Moisture Content to OCR
Moisture Content to OCR
(OP)
Is anyone familiar with a "correlation" from Wn to OCR
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RE: Moisture Content to OCR
f-d
¡papá gordo ain’t no madre flaca!
RE: Moisture Content to OCR
RE: Moisture Content to OCR
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
RE: Moisture Content to OCR
RE: Moisture Content to OCR
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?
RE: Moisture Content to OCR
According to Prof. Duncan - yes, that's what I'm saying.
f-d
¡papá gordo ain’t no madre flaca!
RE: Moisture Content to OCR
DRG
RE: Moisture Content to OCR
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!
RE: Moisture Content to OCR
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
RE: Moisture Content to OCR
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.
RE: Moisture Content to OCR
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!
RE: Moisture Content to OCR
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.
RE: Moisture Content to OCR
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!
RE: Moisture Content to OCR
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!