Under consolidated Clay (OCR<1) 3

[pelelo]

Hello,

I haven't seen many references regarding the settlement computation for "Under Consolidated clays" (OCR <1). In fact, the only reference I have seen is from "Foundation Engineering Handbook" by Robert Day.

My question is, since most of the geotechnical references deal with Normally consolidated and over consolidated soils, is it safe to treat an under consolidated clay as a normally consolidated case?

Please advise.

Thanks.

 

[fattdad]

No, it's not safe.

When the OCR is less than unity, pore pressure remains greater than atmospheric+ZgammaW. If that's the case, then settlement will have to occur in advance of obtaining OCR=1.

How do you know the OCR is less than one? I worked on a job once that had areas of hydraulic fill. I took a 1-in diameter piece of PVC, hand-slotted the lower 2 ft and wrapped it in fabric. I then just pushed the pipe into the hydraulic fill and came back in a few days. Water level was 2 ft above the ground surface.

Obviously, there'd be more settlement to come, irrespective of new loads.

f-d

ípapß gordo ainÆt no madre flaca!

 

[pelelo]

f-d,

Thanks for your response.

It is just in case in the future I run with this type of situation, I just wanted to know how to compute settlement.

Other than "Foundation Engineering Handbook" by Robert Day, do you know any other reference that can provide information settlement computations of under consolidated clays?.

Thanks.

 

[fattdad]

if you have the data. . .

Pore pressure readings show you that you haven't reached equilibrium.
Consolidation data shows you what to expect (don't forget the time readings).
Tube sampling gives you the index properties, void ratio and density.

At that point you'd know whether portions of the stratum have obtained normal consolidation and portions that still have relief underway. Where you have excess pore pressures, time will change the effective stresses. You'd then know the magnitude of the stress change to run the time rate and magnitude of settlement that remains.

Too much? Install wick drains, design a surcharge, or both.
Not too much? Make sure that you have the undrained strength you need for the proposed earthwork and can handle the latent settlement and the new settlement too!

f-d

ípapß gordo ainÆt no madre flaca!

 

[pelelo]

f-d,

After reading again your previous post in which you said you "worked on a job once that had areas of hydraulic fill. I took a 1-in diameter piece of PVC, hand-slotted the lower 2 ft and wrapped it in fabric. I then just pushed the pipe into the hydraulic fill and came back in a few days. Water level was 2 ft above the ground surface".

Just for curiosity, Does that behavior only occurs to under consolidated clays?. If so, how do you think a Normally consolidated and an over consolidated would behave like?.

Thanks.

 

[fattdad]

I have to peel-apart your questions.

Pore pressures greater than atmospheric+gammaW also occur from artesian flow. We were not talking about that so I didn't mention it. When you pile soil on top of saturated clay; however, those pore pressures are all related to the water carrying part of the new load. The relief of those water pressures are related to consolidation and that process changes the effective stress.

Cc and Cr for under consolidated, normally consolidated, and over consolidated soils are (should be) the same. The distinction lies in the value of Pp. In an overconsolidated soil, you'd get about 10 percent the consolidation. . . unless you exceed Pp. Then the load-to-consolidation trends should be the same for both.

Hope that helps?

f-d

ípapß gordo ainÆt no madre flaca!

 

[pelelo]

It did, thanks!.

 

[bdbd]

I can't remember where I read, but basicly: There is no under-consolidated soil. It is still NC. Because, that soil didn't experience any more load than it is experiencing at the moment, this is NC. Settlements are ongoing? OK, then effective stresses are increasing and its preconsolidation pressure is also increasing, still NC.

 

[fattdad]

we use different definitions. The term under-consolidated is used in geotechnical engineering. Not sure the benefit of saying it doesn't exist. When the pore pressures in the soil are greater than atmospheric+ZgammaW, then we call it under-consolidated.

I recognize the point you are trying to make. I just don't see it that way.

f-d

ípapß gordo ainÆt no madre flaca!

 

[LRJ]

There is such a thing as underconsolidated soil. It occurs due to things like rapid deposition and mud volcanoes.

 

[fattdad]

. . . and hydraulic fills. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[bdbd]

Dear all

OCR=Highest stress that soil experienced (or preconsolidation pressure) / Soil's current state of stress

So, if a soil is at the state of consolidation, there is excess pore pressure in the soil. So lets say, preconsolidation pressure of a soil is 2X, and we load it to 4X. Effective overburden stress on the soil is X.

At the start of consolidation: Preconsolidation pressure is still 2X, because load is carried by excess pore pressure. Effective stress is due to overburden and OCR is therefore 2.

At the t50, halfway of the consolidation: Half of the load is carried by soil skeleton, and other half is carried by excess pore pressure. So, therefore total effective stress on the soil is 2X+overburden stress (X) = 3X. Lets get back to the OCR formula, hşghest stress soil experienced? 3X. Soil's current stress? 3X.

Calculations are simple. Underconsolidation is physically meaningless.

 

[LRJ]

Your calculations:
p'₀ = X
p'_c = 2X
OCR = p'_c/p'₀ = 2
Applied load, q = 4X

At t₅₀, half of the excess pressure has dissipated: Δp' = 0.5*4X = 2X
p' = p'₀ + Δp' = X + 2X = 3X > 2X (OCR)

Now let's consider t₁₀:
Δp' = 0.10*4X = 0.4X
p' = p'₀ + Δp' = X + 0.4X = 1.4X < 2X (OCR)

So are you on the C_r (also referred to as C_s) or C_c line for consolidation at this point? For an overconsolidated soil you would be on C_r. For a normally consolidated soil you would be on C_c. This is the essence of the OP's question.