Estimation of OCR by using any soil parameter

[honeywhite]

Hi,

I'm desperately searching some method to estiamte value of OCR by using any other soil parameter as undrained shear strength, effective shear strength, angle of shearing resistance, effective angle of shearing resistance etc.

 

[dgillette]

It's kind of bass-ackward, but you can make a preliminary cut at it by using a measured undrained shear strength and an assumed strength ratio. (Normally, we use OCR to estimate undrained strength.) See C.C. Ladd's Terzaghi Lecture and other publications mentioning SHANSEP. If the OCR is high, "It's high" is about all you will be able to say with any confidence.

That can also be done with the CPT. You can download Peter Robertson's CPT manual from Gregg Drilling's web site - gratis.

http://www.greggdrilling.com/Resources/CPTforGeotechnicalGuide/CPTguiderequestpage.htm

As I recall, Robertson's CPT approach works much the same way as what's described above.

What do you want the OCR for?

Regards,
DRG

 

[fattdad]

it the soil is at its liquid limit it's normally consolidated. If it's less than the liquid limit, there is some overconsolidation.

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

"If the soil is at its liquid limit it's normally consolidated." Not necessarily, fattdad. Under moderately high overburden, like 1 tsf, the water content of saturated NC clay can be substantially lower than LL. I pulled out some old consolidation test data to check on that.

If it's wetter than LL, it's never felt much overburden at all, and quite probably is NC.

 

[fattdad]

O.K. if the clay is a fraction over its liquid limit (somewhat a judgmental test from the onset, I might add) it's normally consolidated. Limitation: Sensitive marine clays.

I do check liquid limit and natural moisture content for this very reason, but am also blessed by having a good lab (well and some experience).

Point taken dg.

f-d

¡papá gordo ain’t no madre flaca!

 

[honeywhite]

Hi,

Thank you both for your replies.

I need OCR to calculate a parameter for Soft Soil Model i Plaxis. I'm writing a diploma about working platforms and I need some calculations in MES. But I have a problem with one parameter - modified swelling index (kappa*).
I want to use an equation 6 Table 7.1a from material manual for Plaxis 8:
kappa* = 2Cr/2.3(1+e)
so that I need a Cr:
Cr = (e1-e2)/log(sigma'p/sigma'1)
In above equation I have everything except sigma'p, but I found out that I can calculate it if I have OCR from equation:
OCR = sigma p/gamma*z

If you have any suggestions how to solve my problem, I'll be really grateful

Regards,
Kate

 

[dgillette]

Hi Kate. I'm a little confused here, and something seems circular:

"Cr = (e1-e2)/log(sigma'p/sigma'1)
In above equation I have everything except sigma'p, but I found out that I can calculate it if I have OCR from equation:
OCR = sigma p/gamma*z

Do you have a consolidation test with rebound? If so, you have Cr and sigma'p as directly as possible.

By definition, OCR is sigma'p/[sigma' at present]. The denominator is (assuming a single layer) gamma*depth - u. The OCR equation you have seems to be neglecting the pore pressure u. If there is no pore pressure, then the present value of sigma' is gamma*z (assuming z is the depth), or total stress = effective stress.

Have never used Plaxis, so I don't know what its soft soil model is like. Does Plaxis understand pore water pressure? Our people use FLAC, and we go for the simplest models we can justify for any particular case, often just elasto-plastic Mohr-Coulomb, but our MC strength is always based on effective stress. (FLAC does understand water tables, gridded pore pressure, etc.)

Also, what is MES?

 

[honeywhite]

Hi dgillette,

Thanks again.I must think over what you've write.

I have results form oedometric test, triaxial shear test and particle-size analysis.

It seems that the oedometric test was made without rebound, but I must ask a person who has done it.

Sorry about MES. I was too fast and write a polish shortcut from finite element method (in polish - Metoda Elementów Sko?czonych).

And about Plaxis - yes it understand pore water pressure. Here you have a general informations about program (2D):

http://www.plaxis.nl/files/files/Plaxis 2D v9.0 - 1 General Info.pdf

More you can find on Plaxis site:

http://www.plaxis.nl/

Regards,
Kate

 

[fattdad]

If you have an odometer test, then you can determine OCR, whether you have a rebound curve or not.

f-d

¡papá gordo ain’t no madre flaca!

