Falling Head Permeabilty Head Scratcher

[caddyr2]

So I am running falling head perms on both remolded and shelby tube samples and I notice that mainly in some shelby tube samples after about a 5 days soak period and when i begin taking readings there is an occation that a sample may not drop. Even after an entire day it does not drop even 1 cm. I am running 13' of head so there should be pleanty of pressure to complete the test. Occationally I notice that if i pull the cover off the sampler, reinstall, and flush the entrapped air out of the lines and mold that the sample drops. Normally this fixes the problem until the sample I have in now. It has not dropped in 3 days! In fact, it has raised up the plastic tubing I use to take readings as if the sample swelled, although it is not a fat clay and should have a low swell. I had previously been told that over time all samples will permeate but after 3 days my previous thought is beginning to change. Anyone else got advice or had this problem? By the way I did scarify both surfaces so that it is not being sealed during trimming. I'm pulling my hair out on this one.

 

[iandig]

What's the predicted permeability of the soil/soil type under test? Falling head tests are suitable for a range of soil permeabilities but not all soils. If you are expecting very low permeabilities i.e. less than 1 x 10-9m/s, you may have to go to a constant head test. As part of this you will go through a saturation and consolidation phase, and even where you get very low rates of flo through the sample, it will perm.

 

[caddyr2]

Well I wasn't expecting a rate less that 1x10^-9 based on past tests on similar soils. I am letting the sample sit for an extra week to see what happens. I predict it at more of 1x10^-8 or faster. I should probably start looking at constant head. Thanks.