you in? Again, I would check out their testing times to failure. To measure pwp in clays, it will take nearly 8 hours or even more to complete one stage for the triaxial. With phi' equals 9deg - either the material you have is "fully disturbed" and you are measuring the residual friction angle - or the test is too fast and the pwp do not fully equalize across the sample. Observe them doing a test - when I confronted the tech in Jakarta back in 1988 who, bhy the way, was a graduate of the Asian Institute of Technology outside Bangkok (good school) - he told me that he failed the pwp test in 20 minutes. I told him it was too fast - he insisted and went to get his book to show me - and I showed him where he had misread - or misunderstood - what the book said.

 

[paganel]

I agree with BigH.
It might caused by disturbance.
Another reason might be the stress history of the soil. Is it highly overconsolidated? The test must be carried out properly.