Correlation between horizontal and vertical permeability coefficients 3

[CHBK]

Hi!

Is there a reference to use for determining a correlation between horizontal and vertical permeability coefficients for different soils?

Thanks,
BK

 

[DBNodurf]

Since we're dealing with soil, its never going to be the same; soil is nonhomogeneous with regard to variation btwn vertical and horizontal permeability. What do you want to do?

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[Focht3]

Bruce is right; although you will see indications in the literature that the "micro-permeability" of soil is as much as an order of magnitude greater in the horizontal direction. I don't subscribe to that view as a rule of thumb, however.

And as Bruce asked, "What do you want to do?"



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[CHBK]

Thank you for your replies. I am currently designing wick drain system to increase the time for primary settlement under an 11.5m high embankment. The soil below is about 7m of clay with a cv (vertical) of 5E-04 cm2/sec. It does not seem realistic to use this value for wick drain design, hence the question. Bowles mentions that the horizontal permeability could be as high as 4 to 5 times the vertical (pg. 354, 5th Ed.) without qualifying the statement.

Any guidance would be appreciated.

Thanks again.

 

[BigH]

I subscribe to the comments that John and Bruce put forth. We would really need to know the type of soil you are talking about and you don't really mention anything specific - other than 7m of clay (original thread just said soils). What are the depositional characteristics of the deposit? This is very important - such as in varved clays where the conglomerate horizontal permeability might be >100 times the conglomerate vertical permeability. In clayey tills, it is not likely to be much different, I would presume. You should do a deeper search than Bowles - look for papers by Bjerrum, Skempton, Bishop, Janbu, as examples - they all had extensive clay soil experience.
Good Luck!

 

[BigHarvey]

For wick drain design it is common to take Ch between 1.5 and 10 times Cv. Generally the choice of the coefficient is done according to the knowledge of the material ( geological description, history, etc ) and the confidence about the lab results, and the economical consequences .
The Cv value you mention has nothing exceptional and I would not go for more than 5 for this type of value.
If your project is really big you can think about a trial zone to check the real consolidation properties.
if it is not the case vertical drains are cheap enough to consider a coefficient of 1.5 or 2. Check also if you want to take a chance if you can put things right again by increaing the height of surcharge if it does not work and consolidation time increases too much.

 

[CHBK]

Hello!

Thanks again. The clay encountered in this region, is glacial in origin, the sand, silt and clay components vary between 20%-40%, which is characteristic of glacial clayey tills. The LL, PL and PI are about 30-35, 15-20 and 15-25 respectiviely.

Hope this helps.

BK

 

[DBNodurf]

So the actual clay MINERAL content may not be all that great. The ironic thing about the Atterberg chart is that CL soils often are mostly silt and fine sand with a lot of kaolinite or a little bit of montmorillonite, or various other possible mixtures. Since the clay is of glacial origin, it probably was originally rock flour, and the clay-SIZED minerals were not clay minerals at all. A really terrific reference to help visualize this is a special version of the Atterberg chart in Mitchell.

The bottom line is that the fewer the actual clay mineral content, the less difference you will see between the vertical and horizontal permeability.

The absolute best way (from a hydrogeologist's viewpoint) is to conduct a pumping test!

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC

 

[VAD]

Hello CHBK:

Can you provide the stratigraphy of the site as it woulfd be of interest to know if there is a drainage layer with depth as well.

 

[npthao121]

Dear CHBK,
This is the table extracted from "prefabricated vertical drain: Vol 1, engineering guidelines" by Rixner J.J, Kraemer S.R & Smith A.D

Soil Kh/Kv

No evidence of layering (partially
dried clay has completely uniform appearance) 1.2+-0.2

No or only slightly developed macrofabric (e.g 1 to 1.5
sedimentary clays with discontinous lenses and
and layers of more permeable soils)

Slight layering (e.g sedimentary clays with 2 to 5
occasional silt dusting to random silty lenses)

Fairly to well developed macrofabric (e.g 2 to 4
sedimentary clays with discontinous lenses and
and layers of more permeable material)

Varved claus in Northeastern US 10 +- 5

Varved clays & other deposits containing embedded 3 to 15
& more or less continous permeable layers
(soft clay is defined as clay with an undrained shear strength of less tha 1000 psf)

In addition, a research done by Prof Bergado (Asia Institute of Technology) for the bangkok soft clay showed there is a big variation between the lab test and field test. as his result Ch(field test)/Ch(lab test) = 4 and Cv(field)/Cv(lab) = 26 for the Banhkok soft clay.
Also he got Kh/Kv = 4-10.

Hope this help

 

[CHBK]

Dear NPTHAO121

Thank you for your post. It is just what I waslooking for.

CHBK

 

[CHBK]

Hi VAD,

In response to your question, the clay/stiff till (N<12) layer is underlain by a 4.2m thick firm glacial clay layer (14<N<50). Although the logs call it out as &quot;sandy&quot; the tests don't seem to support it. Also, there is no evidence that the sand seams if any are sufficiently continuous to permit drainage. Therefore, for consolidation purposes, I have been assuming single drainiage to a granular drainage blanket installed on the surface.

Hope this helps.
CHBK

 

[DBNodurf]

Plug & Chug...

The main thing is not to overestimate Kh; its ok to underestimate it because its ok if the consolidation happens ahead of schedule with less surcharge because that saves money. Your Kv seems really high for a true clay (about 100x if its consolidated), so you could just use your Kv as your Kh, predict required surcharge and time, then monitor actual settlement and calibrate your Kh assumption with it.

D. Bruce Nothdurft, MSCE, PE, PG, M.ASCE, etc, etc,...
Principal Engineer/Geologist
Atlantic Geoscience & Engineering
Charlotte, NC