Help Please - permeability testing above groundwater table

Question

Help Please - permeability testing above groundwater table

Blairsy (Geotechnical)

(OP)

3 Jul 17 17:26

Here is my issue:

I was recently tasked with conducting field permeability tests inside cased/partially cased boreholes at a site with low permeability (glacial till) deposits and a deep groundwater table to aid in appropriately sizing a proposed stormwater infiltration chamber. All of the references I have come across to calculate the horizontal and vertical coefficients of permeability based on variable (falling) head tests are with respect to infiltrating into saturated (i.e. below the groundwater table) soils. However, in my case, the groundwater table is located at great depths below the proposed base of the infiltration system, and I am 100% certain that I was infiltrating into unsaturated soils. A summary of my testing procedure is as follows:
1. Advance boring to a depth of 14 ft. (proposed depth of chamber base) using driven steel casing and rotary drilling methods
2. After the casing was "washed" out, we filled the casing with 2 ft. of filter sand and lifted the casing up 1.5 ft. (leaving 6 inches of sand inside the casing, and 1.5 ft. of sand below the casing)
3. Fill the casing with clean water to the top (reference point) and measure the head drop at specific time intervals

Am I overthinking this? Is is simply a k = Q/A scenario? Any and all advice is much appreciated.

oldestguy (Geotechnical) · Answer 1 · 2017-07-03 21:22:23

No need to double post because most of us look at many rooms.

BigH (Geotechnical) · Answer 2 · 2017-07-04 04:40:40

sorry oldestguy - but I can't find the other post . . .

https://www.rgc.ca/?page=page&id=62 - try this as a start - Infiltration testing in vadose zone.

Blairsy (Geotechnical) · Answer 3 · 2017-07-04 12:42:49

Thanks BigH, but I didn't find any info at that link that helped me. Most of those methods use permeameters that allow a constant head to be applied. In my case, we simply filled the casing and measured the head drop through time.

dik (Structural) · Answer 4 · 2017-07-04 12:51:21

Pretty much like a percolation test for septic system design. I have an article on constructing a constant head percolation 'tester' that I'll try to find.

Dik

BigHarvey (Geotechnical) · Answer 5 · 2017-07-05 08:36:20

Maurice Cassan in his book "Les essais d'eau dans la reconnaissance des sols" (Eyrolles, Paris, 1980) wrote a chapter about permeability tests in dry soils. Basically, it's a Lefranc test with a special interpretation by a russian scientist called Nasberg.

fattdad (Geotechnical) · Answer 6 · 2017-07-05 11:48:42

there are factory tools to help. Guelph and Johnson make permeameters, based on the equations of the USBR.

There is also a method adopted by the county of Fairfax, Virginia that includes a 4- or 5-in diameter pvc pipe placed to the target depth and surrounded by bentonite. After a 24-hr soak, you take measurements of infiltration rate over time.

All of these tests engage very small areal footprints of the soil and conclude some infiltration rate for those boundary conditions. When you scale the test results to the actual area of the infiltration practice; however, the results may not be as intended. . .

To take an area of 10 or 20 ft square and infiltrate, you are assuming that the water has some place to go. Does it? At some point a mound will develop and a mound (i.e., by its very definition) mobilizes horizontal flow, so, the gradient=1 condition is not what's driving infiltration. All infiltration tests above the water table area based on gradient=1.

Good luck.

f-d

ípapß gordo ainÆt no madre flaca!

Blairsy (Geotechnical) · Answer 7 · 2017-07-05 12:58:44

Thanks f-d, I have looked into that method based on some of your other posts: but, unfortunately I have already completed the tests and the procedure I used was not in accordance with the Fairfax method. Would any of Hvorslev's equations apply to the procedure/boundary conditions of my tests?

fattdad (Geotechnical) · Answer 8 · 2017-07-05 13:48:25

Sure, it's worth looking at the Hvorslev boundary conditions and seeing if the equations give meaningful results.

I'd also suggest grain-size correlation to permeability, to get some sense of what you expect. There are correlations to D10 and D20, that I've used with some success.

f-d

ípapß gordo ainÆt no madre flaca!

BigH (Geotechnical) · Answer 9 · 2017-07-06 10:44:10

Some others - may or may not help you - but good to have - especially the Soakaway reference
http://info.ngwa.org/gwol/pdf/891849364.PDF
http://www.eng.utoledo.edu/civil/heydinger/unsatur... (seems to be lab oriented, though)
http://www.robertsongeoconsultants.com/hydromine/t...

BRE Digest 365 - Soakaway Pits
http://idox.bathnes.gov.uk/WAM/doc/BackGround%20Pa... (Design of soakaway pit - talks about infiltration)
http://www.yourspreadsheets.co.uk/soakaway-design.... (a free "lite" spreadsheet is downloadable)

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