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.

 

[oldestguy]

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.,

 

[LRJ]

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.

 

[Sjotroll]

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.

 

[fattdad]

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!

 

[Sjotroll]

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.

 

[Sjotroll]

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?

 

[dik]

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

Dik

 

[oldestguy]

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.

 

[dik]

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

Dik

 

[fattdad]

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!

 

[LRJ]

... 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.

 

[Sjotroll]

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?

 

[oldestguy]

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?

 

[fattdad]

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!

 

[LonelyDeer]

Here we have a section of dumb questions.

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

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:

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?

 

[Sjotroll]

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.

 

[fattdad]

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!

 

[dik]

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

Dik

 

[LonelyDeer]

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 \\