Triaxial test - sample saturation

[dan00]

I'm running a CU test and I'm not sure I fully understand the process of driving the air into solution.
The way I understood it is: same pressure is applied at the top and bottom of the sample and the air driven in the solution is not drained out of the sample. A colleague is telling me that we should create a circuit where water flows from top to the bottom of the sample and carry air dissolved into solution out of the sample.
Thank you

Dan

 

[fattdad]

The triaxial sample allows for water pressure inside the membrane (i.e., where the sample is, or backpressure) and outside the membrane (i.e., the cell pressure). The principal of effective stress allows you to have backpressure just below the cell pressure and leave the state of stress in the sample unchanged. If you increase the backpressure (and correspondingly increase the cell pressure) you will remove air into solution and saturate teh sample.

Flowing water won't do it and it'll take you a long time to realize you failure (i.e., you'll get bad pore pressure measurements during the shearing of the sample.

In summary: You are correct and your colleague is not quite right.

f-d

¡papá gordo ain’t no madre flaca!

 

[jfljoe]

I think the colleague may be referring to the initial saturation of the lines, prior to the saturation of the specimen. However, at this point, an insignificant amount of air may be driven into solution.

Upon setup of the triaxial cell and sample mounting, some air inevitably is trapped within the lines and possibly in the porous stones. A very small gradient along with a very small effective confining pressure and leaving one side (usually top) of the sample open to draining can flush out some amounts of air, alleviating the need for large amounts of back pressure to drive the air into solution during the sample saturation phase. I usually apply 2 or 3 psi cell pressure (depending on OCR), 1 psi back pressure to the bottom and drain the air out of the cell/sample from the top at atmosphere. One burrette of water is sufficient normally.

Partially agree with fattdad: colleague definitely not quite right, but if you have flow during the shearing you are probably not running an undrained test. "Flow" should cease after consoldiation phase is deemed complete.