As it is known, the friction angle is an intrinsic property of the soil while the undrained compression strength is not. The undrained shear strength can be found from the unconfined compression test, and if a relationship with the modified cam clay model it is done, it is possible to obtain the drained friction angle.
This angle friction is obtained to a certain depth using the parameters obtained from the UC test. Now if I go backwards modeling the test with this friction angle I won't be able to get the same strength, because the parameters used to get the friction angle was a k-consolidated soil while the UC test is not confined, and a lower strength is expected.
My question is: How is it possible to successfully model the UC test after getting the friction angle with this K-consolidated soil?
Thank you.
Hi Lissette Janet; Please go through the given links. They will be helpful to you in your research work. Thanks
http://www.uic.edu/classes/cemm/cemmlab/Experiment%2013-Unconfined%20Compression.pdf
http://www.researchgate.net/post/MCC-Unconfined_compression_UC_test_estimations?_tpcectx=qa_overview_loggedout
See Textbook of Soil Behaviour and Critical State Soil Mechanics by Muir Wood (1990).
It is very interesting dicussion that your question is based on the critical state theory of famous cambridge scholar ,easily will be understood by "An introduction to soil mechanics by Atikinson"?
Lissette,
Please write precisely what kind of analysis are you performing. I assume this is one element FEM anaysis. Am I wright?
What parameters do you introduce into your analysis? Especially, what is the elastic part of modified Cam clay model? Two attempts are possible: constant Poisson ratio or constant shear modulus.
Is your soil normally consolidated (NC), which means after Ko consolidation is there any unloading or not?
Check whether you defined properly undrained conditions in your analysis. This should be something like consolidation (flow+deformation) but with very small permeability coefficient (e.g. k=10-10m/s) but high rate of load (i.e. load increase in short time - the shorter the better; try 1 min or 1 hour). Assume flux on boundaries equal to 0.
Maybe you do not need to perform a numerical simulation at all? In the book by David Muir Wood mentioned above and also in the book by Muni Budhu "Soil Mechanics and Foundations" you can find analytical solution (one simple equation) for undrained stress path for normally consolidated clay. There is a simple link between undrained strength and the slope of the critical state line M (cu=0.5qf=Mp'i/2(l-k)/l+1). Here l, k should be substituted by Greek letters lambda and kappa (I'm sure you know what I mean). p'i is the initial mean stress. You start the analysis from p'i and q=0. And certainly you know the relationship between M and the critical friction angle that you are looking for (M=6sin(fi)/(3-sin(fi) ). This is for triaxial compression.
The evaluation of undrained shear strength for overconsolidated clay is according to: cu=0.5qf=0.5Mp'i/(Ro/2)(l-k)/l; where Ro is overconsolidation ratio (Ro=Rp according to Atkinson). It does not matter how the clay was overconsolidated.
Thanks a lot Mr. Krzysztof Sternik, the undrained strength along with the deformations are needed.
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