Converting CBR to Bearing Capacity
Converting CBR to Bearing Capacity
(OP)
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.





RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
Cheers
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
If 1% cbr is your condition, I would be looking for soil exchange or deep foundation solution.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
Thanks.
RE: Converting CBR to Bearing Capacity
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.
RE: Converting CBR to Bearing Capacity
RE: Converting CBR to Bearing Capacity
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!