Constant vs Falling Head Permeability 2

[soileater]

I have samples of clayey sand and clayey gravel (weathered granite) that I want to test for permeability. Which method should i do - falling or constant head?

 

[Gimbli]

If clayey content is so that permeability of material is highly reduced I suggest to perform falling head tests. This method is usually perform for soils having a permeability in the range of 5x10^-5 and 10^-3.

 

[Gimbli]

If clayey content is so that permeability of material is highly reduced I suggest to perform falling head tests. This method is usually perform for soils having a permeability in the range of 5x10^-5 and 10^-3 cm/s.

 

[soileater]

Confused now....surely 10 Exp-5 to 10exp-5 is medium permeability and certainly not low. You recommend falling head for highly reduced permeability, but to quote iandig from aprevious thread: "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." Who's right?

 

[fattdad]

How will you obtain the samples (i.e., drive tubes, recompacted or such)? What is the end game (i.e., how will the permeabilty value be applied)?

My favorite tool for permeabilty testing is the triaxial cell as it allows backpressure saturation. Just not too sure how this would work for a clayey gravel sample.

If you are compacting the sample into some steel mold and then applying a pressure head, I'd likley use a falling head test for lower permeability soils.

If I had the option of performing a field test, that's what I'd consider first (i.e., Boutwell permeameters or slug tests).

Would like to know the context of your study, however. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[soileater]

thanks for reply....the application is for installing fuel tanks underground in a petrol station. we need some idea of whether possible fuel leaks/spills will permeate the soil and affect groundwater or is the soil relatively impervious. The samples are undisturbed but apparently too small for triaxial cell (50mm diameter x 100mm high). I think we may just have to base our eductated guesses on particle size distribution and possibly Hazens formula. I dont like the idea of remoulded samples. What do you think? Is this acceptable.

 

[fattdad]

For the case that you describe the mobility of fuel vertically downward through unsaturated soil may not be properly described by evaluating the saturated permeability in the laboratory (i.e., using the triaxial cell to get full back-pressure saturation and such). You are limited by your sample size whether you consider falling head or constant head, actually. How would you adapt a 2-in diameter sample to the permeabilty cell?

Will the tank bottom be significantly above the water table?

I'd likely do three or four different things: Run a sieve and hydrometer and consider 1) correlation between "texture classification" (USDA soil triangle) and "perc" rate; 2) D10 or D20 correlations to hydraulic permeability (i.e., Hazen's formula); 3) Conduct a field permeability test (i.e., ala Hvorslev); 4) Conduct a standard-issue "perc" test.

Fundamentally, what you want to know is the rate that the permeant will migrate vertically downward at a gradient of 1.0. The perc test doesn't really tell you this as the hole size allows for horizontal spread of the permeant below the point of discharge. That said, it's unlilkely that the ENTIRE tank will fail at once, so a perc test may give you a basis to understand a practical migration rate.

Just some thoughts.

f-d

¡papá gordo ain’t no madre flaca!

 

[iandig]

Just to add to some of the previous comments made above. You cannot ignore the potential difference between a sample tested in the lab and the insitu/field permeability. I agree that some gradings of the material will help, if the clay content is greater than 15% [finer than 2µm] then go for a constant head test in a permeability/triaxial cell. If it is less than this, a falling head test should provide an accurate assessment of the 'lab sample'. In-situ permeabiltiy will be a function of macro as well as micro conditions, so in-situ assessment should also be done. I would also reccomend looking at the in-situ bulk density of the soils and calculating the total voids, satuated voids and air filled voids of the ground [you will need a measured particle density to get an accurate assessment, if the ground has been contaminated already, as suggested, then you need to measure not assume]. This can aid in the assessment of in-situ permeability. Also need to consider the phreatic surface and any perched water that may be there. If the material is very granular, such as a sandy gravel, there is also a 'constant head' test for this, but if your ground is that permeable, you are in a whole different ball game!. We would use a conceptual site model, followed by investigation and plotting of contamination plumes in conjucntion with assessment of the grounds permeability & porosity as partof the assessment process forthe redevelopment.

 

[Ron]

If you are concerned about fuel leaks and permeability of the soil with respect to the fuel, then you need to run the test using fuel or combination of fuel and water. Soils react differently to permeability of petroleum products as compared to water. Permeability of petroleum products is often faster than with water because of the reduced surface tension and the potential to affect the plate attraction in clay particles (van der Waals forces)

If the material is clayey, use falling head test.

 

[molerat2210]

The question may be ASTM D2434 (for granular soils) vs ASTM D5084 (good for 10E-4 cm/sec or less). D2434 is constant head by definition. D5084 gives several options which are more a function of the laboratory apparatus.