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.
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.
RE: Liquid state of fine grained soil.
RE: Liquid state of fine grained soil.
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.
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.
RE: Liquid state of fine grained soil.
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.
Dik
RE: Liquid state of fine grained soil.
RE: Liquid state of fine grained soil.
Dik
RE: Liquid state of fine grained soil.
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.
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.
RE: Liquid state of fine grained soil.
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.
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.
I'm sorry, fattdad, can you explain this part of your previous post?
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:
How the fact that water is perched influence of effective stress?
RE: Liquid state of fine grained soil.
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'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.
Dik
RE: Liquid state of fine grained soil.
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.
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.
After all, thank you very much.
RE: Liquid state of fine grained soil.
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.
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.
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.