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Liquid state of fine grained soil.
2

Liquid state of fine grained soil.

Liquid state of fine grained soil.

(OP)
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.

RE: Liquid state of fine grained soil.

In my view "liquid" means it will flow freely if not confined. Try that with silty sand and gravel in a compact state, yet fully saturated.. Or with a clay that is near the plastic limit and below the liquid limit. You may be right as "not fully saturated", but at 99 percent of voids filled, who cares.,

RE: Liquid state of fine grained soil.

Saturation relates to the void space being full of fluid. Soil does not necessarily 'flow' because all void space is full of fluid; the solid particles can still remain stuck together.

RE: Liquid state of fine grained soil.

(OP)
That makes sense, thanks!
What were you referring to when you said "try that with silty sand..."?
Yeah I know that 100% saturation is unachievable but even so the liquid limit must be below that "fully saturated" water content.

RE: Liquid state of fine grained soil.

2
Clay below the water table may not be fully saturated.
Sensitive marine deposits can and often do have water contents greater than the liquid limit. Watch the Rissa Landslide coverage to see what sensitive clay is like. https://youtu.be/3q-qfNlEP4A
Clay below the water table is typically below the liquid limit, as the voids have been consolidated.

Hope some of this helps?

f-d

ípapß gordo ainÆt no madre flaca!

RE: Liquid state of fine grained soil.

(OP)
LRJ what do you mean? The liquid limit is by definition the water content needed for the soil to pass from plastic state to liquid state. Of course this applies only to fine grained soils.

RE: Liquid state of fine grained soil.

(OP)
Thanks fattdad, that documentary surely is interesting and helpful!
So let me sum up everything to see if I got everything right. Clay under the water table can have water content higher than liquid limit, but as oldestguy said it won't flow because it is confined and undisturbed. Then, clays which consolidated will have smaller void ratio than the void ratio for which the liquid limit was determined, so it is possible that such clays even if fully saturated will have water content below the determined liquid limit. Am I right?
There is still something I don't understand that fattdad mentioned. How is it possible for clays under water table to not be fully saturated?

RE: Liquid state of fine grained soil.

fatdad... I had no idea there were soils like that... excellent clip.

Dik

RE: Liquid state of fine grained soil.

Here is one for you. Do some looking up the general subject of fundamental properties of clays including the crystal graphic units in their make up if you want to explore these details.

RE: Liquid state of fine grained soil.

OG... I assume these are the 'highly flocculated' marine type of clays that were mentioned in undergrad geotechnical courses.

Dik

RE: Liquid state of fine grained soil.

Imagine a clay that is not saturated.

Inundate that clay. Some of the air pockets will be shoved out of the way. Other air pockets will become occluded - completely surrounded by water-filled pores.

How does that air get out of the void? Well, it takes enough water pressure to make it dissolve. In many instances you can be 20 ft below the water table (<10psi) and there's still air in the voids. We know in the soil laboratory it often takes 40 to 70 psi of backpressure to fully saturate a clay!

Now, does it matter? I mean in testing it matters, but in undrained soil mechanics, the bleb of air is constrained by permeability just like the water in excess pore pressure.

Just things to bring to the discussion.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Liquid state of fine grained soil.

Quote (Sjotroll)

... the liquid limit must be below that "fully saturated" water content.

This is not necessarily the case. Indeed the in situ water content of saturated soil can even be below the plastic limit.

RE: Liquid state of fine grained soil.

(OP)
A lot of very interesting things to think about. Do you have some literature to recommend which goes in more details on the subject of clays?

RE: Liquid state of fine grained soil.

Yes. One source is the text by B.K. Hough "Basic Soils Engineering". Ronald Press, 1957.
Why does one de-air a container of water containing the sample to be tested for grain size by the hydrometer method?

RE: Liquid state of fine grained soil.

p.s., this is why I never take at face value the proclamation of perched water when a saturated sand is seen flowing water into an underlying clay (i.e., as in a test pit). Too many folks want to believe what they see, the water is flowing on the sand and not within the clay. Maybe. . . ?

