variance in permeability in horizontal and verical directions

[darrengavin]

is there an established ratio between the permeability of a soft soil,horizontally and vertically and if so could the reference please be submitted

 

[dirtsqueezer]

Hmm. This is just a shot in the dark, but I believe soils will have varying directional 'permeabilities' depending on your material. If you're looking at an exposed soil, permeability, at least vertical, may also have to do with surface conditions- is it cracked, is there ground cover, ect. I suspect it would be possible to test the hydraulic conductivity in the lab, but as far as I know, permeability goes usually only one direction, that is, down. Can you go into your question a little more?

 

[PSlem]

No, horizontal also as you calculate plume spread. I ran some numbers a few years ago and did not believe them. My primary work is piles, tiebacks and grouting. When you pressurize a hole you would expect vertical fracturing as K at rest is less than the overburden pressure. Yet in all instances where there was fracturing and the tieback or grout was uncovered, the grout was in horizontal seams, which suggests natural bedding planes in the residual soils. We can only estimate assuming homogeneous, but I think the numbers are overly conservative in the horizontal.

 

[dirtsqueezer]

So that definitely would be a shot in the dark. Is your issue concerning permeability or fracturing? Without going too far into the logical conclusion of how I don't understand, I'll just go with why. If you're looking at contaminant plumes, that is a model for groundwater flow, and though permeability is the core concept of those mechanics, I believe they are most commonly defined in other terms. As I understand it, as you pressurize a hole, the grout will create an equilibrium with the surrounding media due to differences in pressure and density, creating the seams you encounter. I have to admit, I'm still not sure what you're asking, though I can tell you that flow through a porous media will differ with different soil types, and the best way to determine your situation is to pull an in-situ sample and have it tested. Did you have a more specific question?

 

[PSlem]

I was stating a weaker plane is probably a more permeable plane. So I think the horizontal permeability would be greater than the vertical when looked at in a large scale, but i have not seen any references on this.

 

[jimbo2]

darrengavin:

I agree with PSlem, and the answer to your specific question is no. For example, horizontal and vertical permeability will be near equal in a lacustrine (lake laid) soil deposit if the soil texture is consistent across the zone of interest (i.e all clay or silt size particals) As you encounter any horizontal bedding of different soil partical size, or size distribution differences, the horizontal permeability will likely be higher than the vertical.

I have done a bit of work in varved, fine grained soil deposits with horizontal bedding zones ranging between clay and silt to fine sand. Large scale vertical permeabilities in this deposit are on the order of 1 x 10-7 or 1 x 10-8 cm/sec, and horizontal permeabilities in the deposit are on the order of 1 x 10-4 to 1 x 10-5 cm/sec. I have also seen lacustrine clay deposits with horizontal and vertical permeabilities of approximately 1 x 10-8 cm/sec.

Vertical permeability of soft, fine grained cohesive soils can be measured in the lab on "undisturbed" Shelby tube samples. Horizontal permeability can be measured in-place by slug testing in a properly constructed well or piezometer.

Does that help?

 

[dirtsqueezer]

Is it telling that our entire discussion is taking place on a Saturday? It's true that a soft clay layer will have a permeability through your soil, but regardless its directional properties, permeability is measured as a rate. The properties that make your clay soft have less to do with how fast flow is transmitted than their capacity of absorption. It's possible that the attraction of their respective platelets to their surrounding water molecules may be greater than the pressure from the overburden would apply to seperate them. If this is more the direction you were heading, you may be interested in the shrink-swell expansion test ASTM D-3877. Permeability through this layer may be greater or less than the surrounding layers, which may also be driving your horizontal flow. Best of luck with your endeavor.

 

[BigH]

Do a search - we went through this some several months ago and some good references were given (if my mind has come back to order after the New Year!!). See Terzaghi Peck and Mesri (1995), Article 14.4 "Permeability of Soft Clays", pages 74 to 76 inclusive.

 

[DRC1]

This topic is dicussed in the classic text "Construction Dewatering" by Pat Powers. Permiabilities do vary horizontally & vertically. The text provides a dicussion of how to handle these situautions. Because permiability is so varriable and dependent on so many factors, it is hard to give a general quantification. The best bet is to talk to other hydrologists in the area and see what their experince has been.