OCR and ground water table 1

[haynewp]

From what I understand, usually if the ground water table rises in a soil, then the OCR should increase below the water table (if normally consolidated originally, higher effective stress after table rise).

Could there be a case where the soil's void ratio is very low, and a rise in the ground water table would result in a decrease in effective stress below?

 

[haynewp]

I think I had it backwards, if the water table increases then the effective stress will usually go down at a point below the table due to the increase in pore water pressure.

Can the effective stress ever go up due to a rise in the water table?

 

[GeoPaveTraffic]

I can't see any way for the a rise in groundwater to result in an increase in effective stress.

 

[haynewp]

What is the highest void ratio you have seen?

The reason I was asking is that my company has asked us to give lunch seminars for continuing education.
I was going through a soils book I bought and this question occured to me.

 

[kmerl]

Are you asking for manufactured materials or natural?

The reason the effective stress envelope decreases (put simply) is the buoyancy on each soil particle increases. So in contrast when you lower the groundwater table the upper soil layers can compact (auto-compaction).

Example:

In extreme groundwater extraction within the coastal plain this can lower the water table locally and compact the upper sands. This is a concern because it lowers the permeability of the upper sands creating a shell in the areal extents of the groundwater belly and thereby effecting a reduced recharge in the cone of depression.

So essentially due to the "imaginary" buoyancy force on each soil particle the effective stress will not increase.

Regards,

Keithe J. Merl

 

[haynewp]

So the void ratio remains the same when the soil becomes saturated after a rise in the water table.

The void ratio will only change after the water is drained (autocompaction), or through another means of external compaction when the soil is in a dry state, correct?

 

[kmerl]

Correct, remember the void ratio is the amount of air in the soil, which becomes filled with water when GW rises. Now that is not to say the materials will always have a reduction in void space upon water extraction.



Keithe J. Merl

 

[Focht3]

Void ratio is the amount of non-soil space within the soil volume. It can be occupied by air or water. Degree of saturation is the ratio of space occupied by water compared to the total void space.

Volume changes due to changes in soil moisture seldom follow simple rules. As a general rule, "loose" soils often densify with changes in soil moisture - whether they are dried of wetted. "Medium dense" soils may shrink, expand - or do nothing - with changes in soil moisture, depending on the soil type. "Very dense" soils don't really respond much to soil moisture change unless they contain clay; then the predominant volume change is swell with soil moisture increases.

Now, if I could narrowly define "loose", "medium dense" and "very dense" soils we'd have a neat arrangement. Unfortunately, I can't. Every soil is unique, and seemingly minor variations in the soil unit weight can have a significant influence on soil behavior.

What's the maximum void ratio that "can" occur? Well, it's pretty high - as I recall, some of the montmorillonite clays beneath Mexico City have Plastic limits of around 100 with Liquid limits of up to 400. Natural water contents can range anywhere between, but are often in the neighborhood of 250. You can calculate the void ratio and unit weight ("density&quot of these soils assuming the sample is fully saturated and has a specific gravity of about 2.8 -



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