Perm Saturation Time

[caddyr2]

Ok, here's a new question that I'm sure will have alot of different answers. I am running falling head perms using a compaction mold. How do you judge times that it takes to saturate a recompacted sample? Is there a time of saturation calculation that I can use or do I just have to follow the lab readings and find when the drop levels out? I am fairly new at running these tests and just want to make sure I am giving the samples ample time to saturate before passing/failing the tests. Thanks for any help.

 

[fattdad]

I'll take the first stab at this. There is no way to get saturation just using the method that you are using. Much of this depends on the soil type, but for clays just being below the water table isn't good enough. Typically when the inflow matches the outflow you are as close as you're going to get, but that isn't necessarily saturation. The problem is when fine blebs of air get trapped in a fine grained soil, flowing water just sees these blebs as another obsticle and does nothing to take the place of the air void. When there is suffient water pressure, these blebs go into solution and you do get saturation. In some clays it can take tens of feet of water pressure for this to occur.

Not sure, but hope this helps.

p.s. unsaturated permeability is a lower value than saturated permeability.

f-d

¡papá gordo ain’t no madre flaca!

 

[caddyr2]

I am using 13.5 feet head for my sample to saturate it. The samples are typically for lagoon liners and require as close to 100% saturation as possible. I see where you are getting to and I agree. So basically what you are saying is that there is no way to determine time to saturation? It sounds like this test is more of a "user defined" type of test wheras the user defines when they think saturation has been reached. I wonder if reverse saturation would help at all on this, wheras I introduce the water from the bottom of the sample and force the air out of the top. Thats not how it will work in the field in the actual lagoon but it should help. Only problem with this is that I tried it once for a very lean clay material and it basically pushed my sample upwards compressing my spring and ruined the test. Its frustrating sometimes.

 

[fattdad]

For low ranges of permeability you really need to use a triaxial chamber with appropriate back pressure/cell pressure to get saturation. Whether the flow goes vertical up or down makes no difference for a constant head test, not sure whether it would affect the falling head test - maybe not. . .

Another factor, did you compact the sample on the wet or dry side of optimum? If this is a recompacted sample, you will realize great saturation at the start of the test if you compact it wet of optimum (duh, eh?). The big benefit however is you will also get a MUCH lower saturated permeability value after fully saturated (i.e., as compared to the sample compacted dry of optimum). The difference can be an order of magnitude!

Good luck in your work. You should be able to find a local geotechnical laboratory to do a recompacted perm test for a few hundred bucks, however.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

Maybe the partially saturated K is a more realistic depiction of field permeability than the saturated K. Unless the lagoon is quite deep (so all the air goes into solution, like in a back-pressured triax test), there would still be some air blebs, perhaps more than with bottom-up soaking where air can escape upwards.

Will regulators buy that? I doubt it. So, it's back to back-pressured tests in a triax cell. Besides, if the material has really low K, the tight fit of the membrane is sometimes necessary so the contact between the sample and the mold doesn't leak and give you falsely higher K.