CBR Value.

[Sean213]

I am working on a particular job that has poor ground conditions mainly consisting of aluvial deposits and silty clay. The specification for the access road states that a CBR Value at formation level is to be atleast 3%.

Using a Penetrologger, I have calculated that the bearing capacity of the clay at formation level is on average 1500kN/m^2. Unfortunately I could not find a direct conversion from MPa or kN/m^2 to CBR. Could anyone point me in the right direction if there is one?

Road is to take 300 Tonnes.

I am new this game, still studying at University. But could somebody also explain why when we CFA piled a particular area, 1 of the piles could not obtain any friction and the clay turned liquid?

Regards

 

[fattdad]

You really need to review the requirements for a CBR test. It has nothing to do with "Bearing Capacity" and there is no conversion.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

Agree with f-d....apples and oranges.

Also, a CBR of 3 is very low...I would expect that of relatively soft clay or silt.

To give you perspective, a clean sand will have a CBR of 10 to 20. A clayey sand will typically have a CBR in the range of 20 to 30. A graded aggregate base will typically have a CBR in the range of 70+

 

[fattdad]

I'd also agree that a CBR of 3 is low. That said, in the Piedmont of Virginia (where I formerly worked as a consuntant), we get this all the time, even for soils compacted to 95 percent Std. Proctor. That 4-day soak will kill a CBR value sometimes. . .

We used to design for CBR 3 soils too, depending on the project. Now that I'm working for the Department of Transportation, we'd just undercut it 2 ft and bring in something for better long-term performance.

People just don't like maintenance of their freeways.

f-d

¡papá gordo ain’t no madre flaca!

 

[Sean213]

Since I sent the last message; I have been working on other things so I haven't been able to comment. The client have said that they want a bearing capacity 34.6kN/m^2 for the clay at 1.2m depth (formation depth).

Currently with the tests I have done I am getting a reading of 0.6MPa in one particular area of 100x7m according to my conversion, 0.6MPa is the same as 600kN/m^2? does this not seen extremely high?

 

[Ron]

The bearing capacity of the soil for foundation bearing "is what it is"...and you're not going to change that much if any. You have to work with what you have.

If you want a shallow foundation system, you determine the bearing capacity of the soil and design the foundation accordingly. If the bearing capacity of the soil is too low to get a reasonable foundation size, then you must either lower the foundation to a stronger stratum, use a mat foundation, or go to a deep foundation system.