Converting CBR to Bearing Capacity 3

[PhiIrl]

I am working on a foundation design. The only data I have on the sub base is it is a silt with a CBR of 1%. Does anyone know how I can approximate a bearing capacity from the CBR? Thank you.

 

[Ron]

Is there a typo here? You stated CBR of 1%? CBR is a value, not a percentage. It does not correlate well to a bearing capacity other than in a gross classification sense.

 

[Focht3]

Well, strictly speaking, it is a percentage. You're comparing the resistance of a given material to the resistance of an ideal crushed stone that will support 1,000 psi at 0.1 inch deflection. Read the procedure - the final step is to multiply by 100.

CBRs don't exceed 100 - because they can't exceed the "ideal" material.

We don't normally talk about them as a percentage - hence your confusion.

 

[VAD]

There is a relationship by W.P Black in Geotechnique( British Journal) around 1961. I am going by memory here.

Cheers

 

[1967pradyot]

CBR 1% is a real weak value for a subgrade.

W.P.M.Black in "Geotechnique" (Vol 11, No.1 of 1961 & Vol 12, No.4 of 1962)has shown that the CBR is approx. equal to one-tenth of the ultimate bearing capacity (UBC) of soil (UBC measured in pb/ sq. inch.

 

[ashjun]

Perhaps are we going overboard?

The CBR is normally determined for a thin layer of sub-base (say 6 to 12 inch thick). Whereas the bearing capacity under any footing depends upon the soil conditions up to at least 1.5 to 2 times the width of the footing. True, the correlations may exist between the two, since the CBR is normally determined for 2.5 or 5mm penetration (in a confined mould!) and the bearing capacity determined for 25 or 50mm settlement, but are we correct to follow that?

And yes, CBR greater than 100 is possible using stabilised soils.

 

[Focht3]

ashjun is correct - you can get CBR values of over 100 for stabilized materials. But the AASHTO design guides were developed assuming the CBR of a crushed rock would not exceed 100, so the direct use of values over 100 in a design sense is questionable. Additional assumptions have to be made -

And I was speaking of crushed rock - no modifiers - anyway.

But ashjun is absolutely correct when he brings up the issue of footing size relative to the CBR test. Read the following thread:

thread261-54742

It's long, but worth the time. And it covers the issues that ashjun raised - in more detail.

 

[BigH]

See other thread in this forum - a current one regarding soaked and unsoaked. Is there a way to see if the 1% is erroneous??? I'd hate to even think of correlations based on one value that might be FUBAR. I would be real surprised that a silt has 1%. We are using fly ash - 30% sand size and rest as silt and getting more than 4% regularly although I did see a 2% once but I believe that they were using "slimes" or end of pond dregs.

As a rough thought - and Focht3 is right if the thread he quotes for PLT is the one I am thinking of - there are some correlations given in highway engineering books taking CBR values and getting "k" values. If you know "k" - and are cognizant of the "subject to" caveats, you could get a rough idea of allowable bearing value for a given settlement. weak argument I am sure some would say - me too - but at least it is something to guestimate by.

 

[Focht3]

BigH is right - CBR of only 1% sounds low for a silt.

You really don't have enough to accomplish what you are seeking. However, you could make some "smart guesses" and back them up with a geotechnical study. One way to look at the problem is to evaluate the foundation system assuming the phi angle of the soil is about 20. That's a pretty low value for a granular soil. But you will get pretty good performance with even a moderate amount of footing embedment.

The bottom line is - you need a geotech study.

 

[BigH]

Just to add - our fly ash (as described above) has a phi angle at 95% modified of about 35 degrees!!

 

[LSUDirtEng]

Your foundation design needs to take more into account than the sub base (subgrade) characteristics. One should look at the soils to at least twice the depth of the footing proposed. If indeed you have CBR of 1% I would want to know have far this material extends and what the percent passing the #200 sieve is to gain some idea of its characteristics when wet (be assumptions here). I'll take clay before silt to build on any day.

 

[BigH]

LSUDirtEng: Not if you have Norweigian Quick Clay or Canada's! - I would hope.

 

[Focht3]

The "clay vs silt" thing has a lot to do with our education as geotechnical engineers. Very few of us have a good handle on how to deal with silts. If confined, silts can be 40 to 60 percent stiffer than a very stiff to hard CH clay. With no appreciable long term settlement issues.

 

[LSUDirtEng]

BigH,

I'm sure Norweigian Quick Clay and Canada's have their own nasty behaviors. I use to have to deal with New Orleans Gumbo clay.

Focht3, your right about confined silts; still a pain to get good geotechnical properties to design piles in (was starting to get a handle on using CPT data when I left New Orleans). But the initial problem stated sub base, so I assumed unconfined silt at the surface.

 

[doresh]

Normally, cbr value is determined in specific moisture content and density which ensure certain needed state which is suitable to pavement design. In other moisture contenet and density you can reach higher CBR value, when considering more swelling . CBR value is number that you decide on when considering the desired behaviour of your subgrade. There is not one to one translation to bearing capacity because before you will place your foundation you will not be able to bring the soil to the relation between moisture content and density that are needed to get 1% cbr, because you will be need to compact the layer, moist it or dry it and it will be immpossible in small area.

If 1% cbr is your condition, I would be looking for soil exchange or deep foundation solution.

 

[Focht3]

LSUDirtEng:

You must not be Cajun - cuz dem folks don' move 'way fum da bayou! (I have a lot of friends in south Louisiana - particularly New Orleans. A real experience to live there for even a short time.) And I love Cajun cooking!

Even "shallow" foundations can bear on waterbearing silts and perform well. Go by the Westlake development in west Houston - 20 to 30 story buildings supported on spread footings, strip footings and core mats. The foundations for Westlake III (built in mid-late 1980s) bear directly on a red waterbearing silt. The building has performed quite well for almost 20 years.

 

[PhiIrl]

I would like to thank everyone for replying to my query. Unfortunately, it would appear that it is not possible to derive the bearing capacity from the CBR. However, I have learnt a lot more about CBR which used to be just a value for me.

Thanks.

 

[jdmm]

A useful correlation that I have found to go from undrained shear strength to CBR is:

CBR=Su/23 where Su is the undrained shear strength in kPa.

For your CBR of 1 the undrained shear strength would be about 480 psf. I use this correlation to go from undrained shear strength to CBR for road design but would hesitate to go the other way. The best recommendation is to get better soil information such as Su, Atterberg limits and moisture content rather than rely on a parameter that was not designed to be used to predict bearing capacity.

 

[DBNodurf]

Well John, sum ob dem fochs doo move away fum the bayoo, cuz my great-grandaddy wuz one of dem... he went to KY drilling oil and met my great-grandmother to be on the ole ancestral farm, and the rest is history... btw, this was meant to be as humorous as yours was

 

[Focht3]

Hmmm,

It's like my Mom has always told me: "Wear clean underwear and behave yourself - you never know what will happen, or who will see what you do." Since I didn't give my first name, you must know me (or my Dad or Granddad.) Care to give me a clue about who you are? (And I appreciate the humor!)

And for what it's worth, some of my ancestors came from Louisiana. My 2G-grandfather, Daniel Esquiredge Thomas, fought in the Civil War - and surrendered at Vicksburg. So I have mudbugs in my family tree, too!