back pressure during saturation

[tonette]

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.

 

[GeoPaveTraffic]

Tonette,

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.

 

[tonette]

Hi GeoPaveTraffic. Thanks for putting some thoughts on the matter I raised. Here is a sample of the data I gathered. For instance, increasing the cell pressure from 50 Kpa to 100 Kpa (with back pressure line open), the pore pressure increment is 15.5 KPa, with absolute pore pressure reading of 48.5 KPa (note that this is an instantaneous value, upon application of CP. I didn't let it equalize, I immediately applied the back pressure increment). Immediately after raising the back pressure from 20 KPa to 80 Kpa, the pore pressure change was 22 KPa, with absolute pore pressure reading of 70.5 KPa. Then, after 30 min, the pore pressure rose to about 90 KPa (greater than my 80 Kpa back pressure), then gradually went down to the 80 KPa level. So my question is why does the pore pressure gets as high as 90 Kpa?

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.

 

[GeoPaveTraffic]

My guess is that drainage through the sample is poor, possibly due to the large amount of air in the sample. You have a very low degree of saturation. Also, air in the lines or porous stone between the sample and the measurement points could be causing problems.

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.

 

[morecoffeeplz]

The pore pressure should always increase in reaction to a cell pressure increase. If the pore pressure increases to the cell pressure stress, there is either a leak in the membrane/lines or the specimen is 100 % saturated. I agree with Geopave about the slow drainage through the specimen. Side drains are quite effective in aiding pore pressure dissipation.