 

[TDAA]

Exactly as f-d said, preconsolidation pressure divided by the present effective vertical pressure. You can determine the preconsolidation pressure graphically.

 

[vulcanhammer]

fattdad, glad you brought that up, I just explained that to my Soil Mechanics class yesterday:

http://www.vulcanhammer.net/utc/ence361/s2010/361-sl13.pdf

http://www.pz27.net

 

[honeywhite]

Ok,

Thanks everybody. Tomorrow I'll discuss this problem with my supervisor.

Regards,
Kate

 

[BigH]

You can only determine the OCR "IF" you have taken the consolidation test past the apparent maximum-attained overburden pressure (which, I presume, has been done. Might suggest that to find the maximum past pressure from a plot of logp' vs log e.

You could also estimate, roughly, by looking at the relationship of Su/p' = 0.23 (approx 0.23 - see Terzaghi, Peck and Mesri). If you know po', you can estimate Su(o) as Su(o)/0.23. If you have Su(ocr), you can estimate p'(ocr) similarly, Then Su(ocr)/Su(o) ~ OCR. Su(ocr) = Su in situ. Su(o) = estimated Su at po'. (I think this is right - I've been on holiday for a couple of weeks . . . .)

 

[dgillette]

Big H - that's reasonably reasonable at low OCR, but at higher OCR, strength drops off some as po' is reduced from preconsol pressure.

C.C. Ladd's SHANSEP shows this as an exponent m being less than 1:

Su/sigma'vc = S(OCR)^m

In other words, if OCR is high, Su is a bit less than directly proportional with precon pressure.

 

[fattdad]

SHANSEP: I think of Jamliokowski (sp). I also think 0.2 as a default value (not that 0.23 isn't in the range).

So, you could use 0.2 (or 0.23) and a pocket penetrometer to estimate OCR. I'd think that would be o.k. with stated limitations, that is. . .

Giving props to BigH for mentioning this in the first place, that is. Vacation must have been good for you, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[YannisChasiotis]

OCR is highly dependant on sample disturbance, cementation and many, many other factors. The reliability of obtaining its value from the oedometer is similar to obtaining undrained strength from Unconfined Compression tests...

Since OCR is directly related to geological processes, has anyone try to ask an engineering geologist?

The question is: How much overburden, if any, has been removed?

I 've done it many times...never received an answer...has anyone?

 

[BigH]

Yannis - but one must not forget the "aging" process in the OCR discussion. Soils that have "never" seen a higher O/B pressure show overconsolidation.

 

[geoman110]

Fattdad
The 0.23 factor (as noted by BigH) is correct, not 0.2. More accurately, the factor is 0.23 +/- 0.04.
When you read different references, it is a bit confusing and you have to sort a few things and connect different concepts to understand the reasoning behind the equations. “dgillet” is correct and the equation is valid for low to moderately over consolidated clays with low to moderate PI and for higher degree of consolidation the equation should be corrected by a factor of OCR^m (as noted by dgillet). This equation is applicable for Su obtained from Triaxial or direct shear test. However, the confusing part for me is that the exact same equation is provided in various references for Su obtained from vane shear tests and there is no correction with OCR^m. I even read in one references that for field vane shear test the m is equal with 1. However, there is no explanation for this. Why is m=1 for field vane test and different for other lab tests (note that we are talking about corrected field vane test results). I personally believe the only reason that various references do not mention anything about correction of OCR^m is that in real life nobody conducts the field vane shear test for soil with high OCR. Therefore, the actual tests always happen for low OCR soils. This is just my assumption and I have not yet seen any reference to confirm or disapprove it.

 

[fattdad]

YannisChasiotis: Desiccation in the near surface can provide overconsolidation.

geoman110: O.K. then. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[dgillette]

m might differ from what it is in other tests because of Ko varying with OCR. With higher OCR, the ratio of sigma-h to sigma-v is higher (giving higher p' for a given sigma'v). I haven't done any math to see if that could explain it, however.