Permeability of clean sand vs density

[rj_geo]

Is there a way to see the relative difference in the permeability of clean sand for loose sand (N = 5 to 10) and for very dense sand (N > 60)
Basically there are two sand layers with significant increase in blow counts in the lower layer hinting towards a geologic change. Trying to see if there is any literature showing such denser layer have relatively lower permeability.

 

[GeoEnvGuy]

Permeability K the hydraulic conductivity should be the same if the material is the same only varying the density. The porosity n will be lower with higher compaction.

With a loose deposit which has a higher porosity you will have a higher linear velocity and smaller drawdown cone of influence. To test this you can install 2 wells near each other and conduct a pump test to see the drawdown effects.

 

[rj_geo]

I’d think the void ratio would be lower in the dense sands and the permeability can be lower. I believe Slichter chart also indicates the same thing?
Basically there are two sand layers with significant increase in blow counts in the lower layer hinting towards a geologic change. Trying to see if there is any literature showing such denser layer have relatively lower permeability.

 

[fattdad]

I'd think compaction a 1.5 to 2x factor? I think the precision in ANY permeability test is wider!

Permeability is not a very precise value, in my opinion. . .

f-d

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[EireChch]

f-d agree, there are so many things that affect k. If you are within one order of magnitude you are doing well.

 

[rj_geo]

I think the question is- Is it safe to assume the permeability of denser layer to be half order magnitude lower(one-fifth) than the layer above ? any literature or source to support that?
any available data or experience with results that it's not unrealistic variation?

 

[IRstuff]

Doesn't the effective grain size also come into play? Are the two sand layers of the same grain size and percentage fines?

https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs141p2_024050.pdf

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[rj_geo]

It does depend on D10 (and may on D5) and percent fines, yes. But they do have same grain size (D10 ~ 0.15mm)and %fines (5 to 7%)

 

[Ron]

When you decrease the porosity by compaction, you will increase the head loss of water flowing through those interstices. An increased head loss results in a lower permeability. The difficulty as f-d and EireChch noted is the accuracy of measurement. Might not be able to tell a difference in clean sand. If I can tell the difference between 10^-3 and 10^-6 I'm happy!

 

[rj_geo]

Thank you for the replies all.
I understand the difference is going to be small. But that's what the questions is. Has there been any study to show the difference will and can exist. Is it reasonable to assume the permeability of denser layer to be 1*10-3 as compared to 5*10-3 of loose (less dense) sand layer above.
I guess the consensus is that difference is small to have been studied before? and you guys haven't come across such a study?

 

[IRstuff]

If Slichter's chart is gospel, then a factor 1.5 change in density is roughly 10x difference in permeability, but this paper shows variations in permeability on the order of 1.5 magnitudes at 95% confidence.

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[fattdad]

just for the record a claim of 1x10-3 for dense and 5x10-3 for dense sand is a 500% increase or an 80% decrease. Not sure how 20% equates to a half-order magnitude change?

I'm confident that density is a factor. Don't know how to quantify. I do know if you do a borehole test (with proper boundary conditions), you'll get a better answer.

I'm also mindful of how permeability is non-isotropic in layered soils. We get to consider a factor of 2 to 4, for example, when considering the effect of wicks on Cv/Ch.

Interesting discussion.

f-d

ípapß gordo ainÆt no madre flaca!

 

[rj_geo]

Ok.. not sure what exactly a half order of magnitude is defined as. I took it as 10^0.5 = 3.16. Nearly 300% increase or 30% decrease. I loosely said half order of magnitude, but was using 20% decrease.

Sorry for not being too specific.

Thanks.