Liquid state of fine grained soil. 2

[Sjotroll]

Hi,

I was thinking and I came to something that confused me. I will tell you my line of though so please tell me if I went wrong somewhere, and if not I'd appreciate an explanation.
So, my first assumption was that soil under the water table is fully saturated.
The second assumption was that the liquid limit can't be higher than the maximum water content.
Which means that fine grained soils that are under the water table (saturated) are in a liquid state. And this confused me because I don't think that's the case. So what is actually happening here? Did I go somewhere wrong or is there something I didn't take into consideration?
The only possible anwswer that I see is that soils under the water table aren't fully saturated.

 

[fattdad]

but it's not, "Impermeable!"

Effective stress accounts for the pore pressure, irrespective of saturation. So, technically, if there is negative pore pressure, the effective stress would be greater than the total stress.

We have to judge whether accounting for negative pore pressure is a good thing - 'cause after all during other seasons, that negative pore pressure may be temporal?

We also have to judge whether the pore pressure increases by the unit weight of water even within an, "Impermeable" clay.

There is no clear answer, but in marine clay deposits there may be a head condition or there may not be a head condition and it's up to you (us) to figure out whether it's critical to the geomechanics of the actual problem. I mean in a friction pile analysis it may not matter too much? Don't know?

f-d

ípapß gordo ainÆt no madre flaca!

 

[LonelyDeer]

You're right, fattdad. Maybe this is not so important, but actually, I'm just curious. I've never take part in real geological survey in field. Maybe it sounds dilettante, but I'm really just wonder, what will be if I dig a deep hole in earth and insert piezometer in clay with very low permeability.
After all, thank you very much.

 

[Sjotroll]

Let's see a few scenarios for a soil profile with 2 layers (a sand layer on top of a clay layer) with the water table somewhere down in the clay layer:

If a perched water table forms because of rain and by other means in the sand layer, then you will have positive water pressures inside the perched water zone, while no pressures in the clay layer which has no water (assuming you didn't dig deep enough to enter the suction zone). If you dug deep enough to enter the suction zone then there will be negative water pressure (matrix suction). If you go even deeper and pass the real water table then of course you will have positive pressures as in any other material.

So, the conclusion is that no matter the type of soil, if there is a water table in it then there are pore pressures under the water table which can be detected with a piezometer.

 

[fattdad]

Just a further comment on permeability. It's common to feel that clay soils with low permeability do not convey flow.

That's a flawed thought.

To the OP: Have you ever drawn a flow net? Flow nets are the graphical solution to the LaPlacian second order, non-homogeneous differential equation. When drawing a flow net, there are flow lines and there are equipotential lines. When you are done, you get a shape factor and you can make conclusions about water flow within the domain of the flow net.

No part of the flow net relates to permeability. You see, permeability does not inform any aspect of developing the flow lines or the shape factor. It's only after these details are determined that you run the numbers to calculate the flow. Clearly if the permeability is 10-7 you'll get less flow then if the permeability is 10-4. After the flow net is drawn it's linear. Just scaled by permeability. Doesn't mean that the flow net doesn't influence pore pressures, however.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

LonelyDeer, I am not an expert in groundwater but piezometers measure static groundwater levels. Without loading the soil, the measurements with the piezometer will give you the location of the groundwater. If you load the soil, then you see the excess pore water pressure which at time zero, in theory, it is the same as the applied load. If you have confined aquifers, you can measure also the confined pressure.
So, groundwater flow which relates to permeability is not measured with piezometers, you will need other instruments/methods (

https://pubs.usgs.gov/wsp/1544c/report.pdf).

Also, I agree that sensitive marine deposits can have W% close or above the LL. You can also have an idea if soils are normally consolidated comparing the W% and LL.

 

[Sjotroll]

@fattdad
I did. Sorry but no matter how many times I read it I cannot figure what you wanted to tell me with all that regarding the flow net. Probably I'm missing a connecting piece between my answer and your question. Could you please explain?

@Okiryu

You can also have an idea if soils are normally consolidated comparing the W% and LL.

How would you do this? Is there some specific relation/connection or is it more like: if the W% is near the LL in saturated soil than it is normally consolidated and if not than it is OC, or something like that?

 

[Okiryu]

Sjotroll, look at this old thread. There are good discussions there.

https://www.eng-tips.com/viewthread.cfm?qid=202002

Also, there is not a proved relationship for W%-LL-OCR but if you look at real soil data, you can see that the OCR decreases when the W% is approaching to the LL. Lambe and Whitman's book in Chapter 7 has several examples for soil profiles which shows W%-LL and pre-consolidation pressures. I think that the profiles in L&W book shows this tendency.

 

[Sjotroll]

Thanks Okiryu, I will check out both the book and the thread!

 

[Okiryu]

Sjotroll, for your reference, the attached is from Bowles book.