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Permeability, unloading

Permeability, unloading

Permeability, unloading

Hi fellows

I appreciate it if anyone can refer me to a paper or text book talking about the change of soil permeability during unloading phase of consolidation test (Oedometer, Rowe cell or CRS test)
I believe that permeability in any given soil is only related to the void ratio and stress level doesn't have anything to do with the permeability (other than through changing the void ratio) but couldn't find it clearly mentioned anywhere...

Thanks for your help

RE: Permeability, unloading

there's a direct correlation between Mv and k. You can calculate Mv for a given time of any load increment.

Correlations should be in any geotechnical text book.


¡papá gordo ain’t no madre flaca!

RE: Permeability, unloading

Thanks for your reply f-d
if you mean the equation which has mv and cv, it couldn't be used to directly calculate k since both mv and cv change with effective stress.
it's relatively straight forward to calculate mv (let say based using MCC model)but cv is another parameter which should be determined here.
In my opinion, in any specific soil (the mineralogy and structure is not changing), the permeability is only controlled by the void ratio and any other parameter like effective stress or OCR would change the permeability through their effect on the void ratio. So I'm basically looking for a reference talking about it not calculating the permeability based on cv and mv.

RE: Permeability, unloading

terzaghi and peck (I have the second edition), p. 178:

cv=k/(GAMMA W*mv)

What more is there to say? This is a direct correlation that takes into account the affect of stress on the soil structure under varying loads. As the direct result of an actual test, the relation to stress history is taken into account (after all Pp will affect Cv and Mv). Terzaghi and Peck also have some accompanying text.

I guess I'm not sure what else there is to offer. . .


¡papá gordo ain’t no madre flaca!

RE: Permeability, unloading


I know wot you are saying...k=cv*gamma w*mv...true...but as I said in my last reply, both mv and cv are changing with stress...that's right if one can have mv and cv from a test it's possible to work out k based on this equation...well, after considering the difference between compression and swelling mv and cv.
This question is more about the physics of the problem not the equation...I say the permeability is defined by the size of the "holes" (you read void ratio) that fluid can pass through and the shape of thos holes (structure of the soil)...so if you have same void ratio, regardless of OCR, the permeability should be the same... that's wot I'm looking for a reference for...I might be wrong but I cant see the problem with this simple explanation


RE: Permeability, unloading

overconsolidation can affect cementation, which can lead to lower apparent porosity. Is the permeability of flocculated clay the same as the permeabilty of dispersed clay when the void ratios are the same? Two soils of the same grain size and same void ratio, may not have the same permeability.

You don't need to worry about any of this though if you do a field/lab permeability test. I'd first say that correlations to k from consolidation tests are not too precise, but the mechanics of the test will show you a relationship for THAT SOIL, including its stress history, grain size and apparent porosity.

I'm sure there are formula for this though. . .


¡papá gordo ain’t no madre flaca!

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