Consolidated Undrianed shear strength of NC clays

[ZooKaa]

Hi all,
I have done 3 Consolidated Undrained triaxial tests on typical normally consolidated clay samples. When I drew the 3 Mohr circles for the total stresses I figured out that to get the best tangent line, the M-C line should cut the Y axis in a value > 0.0 and that means that these NC clay samples have both undrained C and ?. I dont know the preconsolidated pressure of the samples yet, but I suspect that the least cell pressure was less than the preconsolidated pressure of the samples in the field therefore the samples became overconsolidated after the consolidation phase and consequently the samples produced C. Am I right? and are there any other possible reasons for that?
Thanks

 

[dgillette]

Start by looking at Chuck Ladd's Terzaghi lecture. It is the best single reference I know of for undrained strength. Although the title refers to staged construction, many of the principles would apply to excavation or one-shot construction of a fill.

Ladd, C.C. (1991), "Stability Evaluation During Staged Construction," the 22nd Terzaghi lecture, ASCE Journal of Geotechnical Engineering, vol.117, no. 8, pp. 1540-615.

If the samples are NC (or close), you should expect to measure Su something on the order of 30 percent of Sigma'C, if you are using CIUC (isotropically consolidated undrained compression) tests. Note however that the stress boundary conditions during consolidation in the ground are not isotropic, and you should expect something less, perhaps 20 to 25 percent, depending on the geometry of your problem.

DRG