Pore pressure response during CIU test

[harrohb]

Hi all,

I have what I believe is a pretty trivial question about PWP response during CIU testing

So at the end of the saturation stage, your B value should be >0.95 so that any increase in the cell pressure is met by a pretty much equal response in pore water pressure, all good.

My question is, looking at tests at the "shearing stage" this no longer seems to be the case - the pwp follows the same trend as the deviator stress, however it does not increase in equal amounts even though the sample is saturated and the test is undrained. Is this because after consolidation, some of the stress is taken up by grain-grain contacts instead of the pore water? Any further explanation would be greatly appreciated

 

[LRJ]

It's to do with excess pore-water pressure. Think of it in terms of the stress path: during an ideal saturation stage, the effective stress would remain constant (i.e. cell pressure minus pore-water pressure is constant), and usually the deviatoric stress would be zero. In contrast, during the shearing stage, excess pore-water pressure develops as the deviatoric stress increases, leading to curvature in the stress path.

Aside from this, it is also worth noting that the constant moisture content saturation you allude to is normally done by application of cell pressure, whereas shearing is normally done with application of deviatoric stress. The displacements during saturation are also intended to be negligible, whereas shearing to a particular strain is required in the shearing stage. So, in short, there are various differences between the stages.