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Moisture Content to OCR 1
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I can't tell from your post what you are trying to predict from what, or why. Regardless, OCR would not correlate with water content directly. Wn would correlate with preconsolidation stress (assuming saturation) with little effect from changing OCR (not much volume change from unloading).
I have seen a correlation between liquidity index and preconsolidation stress: Carrier, W.D. III and J.F. Beckman, Geotechnique vol. 34, no. 2, pp. 211-228 "Correlations between index tests and the properties of remoulded clays." They were working mostly with mine and dredge tailings.
Yes, you WILL need Atterbergs and knowledge of stress history if you really want to predict Wn, or is it vice versa?
fattdad - Are you saying LI of 1 implies OCR of 1?
Regards,
DRG
I have seen a correlation between liquidity index and preconsolidation stress: Carrier, W.D. III and J.F. Beckman, Geotechnique vol. 34, no. 2, pp. 211-228 "Correlations between index tests and the properties of remoulded clays." They were working mostly with mine and dredge tailings.
Yes, you WILL need Atterbergs and knowledge of stress history if you really want to predict Wn, or is it vice versa?
fattdad - Are you saying LI of 1 implies OCR of 1?
Regards,
DRG
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For a given clayey soil, the void ratio is governed almost entirely by the preconsolidation stress. If the soil is assumed saturated, water content comes directly from void ratio and sp gr. If you consolidate it to 1.0 tsf, then unload to 0.5 tsf (OCR=2), the rebound is pretty small, so the void ratio doesn't increase very much compared to the amount of decrease when the initial consolidation goes from 0.5 to 1.0. (Unload-reload loops in the oedometer test are much flatter than the normal consolidation curve.
The whole idea of talking in terms of LI vs Sigma'p instead of Wn vs Sigma'p is so a correlation can be applied to a variety of materials. It works, although not real precisely.
Am I answering the right question here?
The whole idea of talking in terms of LI vs Sigma'p instead of Wn vs Sigma'p is so a correlation can be applied to a variety of materials. It works, although not real precisely.
Am I answering the right question here?
I suppose if you've just mixed the material for the LL test, it would not have experienced any higher past pressure and would have OCR=1, but if you start with a sample that's wetter than the LL, consolidate it until it reaches the LL [and yes, that can be done], then unload it, its OCR would be greater than 1. If Duncan said that, I don't agree with him.
DRG
DRG
DRG, that's an unusual case and no Duncan didn't say that! We were taught to look at the natural moisture content in comparison to the liquid limit to gauge for normally consolidated soils. Like many other things we do, it's an indicator that requires judgement to confirm.
Back to your unusual case. A hydraulic fill (e.g., settling pond at a quarry site) is a soil slurry that's placed at a moisture content greater than the liquid limit. Over time (as settlement occurs), the soil normlizes to an ocr of 1. At that point, the natural moisture content and the liquid limit would be the same (i.e., maybe they'd be "about" the same). If you considered an element of soil at the depth of 10 ft and excavated all but 2 ft of soil cover, the soil element would become overconsolidated. Not sure how much the change in void ratio from the unloading would affect the moisture content, but perhaps enough to make it less than the liquid limit. Interesting questions and I don't have a good answer.
Notwithstanding all that, if I see a soil sample where the natural moisture content is at the liquid limit, I'll continue to see it as normally consolidated, unless I happen to know it's a sensitive marine clay. Better safe then sorry - then again if it was critical, I'd just do an odometer test to confirm.
f-d
¡papá gordo ain’t no madre flaca!
Back to your unusual case. A hydraulic fill (e.g., settling pond at a quarry site) is a soil slurry that's placed at a moisture content greater than the liquid limit. Over time (as settlement occurs), the soil normlizes to an ocr of 1. At that point, the natural moisture content and the liquid limit would be the same (i.e., maybe they'd be "about" the same). If you considered an element of soil at the depth of 10 ft and excavated all but 2 ft of soil cover, the soil element would become overconsolidated. Not sure how much the change in void ratio from the unloading would affect the moisture content, but perhaps enough to make it less than the liquid limit. Interesting questions and I don't have a good answer.
Notwithstanding all that, if I see a soil sample where the natural moisture content is at the liquid limit, I'll continue to see it as normally consolidated, unless I happen to know it's a sensitive marine clay. Better safe then sorry - then again if it was critical, I'd just do an odometer test to confirm.
f-d
¡papá gordo ain’t no madre flaca!
No doubt, a natural water content near LL ordinarily occurs in material with low OCR, but it isn't necessarily so by principles of soil mechanics. As written, your post looked like a blanket statement that LI of 1 implies OCR of 1. I don't think you can even make the generalization that the settled water content is close to LL, especially when dealing with material containing montmorillonites, attapulgite, etc. For those, the settled water content can be a lot higher, and you don't find Wn=LL until pretty deep below the mudline.
In theory, you'd have to consolidate to Wn just below LL to have Wn=LL after unloading. In reality, the amount of rebound after unloading of such soft material is pretty small. It's not zero, but can be neglected within the precision that we can hope to achieve outside the lab.
If the water content > or = LL, it probably doesn't matter what the actual OCR is - it's soft, weak, and highly compressible regardless of OCR.