To really know, just install a piezometer into the clay and another one in the sand. If they have the same head, it's not perched. If they have different heads, then you'll know if there is a gradient - and it may be up or down, eh?

Think if the water is not perched? Your calculations on effective stress will be wrong.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Liquid state of fine grained soil.

Here we have a section of dumb questions.

I'm sorry, fattdad, can you explain this part of your previous post?

Quote (fattdad)

To really know, just install a piezometer into the clay and another one in the sand. If they have the same head, it's not perched. If they have different heads, then you'll know if there is a gradient - and it may be up or down, eh?
Think if the water is not perched? Your calculations on effective stress will be wrong.

Let's consider situation you've described.
If we install piezometer in the clay and other in the sand and they have the same head, then it means that water in the clay have connection with water in the sand (continuum) and effective stress in the clay will be σ_eff = σ_tot - u, where u - is our pore pressure (measured by piezometer).
If we have different heads in the sand and the clay, that means that here we have gradient and water flow in some direction, but effective stress is still σ_eff = σ_tot - u.

But what in case of almost impervious soil (fat clay)? What piezometer head would be in that case? I think zero, but i'm not sure.

An what you mean by this:

Quote (fattdad)

Think if the water is not perched? Your calculations on effective stress will be wrong.
How the fact that water is perched influence of effective stress?

RE: Liquid state of fine grained soil.

(OP)
Hi LonelyDeer, I will try answering that.
First, effective stress depends on the flow of water (gradient):
Sig_eff = Sig_tot - (u + i * u) for water flowing upwards, and
Sig_eff = Sig_tot - (u - i * u) for water flowing downwards.
That is the reason why failure can occur for water flowing upwards with too high gradients - the effective stress will be reduced to 0 and since the shear strength depends on effective stress the frictional component of shear strength will also become 0.

For the second part, I think he means this (correct me if wrong):
The perched water table means that there are 2 water tables (pore pressure = 0) divided by an impermeable layer which might have negative pore pressure. This means that if you detect water table in the first layer it doesn't necessarily mean that the pore pressure will linearly increase with depth.

RE: Liquid state of fine grained soil.

I hadn't even thought of upward water flow and critical gradients, ala piping!

I'm just saying that effective stress is total stress minus water pressure. If you assume that the water is perched (and it's not), you'd calculate an effective stress in the clay that's not correct. That, to me, is a problem.

On the other hand, if the water table is perched, the assumption of hydrostatic water pressure with depth is incorrect.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Liquid state of fine grained soil.

OG: Newtonian liquid, a subset of fluid cannot maintain shear stresses, except at a high velocity.

Dik

RE: Liquid state of fine grained soil.

Thank you, Sjotloll, fattdad, dik, for all your answers. I think I've got it.
But can you answer one more question? If we have a very thick layer of impermeable clay, with no free water (all water bound with clay particles), but with high saturation ratio. What value of pore pressure will we observe? Zero? I mean, does piezometer measure only pressure in free water? Or bonded water will give us some value of pore pressure?

In simple way - piezometer is a tube with graduation, which we insert in soil and observe water head. I think, if we insert tube in impermeable clay, we will get zero(if the clay is impermeable, water doesn't flow through it). But what about special tools? I mean electronic piezometers. Common sense says: "Yep, it will be zero". But... \\ shrug soulders \\

RE: Liquid state of fine grained soil.

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!

RE: Liquid state of fine grained soil.

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.

RE: Liquid state of fine grained soil.

(OP)
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.

RE: Liquid state of fine grained soil.

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!

RE: Liquid state of fine grained soil.

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.

RE: Liquid state of fine grained soil.

(OP)
@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

Quote:

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?

RE: Liquid state of fine grained soil.

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.

RE: Liquid state of fine grained soil.

(OP)
Thanks Okiryu, I will check out both the book and the thread! thumbsup2

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