back pressure during saturation
back pressure during saturation
(OP)
Hi! I am doing saturation of ML soil with plasticity index=7. Size of the sample is 100 mm in diameter and 200 mm in height. The pore pressure (out) I am measuring is greater (sometimes more than 100% greater) than my applied back pressure at each increment of cell pressure and back pressure. Is this normal? Actually, the trend is the pore pressure will increase at a high level then gradually goes down to about equal the applied back pressure. But from what I understand, it should not go higher than my applied back pressure. I would appreciate any comments, please.





RE: back pressure during saturation
I'm trying to fully understand your situation. Could you list some of the data that has been collected? Does the pore pressure increase the to the new cell pressure and then slowly decrease to your backpressure? What is the calculated degree of saturation? Are you getting flow out of both ends of the sample?
It has been a while since I worked in the lab, but will try if you post some more info.
RE: back pressure during saturation
Calculated degree of saturation is 17 percent, drainage only on top, pore pressure measurement at the bottom. Cell pressure applied on top (cell filled with water up to the top plate, then compressed air applied from the top).
Hope I gave you enough details. If there's anything more you want to know, let me know. Again, loads of thanks.
RE: back pressure during saturation
You may just have to leave it for a while, maybe a long while, and get the saturation up before you keep pushing the pressure up. If you don't have filter strips along the sides of the sample, you might want to break the test down and add them. That might help speed up the process.
As to your specific question as to why the pore pressure go all the way up to the cell pressure, my guess is that drainage inside the sample is inhibited. May be the suction pressure due to grain size and the amount of air or something else don't really know. However, it looks obvious that something is not quite right between the top and bottom of the sample.
If you haven't already I would break the test down, check all the lines, and reset it.
RE: back pressure during saturation