DRG
In theory, you'd have to consolidate to Wn just below LL to have Wn=LL after unloading. In reality, the amount of rebound after unloading of such soft material is pretty small. It's not zero, but can be neglected within the precision that we can hope to achieve outside the lab.
If the water content > or = LL, it probably doesn't matter what the actual OCR is - it's soft, weak, and highly compressible regardless of OCR.
DRG
I had to re-read this post several times because of the apparent confusion.
Fattdad, in your original post, you mentioned that an LL (Liquid Limit) of 1 could mean an OCR near 1. Dgillette later questioned fattdad's statement that an LI (Liquidity Index) of 1 implies an OCR of 1. The LL seemed to have changed to a LI.
In dgillette's latest post, he agreed that if the LL (Liquid Limit) is near unity, then the OCR may also be near 1, save for certain cases where the clay mineralogy is highly water loving.
Was this a case of "limit" being replaced with "index" somewhere early on as it relates to fattdad and dgillette's discussion? I don't want to be reading something that isn't there. Please let me know if I'm missing something.
Fattdad, in your original post, you mentioned that an LL (Liquid Limit) of 1 could mean an OCR near 1. Dgillette later questioned fattdad's statement that an LI (Liquidity Index) of 1 implies an OCR of 1. The LL seemed to have changed to a LI.
In dgillette's latest post, he agreed that if the LL (Liquid Limit) is near unity, then the OCR may also be near 1, save for certain cases where the clay mineralogy is highly water loving.
Was this a case of "limit" being replaced with "index" somewhere early on as it relates to fattdad and dgillette's discussion? I don't want to be reading something that isn't there. Please let me know if I'm missing something.
Not to confuse - my point is that if the natural moisture content is at the liquid limit, then the soil is normally consolidated. It is just a math fact that when the natural moisture content is at the liquid limit the liquidity index is one.
I really don't want to argue dgillette's points on the strange behaviors of smectites or other highly active clays. I'm also willing to concede that there are bound to be exceptions to the correlation between LI=1 and ocr=1 (i.e., sensitive marine clays).
f-d
¡papá gordo ain’t no madre flaca!
I really don't want to argue dgillette's points on the strange behaviors of smectites or other highly active clays. I'm also willing to concede that there are bound to be exceptions to the correlation between LI=1 and ocr=1 (i.e., sensitive marine clays).
f-d
¡papá gordo ain’t no madre flaca!
Thanks for the clarification. You're right that when the LL is equal to the nat. moisture content, the LI is unity. I can see dgillette's point too, though, that this case will only occur at one elevation within a given soil stratum of the same LL and PL. This could cause confusion.
For example, the nat. moisture content may be near the LL 5 feet below grade (say, in naturally sedimented clay deposit, not impacted by desiccation). In this case this soil element is NC. Take another soil element 30 feet below grade in the same soil. Because of the consolidation pressure at that depth, the natural moisture content may be much less than the LL, yet still NC based on an analysis of effective overburden pressure versus past effective stress. The LI in this case may be 0.25, yet the soil is still NC.
To circumvent the possibility of this confusion, I've always liked to evaluate a preliminary OCR by looking at the SHANSEP relationship: su/sig'v = +/-(S)OCR^m.
The effective overburden pressure can be estimated fairly well, and su can be estimated too in many cases. With this relationship, it's usually pretty easy to tell if you're dealing with an NC or lightly OC soil, as opposed to a highly OC soil. This relationship is also a good reminder that undrained shear strength varies with the effective stress at which it's consolidated.
For example, the nat. moisture content may be near the LL 5 feet below grade (say, in naturally sedimented clay deposit, not impacted by desiccation). In this case this soil element is NC. Take another soil element 30 feet below grade in the same soil. Because of the consolidation pressure at that depth, the natural moisture content may be much less than the LL, yet still NC based on an analysis of effective overburden pressure versus past effective stress. The LI in this case may be 0.25, yet the soil is still NC.
To circumvent the possibility of this confusion, I've always liked to evaluate a preliminary OCR by looking at the SHANSEP relationship: su/sig'v = +/-(S)OCR^m.
The effective overburden pressure can be estimated fairly well, and su can be estimated too in many cases. With this relationship, it's usually pretty easy to tell if you're dealing with an NC or lightly OC soil, as opposed to a highly OC soil. This relationship is also a good reminder that undrained shear strength varies with the effective stress at which it's consolidated.
This is another good way to look at OCR, but is difficult when the OCR is slight. If you have a saturated clay at teh depth of 25 ft, sig'v would be about 1,200 psf. For an Su/p of 0.2 that would equate to an undrained shear strength of 240 psf (0.12 tsf) or an unconfined compressive strength of 0.25 tsf - the limit of a pocket penetrometer.
¡papá gordo ain’t no madre flaca!
You're right about that, fattdad.
It works best in the upper, say, 10 feet where the soil has an OCR much greater than 2 or 3. As you evaluate the OCR with depth, you can usually get a good idea where the soil becomes NC or lightly OC. For one thing, like you said, you won't be able to evaluate su using a pocket penetrometer or torvane anymore!
It works best in the upper, say, 10 feet where the soil has an OCR much greater than 2 or 3. As you evaluate the OCR with depth, you can usually get a good idea where the soil becomes NC or lightly OC. For one thing, like you said, you won't be able to evaluate su using a pocket penetrometer or torvane anymore!
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